Kinetic Isotope Effects in Free Radical Chemistry. Kenneth George Kneipp Louisiana State University and Agricultural & Mechanical College

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1971 Kinetic Isotope Effects in Free Radical Chemistry. Kenneth George Kneipp Louisiana State University and Agricultural & Mechanical College

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72-17,778

KNEIPP, Kenneth George, 1944- KINETIC ISOTOPE EFFECTS IN FREE RADICAL CHEMISTRY.

The Louisiana State University and Agricultural and Mechanical College, Ph.D., 1971 Chemistry, organic

University Microfilms, A XEROX Company, Ann Arbor, Michigan

THIS DISSERTATION HAS BEEN MICROFILMED EXACTLY AS RECEIVED.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. KINETIC ISOTOPE EFFECTS

IN FREE RADICAL CHEMISTRY

A Dissertation

Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of the requirements for the degree of

Doctor of Philosophy

The Department of Chemistry

Kenneth George Kneipp B.S., Tulane University, 1966

December, 1971

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University Microfilms, A Xerox Education Company

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGMENTS

To Professor William A. Pryor, whose interest in his students

distinguishes him among educators, for his guidance during the

course of this investigation,

to Mr. Lynn Lasswell, Dr. Tz-Hong Lin, and Dr. Yvonne Rees for

many helpful discussions,

to my wife, Sandy, for her help in preparing the Dissertation,

and for her patience,

to my parents, for their continual encouragement,

to the National Institutes of Health, for a research assistant-

ship awarded through Louisiana State University, I9 68 -I97 I,

to the Charles E. Coates Memorial Fund of the Louisiana State

University Foundation for financial assistance in preparation of

this Dissertation,

the author wishes to express his sincere appreciation and

thanks.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS ...... i

LIST OF TABLES...... v

LIST OF F I G U R E S ...... ix

ABSTRACT...... xi

CHAPTER I. INTRODUCTION ...... 1 References for Chapter I ...... 10

CHAPTER II. THEORETICAL...... 13 Quantum Mechanical Tunneling...... 16 Transmission Coefficient ...... 18 Activity Coefficient ...... 19 Partition Functions ...... 20 References for Chapter I I ...... 34

CHAPTER III. EXPERIMENTAL...... 37 Procedures for Thermal R u n s ...... 37 Procedures for Photochemical R u n s ...... 40 Procedure for Reaction of DPPH with Mercaptans .... 43 Materials...... 4-4 Analytical...... 43 General Determination of Tritium Absolute Activities General Method Liquid Scintillation Counting of Mercaptans Effect of Scintillator Composition Activity of Mercaptan by Proportional Tube Flow Counter Techniques Combustion Analysis of Jt-butyl Mercaptan Conclusion Liquid Scintillation Counting of Nitro benzene from NAT

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Oxidation of Aromatic Hydrocarbons...... #+ The Benzyl Radical The Diphenylmethyl Radical The Trityl Radical Kinetics of Initiator Decomposition ...... 67 Mechanism of Initiator Decomposition ...... 68 Synthesis of _t-butyl-a-deuteriocyclooctaneperoxycarboxylate Preparation of a-deuteriocyclooctanecarboxylic Acid Preparation of a-deuteriocyclooctanoyl Chloride Preparation of Sodium £-butyl Peroxide Preparation of the Perester Analysis of t:-butyl-a-deuteriocyclooctaneperoxycarboxylate Analysis of cyclooctanecarboxylic acid and a- deuteriocyclooctanecarboxylic acid by nmr Analysis of cyclooctanoyl chloride and a- deuteriocyclooctanoyl chloride by nmr Analysis of the £-bromophenacyl ester of a- deuteriocyclooctanecarboxylic acid by nmr Viscosity Dependence of Bond Homolysis Secondary Isotope Effects in Initiator Decomposition The Use of Scavengers in Homolytic Initiator Decom position Radiolytic Generation of Free Radicals ...... 79 References for Chapter I I I ...... 85

CHAPTER IV. RESULTS...... 88 Isotope Effect Data ...... 88 Analysis of the D a t a ...... 130 Arrhenius Activation Parameters ...... 133 Kinetics of Initiator Decomposition ...... 137 References for Chapter I V ...... 139

CHAPTER V. DISCUSSION...... l4l Abstraction from Tritiated t>butyl Mercaptan by Free Radicals...... 1^1

iii

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Abstraction from Deuterated t>butyl Mercaptan by the Methyl Radical...... 1^2 Abstraction from Deuterated J:-butyl Mercaptan by the Hydrogen A t o m ...... 1^3 Isotope Effect for Photolysis of Deuterated J:-butyl Me r c a p t a n...... 1^5 Abstraction Isotope Effect D a t a ...... ll)-9 References for Chapter V ...... 169

APPENDIX. PART I - MECHANISM OF PERES TER DECOMPOSITION . . . 171 Activation Parameters as a Test for Concerted Decomposi tion ...... 172 Activation Volumes for Homolytic Scission Reactions . . . 1 7 3 The Viscosity Dependence of Bond H omolysis...... 174

Secondary Isotope Effects in Initiator Decomposition . . 1 7 9 The Use of Scavengers in Homolytic Initiator Decomposi tion ...... 180 Conclusion...... 181 References for Appendix-Part I ...... I85

APPENDIX. PART II - RADIOLYTIC GENERATION OF FREE RADICALS . 188 References for Appendix-Part II ...... 210

VITA ...... 213

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Page

CHAPTER I. INTRODUCTION TABLE I Isotope Effects for Hydrogen Abstraction from thiols by Various Radicals ...... 3 TABLE II Activation Energy for Abstraction by Methyl and Trifluoromethyl Radicals . . . 6 TABLE III Comparison of the Reactivities of Methyl and Trifluoromethyl Radicals at 164°C . . 7

CHAPTER II. THEORETICAL TABLE I Calculated Approximate Isotope Effects (k^/k^) on Breaking a C-H(d ) or S-H(D) Bond in the Transition S t ate...... 27 TABLE II Isotope Effects for Methyl Radical Attack on C-H and C-D Bonds...... 28 TABLE III Methyl Radical Attack on Primary, Secon dary, and Tertiary C-H Bonds...... 32

CHAPTER III. EXPERIMENTAL TABLE I Effect of Initiator Concentration on the Measured Isotope Effect for Abstraction by the Cyclohexyl Radical at 100°C 39 TABLE II Effect of Length of Photolysis of t.-butyl Peroxyformate in _t-BuSH(D) - 3^00 A lamps. 42 TABLE III Mass Spectrometric Measurement of Stan dard Mixtures of Hydrogen and Hydrogen- dx ...... 48 TABLE IV Comparison of nmr and Mass Spectrometric Methods for Determination of the Extent Deuteration of _t-butyl Mercaptan...... 51 TABLE V Data Cards for Use of Activity Computer P r o g r a m ...... 54 TABLE VI Flow Counter Calibration Using Tritiated Toluene and Cyclohexane ...... 58 TABLE VII Determination of Absolute Activity of Tritiated t:-butyl Mercaptan Using Flow C o u n t e r ...... 59 TABLE VIII Activity of J:-butyl Mercaptan Determined by Combustion Analysis...... 6 l

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TABLE IX NMR Analysis of a-deuteriocyclooctanecarboxylic Acid - Calculation of Extent Deuteration ...... n TABLE X NMR Analysis of a-deuteriocyclooctanecarbonyl Chloride - Calculation of Extent Deuteration ...... 75 TABLE XI NMR Analysis of the £-bromophenacyl Ester of a-deuteriocycloctanecarboxylic Acid - Calculation of Extent Deuteration .... 77 TABLE XII Fricke Dosimetry. Ferric Ion Absorbance as a Function of the Time of Cobalt-60 Irradiation ...... 82

CER IV.. RESULTS TABLE I Abstraction from t-BuSH(D) by the Hydrogen A t o m ...... 91 TABLE II Abstraction from J:-Bu SH(t ) by the Phenyl Radical ...... 96 TABLE III Abstraction from £-BuSH( T) by the 1- Adamantyl Radical ...... 98 TABLE IV Abstraction from £-BuSH(T) by the Cyclo- hexyl Radical ...... 100 TABLE V Abstraction from J:-Bu SH(d ) by the Methyl Radical ...... 102 TABLE VI Abstraction from jt-BuSH(T) by the £- Nitrophenyl Radical ...... 10if TABLE VII Abstraction from H£S/D2 S by the Trifluoro- methyl Radical ...... 106 TABLE VIII Abstraction from £-Bu SH(t ) by the 1-nonyl Radical ...... 108 TABLE IX Abstraction from Jt-BuSH(T) by the 3-Heptyl Radical ...... n o TABLE X Abstraction from n-BuSH(D) by the Poly- styryl Radical ...... 112 TABLE XI Abstraction from J:-Bu SH(t ) by the Tri- ethylmethyl Radical ...... n 4 TABLE XII Abstraction from £-Bu SH(t ) by the Benzyl Radical ...... 1 1 6

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Page TABLE XIII Abstraction from t-BuSH(T) by the Diphenylmethyl Radical ...... 118 TABLE XIV Abstraction from J:-BuSH(t ) by the Trityl Radical ...... 120 TABLE XV Abstraction from t-BuSH and J:-BuSD by DPPH ...... 122 TABLE XVA The Disappearance of DPPH in a Benzene Solution of J:-butyl Mercaptan at 10.5°C . 124 TABLE XVB The Disappearance of DPPH in a Benzene Solution of t:-butyl Mercaptan at 35-5°C . 126 TABLE XVC The Disappearance of DPPH in a Benzene Solution of j:-butyl Mercaptan at 50.0°C . 128 TABLE XVI Data Cards for Use of Least Squares Computer P r o g r a m ...... 131 TABLE XVII Kinetic Isotope Effects on Hydrogen Atom Abstraction from ^>butyl Mercaptan at 6 o ° c ...... 132 TABLE XVIII Zero-point Energy Differences and Pre exponential Ratios for Hydrogen Atom Abstraction from J:-butyl Mercaptan at 6 o ° c ...... 136 TABLE XIX Collected Kinetic Data on Peresters . . . 138

CHAPTER V. DISCUSSION TABLE I Photolysis of _t-butyl Peroxyfornate. Evaluation of Isotope Effect for Abstrac tion by the Hydrogen A t o m ...... 146 TABLE II Isotope Effect for Photolysis of J:-butyl Mercaptan in Quartz at 3 5 ° C ...... 150 TABLE III Bond Dissociation Energies. Relative Stability of Free R a d i c a l s ...... 153 TABLE IV Ionization Potentials. Relative Stabil ity of Carbonium I o n s ...... 154 TABLE V Dissociation Energy of Bond Formed Versus Measured Isotope Effect ...... 157 TABLE VI Decomposition of cis- and trans-4-t- butylcyclohexyl Hypochlorites in Carbon Tetrachloride. Formation of cis- and trans-4-t-butylcyclohexy1 Chloride . . . 165 TABLE VII Kinetics of the Thermolysis of Jt-butyl Peresters of Bridgehead and Ordinary Tertiary Carboxylic Acids ...... 168

APPENDIX. PART I - MECHANISM OF PERESTER DECOMPOSITION

vii

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TABLE I Activation Volumes for Homolytic Scission Reactions ...... 175 TABLE II Observed Rate Constant for Decomposi tion of Carbo-_t-butylperoxycyclohexane in Hydrocarbon Solvents at 79.31°C ...... 177 TABLE III Observed Rate Constant for Decomposi tion of jt-butyl Cyclooctaneperoxycar- boxylate and t-butyl-a-deuteriocyclooctaneperoxycarboxylate in Hydro carbon Solvents at ...... 178 TABLE IV Effect of Scavenger on Yield of Carbon Dioxide in the Decomposition of t - butyl Cyclooctaneperoxycarboxylate . . 182 TABLE V Collected Data on the Mechanism of Perester Decomposition ...... iBk

APPENDIX. PART II RADIOLYTIC GENERATION OF FREE RADICALS TABLE I Y-Radiolysis of Cyclohexane. Isotope Effect for Abstraction by the Cyclo- hexyl Radical at 3k°C ...... l° k TABLE II Results of Elemental Analysis of N,N- Dimethylglycine Hydrochloride ...... 203 TABLE III Determination of Position of Tritium Labelling in the ethyl ester of N,N- Dimethylglycine. Ester Activity Compared to Activity of N,N-Dimethyl- glycine Hydrochloride ...... 205 TABLE IV y-Radiolysis of N,N-Dimethylglycine ethyl ester. Isotope Effect for Abstraction by (CHs^N-CH-COaCaHs at

3 k ° c ...... ’ ...... 206 TABLE V Kinetic Isotope Effects on Hydrogen Atom Abstraction from _t-butyl Mercaptan at 34°C ...... 208

viii

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Page

CHAPTER I. INTRODUCTION

CHAPTER II. THEORETICAL FIGURE I Potential Energy Diagram for an Exothermic Hydrogen Abstraction from t-butyl Mercap tan ...... 15 FIGURE II The Hammond Postulate. Potential Energy vs. Reaction Coordinate for Exothermic, Endothermic, and Thermoneutral Reac tions ...... 51

CHAPTER III. EXPERIMENTAL FIGURE I Mass Spectrometer Standardization; [HD]/- LHs] vs. (m/e 3 )/(m/e 2 ) ...... 47 FIGURE II Fricke Dosimetry; Absorbance vs. Time . . 85

CHAPTER IV. RESULTS FIGURE I Abstraction from ^-BuSD(D by the Hydrogen A t o m ...... 92 FIGURE IA Abstraction from _t-BuSH(D by the Hydrogen Atom at 10.5°C ...... 95 FIGURE IB Abstraction from j>BuSH(D by the Hydrogen Atom at 40.0°C ...... 94 FIGURE IC Abstraction from £-BuSH(D by the Hydrogen Atom at 70.0°C ...... 95 FIGURE II Abstraction from J:-BuSH(D by the Phenyl Radical ...... 97 FIGURE III Abstraction from Jt-BuSH(D by the 1 - Adamantyl Radical . . . 99 FIGURE IV Abstraction from j>BuSH(D by the Cyclo- hexyl Radical ...... 101 FIGURE V Abstraction from _t-BuSH(D by the Methyl Radical ...... 103 FIGURE VI Abstraction from _t-BuSH(D by the j>- nitrophenyl Radical . . 105 FIGURE VII Abstraction from HaS/D2 S by the Trifluoro- methyl Radical ...... 107

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Page FIGURE VIII Abstraction from t-BuSH( D) by the 1- Nonyl Radical...... 109 FIGURE IX Abstraction from _t-BuSH(D) by the 3" Heptyl Radical ...... Ill FIGURE X Abstraction from n-BuSH(D) by the Poly- styryl Radical ...... 113 FIGURE XI Abstraction from J:-BuSH(d ) by the Tri- ethylmethyl Radical...... 115 FIGURE XII Abstraction from J:-BuSH(D) by the Benzyl Radical...... 117 FIGURE XIII Abstraction from J:-BuSH(D) by the Diphenylmethyl Radical ...... 119 FIGURE XIV Abstraction from j:-BuSH(D) by the Trityl Radical...... 121 FIGURE XV Abstraction from _t-BuSH(D) by DPPH . • . 123 FIGURE XVA The Disappearance of DPPH in a Benzene Solution of J:-butyl Mercaptan at 10.5°C . 125 FIGURE XVB The Disappearance of DPPH in a Benzene Solution of t.-butyl Mercaptan at 35.5°c ...... 127 FIGURE XVC The Disappearance of DPPH in a Benzene Solution of _t-butyl Mercaptan at 50*0°C . 129 FIGURE XVI Potential Energy vs. Reaction Coordinate Effect of Deuterium Substitution on Zero- point Energy Levels ...... 135

CHAPTER V. DISCUSSION FIGURE I Isotope Effect for Photolysis of J:-butyl Mercaptan in Quartz at 3 5 ° C ...... 151 FIGURE II Dissociation Energy of Bond Formed vs. Isotope Effect ...... 158 FIGURE III Potential Energy Diagram for the Endo- thermic Abstraction of a Hydrogen Atom from J:-butyl Mercaptan by D P P H ... l6 l FIGURE IV The Conformation of the Cyclohexyl Radical...... 163

APPENDIX. PART I - MECHANISM OF PERESTER DECOMPOSITION

APPENDIX. PART II - RADIOLYTIC GENERATION OF FREE RADICALS

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT

Kinetic isotope effects for hydrogen atom abstraction from

mercaptans by free radical species have been investigated. A

variety of structurally different radicals have been generated by

thermolytic, photochemical, and radiolytic techniques and have been

allowed to abstract hydrogen, deuterium, or tritium atoms from the

S-H position of a mercaptan molecule. The rate ratio, or isotope

effect, kjj/kjj or k^/k^, has been measured in all cases as a func

tion of temperature. A useful relationship has been developed

whereby the magnitude of the observed isotope effect, or isotopic

selectivity, for a given radical may be correlated to its stability

or reactivity. Such a pattern of isotope effects for abstraction

should prove useful as a diagnostic test for radical species whose

reactivity may otherwise be difficult to assess.

Secondary deuterium isotope effects on thermolysis of perester

initiators have been measured in order to elucidate the mechanism

of decomposition of these free radical precursors. The results

are shown to be in good agreement with similar secondary isotope

effect measurements reported in the literature and the mechanistic

implications are consistent with results obtained using a variety

of investigation techniques.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. INTRODUCTION

A plethora of experimental observations must be brought to bear on

the problem of discerning the mechanism of a chemical reaction. For

example, considerable information is gained from studies of the products

formed, their stereochemistry in comparison to that of the reactants,

the detection of short-lived intermediates, either spectroscopically or

by actual trapping, and the change of the position of isotopic labels in

going from reactants to products. Additionally, the science of chemical

kinetics often provides the most general method of determining the

mechanism of a reaction. The early study of Arrhenius1 led to a quanti

tative formulation of rate constant data and an evaluation of the energy

barrier, or activation energy, in going from reactants to products.

Measurements of these parameters frequently provide valuable supporting

evidence for the justification of a particular mechanism.2 ’3

Any proposed reaction mechanism and the mental picture of the

transition state must be tested experimentally. The introduction of

substituents at some position outside the reaction center in one of the

reactant molecules and the measurement of relative rates is sometimes a

useful test. However, such a substitution often creates too many

variables in the reaction itself to allow a dependable comparison of

the rate effect. A much more subtle change is isotopic substitution.

This type of substitution, when made at a position in the molecule

outside of the reaction center, will have almost no influence on the

reaction itself or on the quantitative measure of rate. There will be

an appreciable rate effect, however, when isotopic substitution is made

at the position of reaction where bonds are being formed and broken.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 The reaction under consideration in this study is one of primary

importance in any discussion of free radical chemistry, namely the atom

transfer, or, more specifically, the hydrogen atom abstraction reaction. 4

The reactions of interest are shown with their appropriate rate constant

notation in eq. (1) - (3 )' hi R* + QH >RH + Q- (1) k^ R. + QD > RD + Q- (2)

kT R- + QT > RT + Q- (3)

where D and T are used to indicate, respectively, deuterium and tritium

substitution. In these studies, the hydrogen atom donor QH is a mercaptan

molecule, primarily Jt-butyl mercaptan, where the position of attack is

the S-H bond. 5 Mercaptans are chosen as model QH compounds because of the

lability of the S-H bond to radical attack compared to other C-H bonds

in themolecule, their ready availability, ease of handling, and

facility whereby theymay be isotopically substituted. Very little

data are available in the literature on isotope effects for hydrogen

abstraction from mercaptans. A summary is given in Table I.

On the basis of the usually observed pattern of reactivity versus

selectivity,10 it would be expected that the most reactive radical is

the least selective, i.e., exhibit the smallest isotope effect for

abstraction. The data in Table I point out two discrepancies. First,

one would expect phenyl groups in the growing polystyryl radical chain

to delocalize the odd electron on the or-carbon atom, and thus increase

the stability (decrease the reactivity) of the polystyryl radical when

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table I

Isotope Effects for Hydrogen Abstraction from thiols by Various Radicals

Ref. Q* RSH VC

polystyryl n-butylthiol 6

tetralin 2,4,6-tri-t- 2 .9^ 7 peroxy butyl thiophenol

methyl hydrogen sulfide k.0$- 8

trifluoromethyl hydrogen sulfide 2 .5- 9

-6o°c

—calculated from reported isotope effects at other temperatures

—calculated from reported values of (E^-E^) and see page 1 3 3 *

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. k

compared to the methyl radical. 11 Thus, one would predict the isotope

effect for abstraction by polystyryl to be larger than for methyl.

This discrepancy cannot readily be attributed to a significant differ

ence in the donor properties of the thiols used in the two studies.

The dissociation energy for hydrogen sulfide is nearly identical to

that for n-butylthiol: D(HS-H) — 90 kcal mole 1; D(n-BuS-H) — 88 kcal

mole •

The second discrepancy lies in the comparison of the methyl and

trifluoromethyl radicals. It has been suggested13 that trifluoro

methyl radicals are considerably more reactive than methyl on the basis

of hydrogen atom abstraction reactions with hydrocarbon substrates.

As discussed below, methyl should be more reactive than trifluoro-

methyl in abstracting hydrogen from hydrogen sulfide or hydrogen

chloride. It would be expected, therefore, that the isotope effect for

abstraction by methyl should be smaller than that for abstraction by

trifluoromethyl in the case of these two substrates. (See Table i).

Two different approaches have been used to explain the different reac

tivities of the two radicals.

The first explanation involves enthalpy changes in the two reac

tions being considered, eq. (4) and (5)»

CF3 * + R H > CF3H + R- (k)

CH3- + RH > CEt + R* (5)

The difference in enthalpies (AH„_ - AH,™ ) is a reflection of the KjC q 0 r i3 difference in dissociation energy of the bonds formed in eq. (^) and

(5) since the same (R-H) bond is broken in both cases. The

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. difference in enthalpies is near 2 kcal mole" 1 since D( CF3 -H) — 106 + 1

kcal mole 1 and D( CH3 -H) 2= 10k + 1 kcal mole 1 . 12 The effect of this

enthalpy difference is to slightly lower the activation energy for

abstraction from hydrocarbon substrates in the trifluoromethyl case.

Table II gives a compilation of the activation energies for abstraction

for the two radicals.

The second and more important explanation for the difference in

radical reactivities involves polar effects and electronegativities.

Since fluorine is more electronegative than any other element, the tri-

fluoromethyl radical is strongly electrophilic. This fact has been used

to explain its increased reactivity toward hydrocarbons.17 In attack

on polar bonds, however, trifluoromethyl radicals are not favored

because of repulsions from polar interactions. In the case of tri-

fluoromethyl radical attack on polar bonds, activation energies are

raised relative to those for attack by methyl radicals.“ Compared to

hydrocarbon substrates, there is an inversion of the order of activa

tion energies for the two radicals in attack on hydrogen bromide and

hydrogen sulfide as seen in Table II. Dipole-dipole repulsions are

considered to account for this pattern of reactivities.18’20 Similari

ties in bond dissociation energies, bond lengths, and dipole moments in

these two polar substrates should make the order of magnitudes of acti

vation energies for hydrogen abstraction from the two molecules

similar.20’23

There is considerable precedent in the literature for involvement

of dipole-dipole repulsions in explaining this observed pattern of

reactivities toward hydrogen sulfide. Table III shows the rate

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table II

Activation Energy for Abstraction by Methyl and Trifluoromethyl Radicals

kcal mole 1 Substrate E( CH3) E(CF3) Ref.

methane I k . 7 10.3 14, 15

ethane 1 1 .8 7-5 1 5 , 16

butane 9.6 5-3 16, 17

hydrogen 1.4 3.0 1 8 , 19 bromide

hydrogen 2 .6 3-9 2 0 , 21 sulfide

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table III

Comparison of the Reactivities of Methyl and Trifluoromethyl Radicals at l64°C

Substrate Reference kCF3/kCH3

Sittj 8 .0 24

(CH3)3-Si-H 100 24

Cl3-Si-H 0.4-1.2 25

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8 constant ratios at l64°C for the two radicals in abstraction from a

series of substituted silanes. Methyl substitution of silane enhances

trifluoromethyl attack on the Si-H bond. 24 Chlorination of silane has

the opposite effect. 25 In addition, Cheng and Szwarc26 have reported

that at 180°C, for trifluoromethyl radical attack on Si(CH3)4 ,

ClSi(CH3 )3 , Cl2 Si(CH3 )2 , and Cl3Si( CH3), the carbon-hydrogen bond reac

tivity decreases with successive substitution of chlorine atoms.

The purpose of this study is to further probe isotope effect

data for hydrogen atom abstraction from thiols by free radicals and to

reassess the discrepancies presented in Table I. To this end, a number

of structurally different free radicals have been generated by thermo-

lytic, photochemical, and radiolytic techniques and have been allowed

to abstract hydrogen, deuterium, or tritium atoms from the S-H position

of a thiol molecule. The rate ratio, or isotope effect, k^/k^ or

k^/k^,, has been measured in all cases as a function of temperature. A

useful relationship has been developed whereby the magnitude of the

observed isotope effect, or isotopic selectivity, for a given radical

may be correlated to its stability or reactivity. Such an isotope

effect pattern should prove useful as a diagnostic test for radical

species whose reactivity profile may otherwise be difficult to assess.

The measurement of isotope effects on reaction rates gives the

chemist a powerful tool for the elucidation of a reaction mechanism

and provides a sensitive test of transition state theory. A number

of attempts have been made27 31 to account for observed isotope

effects for various reacting systems by assuming a reasonable

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9

transition state model and potential energy surface for the system.

By testing the validity of a particular assumed transition state

configuration, these methods have generally proved fruitful in that

they often lead to satisfactory quantitative predictions regarding

magnitudes of isotope effects, and provide a means for explaining

subtle steps in a reaction sequence.

In the hydrogen atom abstraction reaction considered here, the

transition state model is usually assumed to be linear with the S-H

(D,T) bond being broken in the rate determining step. Section II

will be devoted to a theoretical development of kinetic isotope effect

theory using this assumed model in an attempt to substantiate its

validity. Such a model will be useful in predicting isotope effects

and correlating these calculated values to experimentally observed

values for a number of reacting systems described in the literature.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. REFERENCES - I

1. S. Arrhenius, Z. Physik. Chem. (Leipzig), 226 (188 9).

2. A.A. Frost and R.G. Pearson, "Kinetics and Mechanism", John Wiley

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3. J. March, "Advanced Organic Chemistry: Reactions, Mechanisms,

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4. W.A. Pryor, "Free Radicals", McGraw-Hill Book Co., New York, 1 9 6 6 ,

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V j. P.B. Ayscough, J.C. Polanyi, and E.W.R. Steacie, Can. J. Chem.,

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16. J.R. McNesby and A.S. Gordon, J. Amer. Chem. Soc., 7^, 3570 (1956 ).

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21. N. Imai and 0. Toyama, Bull. Chem. Soc., Japan, 3^5> 652 (i960 ).

22. J.M. Tedder, Quart. Rev., lA, 336 (i960 ).

2 3 . E.R. Morris and J.C.J. Thynne, Trans. Faraday Soc., 6 ^, 2^70 (1967 ).

2k. a) E.R. Morris and J.C.J. Thynne, J. Phys. Chem., 73/ 329^- (1969)*

b) E.R. Morris and J.C.J. Thynne, Trans. Faraday Soc., 6 6 , 183

(1970).

2 5 . a) J.A. Kerr, D.H. Slater, and J.C. Young, J. Chem. Soc., A,

io4 (1966 ).

b) J.A. Kerr, A. Stephens, and J.C. Young, Int. J. Chem.

Kinetics,— —■ — .i.i ^ 1, 371 (1969 ).

c^/ T.N. Bell and B.B. Johnson, Austral...... J. Chem.,. 20, 15^-5

(1967). 26. W.J. Cheng and M. Szwarc, J. Phys. Chem., 72, 49^ (1968).

27. F.H. Westheimer, Chem. Rev., 6 l^ 265 (I96 I).

28. L. Melander, "Isotope Effects on Reaction Rates", Ronald Press

Co., New York, i9 6 0 , p. 5»

2 9 . R.E. Weston, Jr., Science, 1^, 332 (1 9 6 7 ).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 12

30. R.A. More O'Ferrall and J. Kouba, J. Chem. Soc., B, 985 (I967 ).

31. M. Salomon, Int. J. Chem. Kinetics, 2, 175 (19T0).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. THEORETICAL

In thereactions considered here, either a hydrogen or deuterium

atom is transferred according to the reactions:

k H R- + J:-BuSH - 2 -> RH + t-BuS- (1)

k R- + J:-BuSD -=*-> RD + t-BuS- (2)

£ which proceed via the transition states (£-BuS H R) and (j;-BuS-- $ * D R), respectively. In the theory of absolute reaction rates, it is

assumed that the initial reactants are always in equilibrium with the

activated complex or transition state, and that the latter decomposes

to give products at a definite rate. The position of the activated

complex along the reaction coordinate is assumed to be at the point of

highest potential energy along the energetically most desirable path

from reactants to products. Thus, more complete equations for the

reactions being considered here are shown by the following:

+ R- + t-BuSH ~— * ( t-BuS---H---R) ---- > t-BuS* + RH (3)

R- + t-BuSD ---^ ( t-BuS---D---R)----- > t-BuS- + RD (4) \“

A potential energy diagram is shown in Figure I. The reaction

depicted by this energy diagram is shown to be exothermic, i.e., the

* The superscript cross of Lorraine (double dagger) will be used throughout to denote the transition state.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. dissociation energy of the R-H bond is greater than that for the S-H

bond in jt-butyl mercaptan.

The theory of absolute reaction rates has been developed in the

classical work of Glasstone, Laidler, and Eyring.1 The net rate con

stant for the reaction is determined by the average velocity of the

activated complex over the top of the potential energy barrier, and is

given by the following expression:

k (5)

1 hvI 2 The expression ) ] is a correction for "tunneling" or

"leakage" through the energy barrier. According to the classical

mechanical treatment, the reactant molecules must pass over the energy

barrier before reaction may occur. Quantum mechanics, however,

predicts there is a finite probability that molecules with a smaller

amount of energy will succeed in getting from the initial to final

state. The quantity v is the imaginary frequency of the stretching

vibration along the reaction coordinate leading to decomposition of

the activated complex. Thus, v2 is negative, and the rate is greater

if tunneling occurs. The transmission coefficient, y, is the factor

which allows for the possibility that not every activated complex

reaching the top of the barrier moving along the coordinate of decom'

position will lead to reaction. The constant k is Boltzmann's

constant, h is Planck's constant, and T is the absolute temperature. $ The constant K is the thermodynamic equilibrium constant for the

equilibrium between activated complex and initial reactants. The

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. R* + t-BuSH

•H

RH + t-BuS*

Reaction Coordinate

FIGURE I. Potential Energy Diagram for an Exothermic Hydrogen Abstraction Reaction from J:-butyl mercaptan.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 16

a terms denote activity coefficients, which allow for influences of

the molecular environment on reactions in solution. The realization of an isotope effect on reaction rate is a result

of changes in the magnitude of terms in equation (5) on isotopic substi

tution. It is instructive to examine which of these terms are most

dependent on isotopic substitution.

1) Quantum Mechanical Tunneling

The usual method of applying the tunneling correction to reaction

rate theory was first advanced by Wigner2 who showed that, to a first

approximation, the correction may be given by:

« - “ S f s s g & r - a - I f e * > 1 <6>

This expression is generally found satisfactory when the correction

is small, but Bell3 has pointed out that this will not always be the

case in practice. In a later paper, Bell4 derived a more complete

expression for the correction to be applied, given in equation (7 ):

<}' = fc ut/sin(fc ufc) (7 )

where ufc = hvt/kT

vt = E^/na(2m)^

E = height of the energy barrier (usually considered to be parabolic)

2a = width of the barrier

and m = mass of the particle vibrating with frequency vfc

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 17

For a reaction in which tunneling occurs, the experimentally

observed Arrhenius activation energy, E , will differ from the exp classically defined, temperature independent activation energy, E , C JL 3 S S Bell, Fendley, and Hulett5 have defined the following relationship:

Eexp = ^ ^ where (3 = 2Tl2a (2mE)^/h

and a = Eclass/kT

Equation (8) may be rearranged to give:

E /E .. = 1 - L(hkT)/{kT2n2 a(2mE)^ - Eh}] (9) exp class

Equation (9) is derived for a reaction whose energy barrier is

assumed to be parabolic. Several qualitative predictions may be made

from the examination of eq. (9) regarding the nature of quantum

mechanical tunneling:

A. E < E , exp class The difference between the observed and classically defined

activation energy may be used to approximate the shape of the energy

barrier. An account of this has been given by Hulett. 6

B. As the temperature decreases, Efi decreases.

C. For particles of lower mass E differs from E . by a 6 xp class greater amount. This leads to the experimentally observed increase

in the isotope effect, k^/kp, when tunneling is significant because

tunneling contributes a greater rate enhancement to k^ than to k^.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 18

Conditions (B) and ( C) lead to the prediction that EeXp will be

dependent on temperature and that non-linear Arrhenius plots will be

observed in reactions where tunneling occurs. This predicted non-

linearity would be anticipated at low temperatures and has been

observed experimentally in a number of systems. 7

The theory of quantum mechanical tunneling is well advanced, but

the effect seems to be unimportant in most ordinary chemical reactions

at ordinary temperatures. 8 Thus, the changes in the magnitude of the

tunneling correction factor in eq. (5) on isotopic substitution will

be ignored.

2) Transmission Coefficient

The transmission coefficient, y> is h*16 fraction of activated

complexes passing the energy barrier in the forward direction which

will lead to completed reaction. Hirschfelder and Wigner9 pointed

out that this quantity cannot be evaluated by classical mechanics

because, in many cases, it is a rapidly fluctuating function of the

energy of the system. If the temperature distribution of the energy

is sufficiently broad, i.e., at all except very low temperatures, it

is concluded that changes in the average transmission coefficient on

isotopic substitution will have little effect on the relative rates

of reaction. At temperatures considered in this work, it is assumed

that differences between transmission coefficients in the hydrogen

and deuterium abstraction cases introduce no isotope effect.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 19

3) Activity Coefficient

In order to compare reaction rates in one medium to that in another,

care must be taken in defining activity coefficients. If rates for a

reaction in onesolvent are being studied, it is convenient to compare

those rates tothe rate in the gas phase. Thus, theactivity coeffi

cient, a, in solution must be defined with respect to the ideal gas at

one atmosphere pressure as the standard state. 10 For a first approxi

mation to the isotope effect, it is generally assumed that isotopic

substitution will have only a negligible effect on relative activity

coefficients and that all the activity coefficients are equal to unity.

Therefore, the main contribution to relative rates on isotopic

substitution is due to changes in K , the thermodynamic equilibrium

constant for the equilibrium between activated complex and initial

reactants. The isotope effect may thus be expressed by equation (10):

1‘ h _ 1' h ,10l h ' * ? ( w )

where the values for *1* and 4* are for the following equilibria:

a h + R- (i^)* (ii)

k/ * ^ + R* ( V (12)

In our case, and A^ represent protium and deuterated t-butyl

mercaptan, respectively.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 20

For an ordinary equilibrium, the equilibrium constant may be

expressed in terms of total partition functions per unit volume, Q°,

as follows:

. Q° * 4 = — — («> VS-

q O * and k! = — ---- (14)

Therefore:

kH 4 % Q^Qg. q°*

(15)

The total per volume partition function gives the total probability

of the occurrence of a particular atomic or molecular species per

unit volume; it may be defined as follows:

V -e./kT )° =L g, e 1 (16 ) i where is the energy of the i th quantum state of g-fold degeneracy.

The energy contains an appropriate term for each type of energy,

so that the complete partition function is defined as follows:

0x ° = xtransQ Qxrot Qxvibr .. Qxel , QHnuc (-)'v' (17)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 21 If it is assumed that the different forms of energy distribution

are independent of each other, the following substitutions for separate

partition functions may be made: 11

Oo = ( 2TTtnkT)^v 8^ 8tt3ABC)^( kT)^ V hd x ah3

i=k -hv./2kT -hvi/kT TT /I - x ^el ^nuc (18) i=l

where,

A, B, and C are the principal moments of inertia for a

polyatomic molecule,

a is the symmetry number corresponding to the number of

undistinguishable ways of orienting the molecule in

space,

is the fundamental frequency of the ith vibrational

degree of freedom in a molecule having k vibrational

degrees of freedom,

g ^ accounts for the electronic statistical weight of the

ground state. is usually equal to g ^ since

there is essentially no excited electronic state

population at ordinary temperatures, I g accounts for degeneracy due to different orientations nuc of the nuclear spin, and,

the remaining symbols have their usual meaning.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 22

As developed by Bigeleisen, 12 14 the expressions for the total

partition functions described in eq. (18) may now be substituted into

eq. (15) to give an expression for the isotope effect, k^/k^.

3n-6 [l - exp( -U*) ] V dcd J [l - exp(-Uj)] “h [W hJ h£J r± ^ i' [1 - exp(-U^)] *b W d 3n-7 * £ $ IT i [1 - exp(-U^)] _ w L k J 3n-6

exp Z (vt-v. ( W /g (19) 3n*-7

exp I ( U ^ ) 12

where U 1 = hv1/kT i v a normal vibrational frequency *i v ~ = the imaginary frequency of the transition state, sometimes

referred to as the frequency of decomposition.

The reactants' sums and products are taken over 3n-6 real vibrational

frequencies; the transition state has 3 * ^ -7 real frequencies, since

one mode of vibration in the transition state leads to decomposition

to give products.

The first term in eq. (19) arises from translational and

rotational partition functions and is designated the mass moment of

inertia term, or MMI. The second term io eqi (19) arise* from

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 23 vibrational excitation and is labelled EXC. The final term arises

from the vibrational zero-point energy difference, and is designated

ZPE. Thus, eq. (19) may be expressed more simply as

1^/k^ = MMI x EXC x ZPE (20)

Wolfsberg and S t e m 15’ 10 have carried out calculations for

several model systems to approximate the relative importance of the

three terms in equation (20). In order to use eq. (19) or (20) for

the calculation of isotope effects, a knowledge of vibrational + + frequencies (v^, Vp> an(* vp) must be assumed. The calculations

from their model systems indicate that both MMI and EXC terms are

negligible when compared to the ZEE term at normal temperatures. Thus,

eq. (20) may be conveniently approximated by the following:

3n-6 V exp A («*-u£ ) /2 Cirti = 3n*-7

exp

(Melander17 has pointed out that it is not valid to neglect any of

the terms in eq. (20) when isotopes heavier than hydrogen are

considered. In these cases, deviations in the MMI term on isotopic

substitution will be of the same order of magnitude as the measured

isotope effect.)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2k

Since very little is generally known about the vibrational

frequencies in the reactants and transition state, the usual approxi

mation is to assume that all but one of the vibrational contributions,

namely, the stretching mode of the bond to be broken, disappear by

cancellation. 18 This should be an excellent approximation for those

parts of the molecule and transition state which are not isotopically

substituted and which do not undergo drastic bond reorganization

during reaction. This, then, leaves the isotopically dependent

stretching vibration for the bond to be broken in the reaction (U„,

Up). Eq. (21) may therefore be rewritten as follows:

This equation sometimes appears in the form

^ sinh(£ UH) % ' °inh(i »D) (25)

where the hyperbolic sinh function is defined by the following: V 2 -V2 sinh (| UR) - - (2k)

Thus,

U„/2 -U„/2

2 -Ud /2 (25) kD =

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 25

where e “V1 2 and e "V 2 are negligible at normaltemperatures. The

final equation based on the assumption discussed above for the kinetic

isotope effect is:

k sinh (| U ) = ZPE = e*P[(UH-UD)/2] = slnh (j, Md) (26)

or,

kH < V V hc ^ - exp --a - (27)

Eq. (27) may be used to calculate approximate isotope effects.

In calculations using this equation, the stretching vibrations of

the bonds attached to the isotopic atom are considered responsible for

the observed isotope effect and contributions from the bending modes

are ignored. It is instructive to use eq. (27 ) for the calculation

of predicted approximate isotope effects on breaking a X-H (d) bond.

Calculations of this type were first carried out by Bell19 and later

by Egger20.

In order to perform such a calculation, the stretching frequen

cies v„ „ and v must be known. Consider first the case in which a X-H X-D C-H and C-D bond is broken in the transition state. An average value

for y in an alkane is 2900 cm 1 21 and v_ _ is calculated to be C-H C-D 2148 cm 1 by the following equation: 22

- ^ = 1.35 (28) X-D

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 26

Values for the remainder of the parameters in eq. (27) are given

below:

k = Boltzmann's constant = I .38 x 10 16 erg deg 1

c = speed of light = ) x 1010 cm sec 1

h = Planck's constant = 6.62 x 10 27 erg sec

If the bond being broken is the S-H(D) bond, the corresponding

stretching frequencies vc and vc must be known. The value for o-n o“D Vo u was determined to be 2587 cm 1 by infrared analysis of neat S “H _t-butyl mercaptan, and v was determined to be 1875 cm 1 by infrared D "U analysis of the deuterated mercaptan. Using eq. (28) and the measured

value for 7J , vc ^ is calculated to be I916 cm x, in good agreement s“H d-d with the measured value for \J . Table I gives a compilation of b —D predicted approximate isotope effects calculated using eq. (27 ).

These predicted values may be compared to experimentally measured

values for methyl radical attack on various hydrocarbons. These

isotope effects for breaking a C-H or C-D bond are collected in

Table II. 23 The measured isotope effects in Table II are all

corrected to the common temperature of l6 ^°C. The agreement between

calculated and experimental isotope effects is satisfactory, when

one considers the many approximations and simplifications made in

deriving eq. (27 ).

Laidler, 24 has derived an equation which allows for the calcu

lation of the "maximum" expected isotope effect. This "maximum"

isotope effect will be obtained if there is no isotope effect in the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE I

Calculated Approximate Isotope Effects (4^/k^) on Breaking

a C-H(d ) or S-H(d ) Bond in the Transition State

Bond Broken Temperature (°C)

6o° 100° 164° 200°

c -h ( d ) 6.3 5.1 4.0 3.6

s -h (d ) 4.7 4.0 3-2 3.0

*eq- (27)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE I I

Isotope Effects for Methyl Radical Attack on

C-H and C-D Bonds

Substrate kjj/kjj (I6 if°c) Ref.

CH4, CD4 6 .2 2 5 , 26

C2H6 > C2P6 5 .6 27, 28

CH3CD3 6 .3 28

CH3CH3CH3, CH3CD2CH3 5 .1 2 8, 29

(CH3)3CH, (c h 3)3c d 5 .0 28

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 29 activated complex, i.e., there is no bonding of the isotopic species

in the transition state. Since this condition of no bonding in the

transition state will never be obtained in practice, the calculated

"maximum" may be considered to be the largest isotope effect which

is theoretically allowable. The estimated "maximum" at 25°C for

breaking a C-H and C-D bond is given24 as 18.

The experimental results in Table II point out an interesting

comparison of the reactivities of primary, secondary, and tertiary

C-H bonds. Westheimer30 and Yokota and Timmons33 have pointed out

that the isotope effect should be a maximum when the transition

state for the reaction is most nearly symmetrical. Symmetry is

defined in terms of bond distances, force constants, and masses of

the end atoms involved in the transition state. In the case of the

most symmetrical transition state, the bond being broken is half

broken and the bond being formed is half formed, and the effect of

isotopic substitution on the stretching frequency of these two partial

bonds should be a maximum. The fact that the most symmetrical tran

sition state occurs in the case of the most nearly thermoneutral

reaction can be deduced from the Hammond31 postulate. The effects of

this postulate can be seen most clearly in the potential energy

diagram shown in Figure II.

In Figure II,curve A represents the potential energy diagram for

an exothermic reaction. It is seen that going to the transition

state involves relatively slight progress along the reaction coordi

nate and therefore the reactants should most closely resemble the

transition state. The opposite is true of curve B when the products

most closely resemble the transition state of an endothermic

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 30

reaction. Curve C, the potential energy diagram for a thermoneutral

reaction is observed to give the most symmetrical transition state

which is relatively unlike both reactants and products.

It is interesting to compare the magnitude of measured kinetic

isotope effects to the positions along the reaction coordinate for a

homologous reaction series. This can be done by examining the data

for methyl radical attack on primary, secondary, and tertiary C-H

bonds given in Table II.

In the case of methyl radical attack on methane or ethane

(primary C-H bonds) the dissociation energy of the bond broken is

identical to or very nearly the same as the dissociation energy of the

bond formed. Thus, the attack on primary C-H bonds is nearly thermo

neutral and is best described by the potential energy curve C in

Figure II. In going to the rupture of secondary and tertiary C-H, the

reaction becomes more like curve A, and the transition state becomes

increasingly less symmetrical. On this account, it could be expected

that methyl radical attack on primary C-H bonds would exhibit the

largest isotope effect and that tertiary C-H bonds should show the

smallest isotope effect. The compilation of data in Table III shows

that this is, in fact, the case.

In this discussion, the attacking radical is the same, namely

methyl, and the substrate being attacked is varied. In the reactions

of interest in this dissertation the attacking radical is varied and

t^-butyl mercaptan is the substrate used throughout. It will be seen

in a later discussion that a similar relation to that described

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Energy

Reaction Coordinate ->

FIGURE II. The Hammond31 Postulate. Potential Energy vs. Reaction Coordinate for Exothermic (A), Endothermic (B), and Thermoneutral (c) Reactions.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE I I I

Methyl Radical Attack on Primary, Secondary,

and Tertiary C-H Bonds.

Type of C-H Dissociation Energy Dissociation V kD Bond Broken of Bond Formed Energy of Bond Broken (kcal/mole) 32 (i64°c)

* Primary 103.9 9 8 .0 - 103.9 6 .0

Secondary 103.9 9^-5 5.1

Tertiary 103.9 9 1 .0 5-0

Average value for methane and ethane shown in Table II.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 33

above in which the observed isotope effect for abstraction is correlated

to the dissociation energies of the bonds broken and formed may be

developed for the case of radical attack on mercaptan.

In this section, the validity of the linear three-atom model for

the transition state of a hydrogen transfer reaction has been demon

strated. The breaking of the S-H (D,T) bond occurs in the rate deter

mining step. Transition state theory permits satisfactory quantitative

predictions of kinetic isotope effects, as seen in the case of attack

on C-H bonds. A comparison between predicted and measured isotope

effects for radical attack on t-butyl mercaptan as well as a discussion

of the utility of the measured values will be presented in the

following sections.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. REFERENCES - II

1. S. Glasstone, K.J. Laidler, and H. Eyring, "The Theory of Rate

Processes", McGraw-Hill Book Co., New York, 1941, pp» 184-201.

2. E.P. Wigner, Z. Physik. Chem., B, 1£, 203 (1933)•

3. R.P. Bell, Proc. Royal Soc., A, 1M3, 24l (1935)*

4. R.P. Bell, Trans. Faraday Soc., 5^, 1 (1959)*

5. R.P. Bell, J.A. Fendley, and J.R. Hulett, Proc. Royal Soc., A,

235 , 453 (1956).

6 . J.R. Hulett, Proc. Royal Soc., A,2^1^, 274 (1959) •

7* a) H.S. Johnston and D. Rapp, J. Amer. Chem. Soc., 8^, 1 (1961 ).

b) W.M. Jackson, J.R. McNesby, and B. deB. Darwent, J. Chem.

Phys., 1610 (1962).

c) E.F. Caldin and M. Kasparian, Disc. Faraday Soc., 3£, 25

(1965).

d) J.R. Hulett, Quart. Rev., 18, 227 (1964)*

e) K.J. Laidler and J.C. Polanyi, Progr. React. Kinetics, 3, 30

(1965).

8 . a) Ref. 1, p. 191.

b) L. Melander, "Isotope Effects on Reaction Rates", Ronald

Press Co., New York, i960 , p. 11.

9. J.O. Hirschfelder and E. Wigner, J. Chem. Phys., "JJ 6l 6 (1939)*

10. Ref. 1, p. 403.

11. H. Eyring, J. Walter, and G.E.Kimball, "Quantum Chemistry",

John Wiley and Sons, Inc., 1944, pp. 292-296 .

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 35 12. J. Bigeleisen and M.G. Mayer, J. Chem. Phys., 1^, 26l (1947).

13. J. Bigeleisen, J. Chem. Phys., 17 , 675 (1949)-

14. J. Bigeleisen and M. Wolfsberg, Advan. Chem. Phys., 1, 15 (1958).

15. M. Wolfsberg and M.J. Stern, Pure Appl. Chem., 8^, 225 (1964).

16. M.J. Stern and M. Wolfsberg, J. Chem. Phys., 26l8 (I966 ).

17. L. Melander, "Isotope Effects on Reaction Rates", Ronald Press

Co., New York, i960 , p. 32-33*

18. Ref. 17, p. 16, 17, 20.

19. R.P. Bell, Disc. Faraday Soc., 3£, 16 (1965 ).

20. K.W. Egger, Int. J. Chem. Kinetics, 1, 459 (1969)*

21. J.D. Roberts and M.C. Caserio, "Basic Principles of Organic

Chemistry", W.A. Benjamin, Inc., New York, 1965 , p. 32.

22. A. Streitweiser, Jr., R.H. Jagow, R.C. Fahey, and S. Suzuki, J.

Amer. Chem. Soc., 80, 2326 (1958).

2 3 . For a more extensive compilation, see P. Gray, A.A. Herod and A.

Jones, Chem. Rev., in press.

24. K.J. Laidler, "Chemical Kinetics", McGraw-Hill Book Co., 1965 ,

P- 57. 25. F.S. Dainton, K.J. Ivin, and F. Wilkinson, Trans. Faraday Soc.,

5£,> 929 (1959). 26. G.A. Creak, F.S. Dainton, and K.J. Ivin, Trans. Faraday Soc.,

5 8, 326 (1962 ).

27. J.R. McNesby and A.S. Gordon, J. Amer. Chem. Soc., 77, 4719

(1955).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 36

28. W.M. Jackson, J.R. McNesby, and B. deB. Darwent, J. Chem. Phys.,

37, 1610 (1962).

29. W.M. Jackson and J.R. McNesby, J. Amer. Chem. Soc., 8^, 4891

(1961).

30 . F.H. Westheimer,* Chem.11 ■ Rev., 6l, 265 (I96I).

31. G.S. Hammond, — J. -Amer. ■ Chem. Soc., 1 77 /VN/ > 334 (1955)•

3 2 . J.A. Kerr. Chem. Rev., 66 , 465 (1966 ).

33- T. Yokota and R.B. Timmonsy,Int. J. Chem. Kinetics, 2, 325

(1970).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. EXPERIMENTAL

PROCEDURES FOR THERMAL RUNS

All of the thermal runs in which an initiator was decomposed in

tritiated jt-butyl mercaptan (except the thermolysis of j:-butyl perace-

tate, which will be treated separately) were done in 5 nil Pyrex ampoules

constructed with an extension of 10 mm 0D tubing. Each ampoule could

be re-used by replacing the extension.

The samples were prepared by weighing the initiator into a volume

tric flask and diluting with freshly distilled mercaptan. The concen

tration of initiator was about 5 x 10 2 M in all cases. Control experi

ments, using initiator concentrations ranging from 1 x 10 2 M to 5 x

10 1 M, indicated that the measured isotope effect was constant over

this concentration range. This was verified by studying the effect of

concentration of the J:-butyl perester of cyclohexanecarboxylic acid on

the measured isotope effect for abstraction by the cyclohexyl radical

at 100°C. These data are shown in Table I. The concentration 5 x 10 2

M was chosen for the remainder of the initiators studied because it was

low enough to eliminate the concern of induced decomposition, but high

enough to yield sufficient product to analyze. The solution was trans

ferred to ampoules with a disposable pipette. The ampoules were

degassed and repressurized with nitrogen three times by the freeze-

pump-thaw technique, and then sealed under vacuum. The sealed samples

were kept in cold storage until they were reacted.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 38 Each sample was reacted to completion (ten half-lives of

initiator) in a constant temperature bath. Baths at several tempera

tures ranging from 60°C to 130°C were available, and were controlled

to within + 0.05°C by Sargent Thermonitors.

Following reaction, the ampoules were removed from the heated

baths, quenched in dry ice-acetone, and kept in cold storage until

they were analyzed. Analysis of each sample to determine the amount

of hydrogen abstraction product (RH from R-COa^Jt-Bu) was performed by

gas chromatographic techniques. (See ANALYTICAL) Such an analysis

was necessary because, before the activity of RH could be determined,

the sample was diluted with a pre-weighed amount of unlabelled RH. This

isotopic dilution technique greatly increased the amount of substrate

so that its subsequent isolation and purification was made easier.

Following chromatographic analysis and isotopic dilution, the

substrate RH was purified by multiple extraction with 2N aqueous sodium

hydroxide to remove mercaptan, followed by extraction with distilled

water, drying over anhydrous magnesium sulfate, and, finally, either

multiple recrystallization or distillation followed by preparative gas

chromatography. In each case, repurification of the substrate led to

no change in specific activity. This indicated that the extraction

with base did not exchange tritium from the substrate and that all

radioactive impurities had been removed.

The thermolysis of t,-butyl peracetate in j;-butyl mercaptan was

treated differently due to the formation of gaseous methane as a

product. In the case of £-butyl peracetate decomposition, the methyl

radicals formed were allowed to compete for hydrogen or deuterium

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table I

Effect of Initiator Concentration on the Measured

Isotope Effect for Abstraction by the Cyclohexyl

Radical at 100°C

Perester t-BuSH( T) Cyclohexane Measured Concentration Activity Activity V kT M in t-BuSH(T) (dpm/mole) (dpm/mole) x 102 x 10_1° x 10"10

0.89 ^•550 1.888 2 .ifl

5.13 550 1.912 2.38

2 0 .6 if.550 l.$kh 2.34

U8.7 ^•550 1.90if 2.39

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ko

atom abstraction from either the protium or deuterated mercaptan,

and the relative amounts of methane and methane-di were measured by

mass spectrometry. The ampoules were constructed with a break seal

so that the methane could conveniently be introduced into the mass

spectrometer.

PROCEDURES FOR PHOTOCHEMICAL RUNS

The only experiment in which a free radical was photochemically

generated was the case of the photolysis of t-butyl peroxyformate

(BUP). Pincock1 has reported that the thermolysis of this compound

in cumene at l40°C leads to decomposition via an ionic pathway and

yields no hydrogen. It has recently been proposed, however, that

photolysis of BUP provides a source of hydrogen atoms in solution.2

Solutions of Jt-butyl peroxyformate in concentrations about 1 x 10 2

M in a mixture of protium and deuterated _t-butyl mercaptan were photo-

lyzed using 36OO A lamps (General Electric Model F8T5/BL) in a Rayonet

Srinivasin - Griffin Photochemical Reactor. A thermostated region was

constructed by placing a condenser made of Vycor 79^3 vertically in the

center of the reactor and fixing the sample within the condenser. The

temperature of the solution was controlled to within + 0 .5°C by means

of a Lauda-Thermostat pump, which circulated water at the desired

temperature through the condenser in which the tube was fixed. The

tubes used to contain the samples were constructed from pieces of 8 mm

0D pyrex tubing about 25 cm in length. One end of the tubing was test-

tubed and a removable vacuum stopcock was attached to the other end by

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 41 means of a 10 /3 0 standard taper joint. These tubes could easily be

mounted within the thermostated condenser by means of a rubber

stopper. The tubes were degassed through three freeze-pump-thaw

cycles and sealed under vacuum prior to reaction.

Several control experiments were necessary to establish the

validity of this BUP/mercaptan system. Using BUP concentrations in

the range of 1 x 10 2M to } x 10 ^ in _t-butyl mercaptan, it has

been shown2 that no induced decomposition of BUP is detectable.

These concentrations, however, are sufficiently high to produce

enough hydrogen to quantitatively measure with accuracy for a pho

tolysis time of two hours, the time used in most of the runs. The

fact that the mercaptan does not photolyze under the reaction condi

tions was established by the photolysis of neat protium and deuter

ated t^-butyl mercaptan for a period of four times longer than the

BUP/mercaptan solutions were normally photolyzed. Under these con

ditions of longer photolyses, essentially no gaseous hydrogen or

deuterium was detected by mass spectrometry. The fact that the

relative ratios of [H^I/Ch d ] were independent of the duration of

photolysis was established by photolyzing identical samples for times

of two and eight hours. These data are shown in Table II.

Following reaction, the relative amounts of [H2] and [HD] were

measured by introducing the sample through the vacuum stopcock into

the mass spectrometer. (See ANALYTICAL)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table II

Effect of Length of Photolysis of J:-butyl Peroxyformate

in _t-BuSH(D) - 36OO A Lamps

Temp., °C Peroxyformate Time, [h 2 ]/[h d ] [_t-BuSH]/[t-BuSD] conc. M x 102 hr.

11.0 1.441 2.0 1.361 1.220 11.0 1.441 8 .0 1.3 6 1 1.220

11.0 1.644 2.0 0.724 0.665 11.0 1.644 8.0 0.724 O.665

11.0 1.536 2.0 0.407 0.332 11.0 1.536 8.0 o.4oo 0.332

35.0 1.441 2.0 1.275 1.220 35-0 1.441 8.0 1.275 1.220

35-0 1.644 2.0 0.718 0.665 35-0 1.644 8.0 0 .7 1 9 0.665

35.0 1.536 2.0 0.356 0.332 35-0 1.536 8.0 0.345 0.332

55.0 1.441 2.0 1.240 1.220 55-0 1.441 8.0 1.240 1.220

55.0 1.644 2.0 0 .650 0.665 55-0 1.644 8.0 0.646 0.665

55.0 1.536 2.0 0.327 0.332 55-0 1.536 8.0 0.327 0.332

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. PROCEDURE FOR REACTION OF DPFH WITH MERCAPTANS

The rate constant for hydrogen atom abstraction by the diphenyl-

picrylhydrazyl (DPPH) radical from J:-butyl mercaptan and from deuterated

j:-butyl mercaptan was measured by following the rate of disappearance

of DPPH under pseudo- first order reaction conditions. In all cases,

the concentration of mercaptan is in sufficient excess to be considered

constant. The reaction was carried out in Beckman pyrex cuvettes which

were degassed through three freeze-pump-thaw cycles and sealed under

vacuum.

The rate of disappearance of DPPH was monitored by measuring

changes in absorbance of the solution at 520 mp, using a modified

Beckman DU spectrophotometer which had been equipped with a Gilford

Model 222 absorbance photometer and dual regulated lamp power supply.

Up to four cuvettes were automatically positioned in the measuring

beam in a timed cycle by using the Gilford Model 210-D Automatic

Cuvette Positioner, thus making possible sequential recording of up to

four absorbance measurements during a single run. The reaction tem

perature was kept constant (+ 0 .5°C) by means of a Lauda-Thermostat

pump, which circulated water at the desired temperature throughout the

cuvette compartment.

The isotope effect for abstraction by DPPH from _t-butyl mercaptan

was calculated from the ratio of absolute reaction rates determined

using protium and deuterated mercaptan. The data are shown in Chapter

IV.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 44 MATERIALS

Pcrcstcrs. All peresters were prepared and purified by the

method of Bartlett and Riichardt3 except ^-butyl peracetate, which

was purchased from Lucidol (Wallace and Tiernan, Inc.) and purified

by distillation, and J:-butyl peroxyformate, which was prepared by

the method of Pincock.1’4 Carbon-hydrogen analyses were all consistent

with expected values and infrared analysis indicated no hydroxyl-

containing impurities.

£-Nitrophenylazotriphenylmethane (NAT). The hydrazo precursor

to NAT was prepared by the method of Cohen and Wang5 and oxidized by

refluxing in ether for one hour with a two-fold excess of isopentyl

nitrite.6 The crude NAT was recrystallized from a 2 :1 mixture of

petroleum ether (30-40°) and methylene chloride.

Phenylazotriphenylmethane (PAT). This material was purchased

from Eastman Chemical Co. and recrystallized from benzene-petroleum

ether (30 -40 °).

Methane-di. This material was prepared on a vacuum line by the

slow hydrolysis of the Grignard reagent, CH3MgI, by D2O. The Grignard

reagent was prepared in anhydrous ether by a standard procedure7 and

was hydrolyzed in situ. The collected material was shown to contain

less than 2$> methane as impurity by the mass spectroscopic analysis

at low voltage using the Varian M -66 mass spectrometer.

Hydrogen-di. This material w^s prepared on a vacuum line by the

method of Wender, Friedel, and Orchin.8 Mass spectroscopic analysis

(CEC Model 21-620 ) indicated isotopic purity greater than 95$«

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ^5 Tritiated J:-butyl mercaptan. One ml tritiated water containing

50 mc/ml tritium was added to 100 ml jt-butyl mercaptan (Matheson,

Coleman, and Bell). The mixture was allowed to stir overnight and

the mercaptan was then dried over anhydrous magnesium sulfate,

decanted, and distilled. (B.P. 64°C)

Additional Materials. All substrates (RH from R-C03~jt-Bu)

that were added in the isotopic dilution procedure were commercially

available and were either reagent grade or were purified before

use.

ANALYTICAL

A. General

Gas chromatographic techniques were employed for the quantita

tive determination of substrate (RH from R-C03^t-Bu) in each run

except those in which gaseous products were formed. In the earlier

experiments, the analysis for RH was made using a MicroTek Model

2000R gas chromatograph equipped with dual 8 to 12 ft. stainless

steel columns, flame-ionization detector, and Westronics recorder

with DiscChart integrator. In many of the later experiments, a

Glowall Model 320 gas chromatograph equipped with dual 6 ft. glass

columns, flame-ionization detector, and Varian Model G-2000 inte

grating recorder was used. The Glowall chromatograph is preferred

by the author because of its simplicity of design, ease of opera

tion, reliability, and the facility whereby the glass columns may

be inspected and repacked. In each case, quantitative

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. h6 determination of the amount of product RH was made by the analysis

of a series of standards composed of various amounts of RH in

mercaptan. In those cases in which RH could be conveniently puri

fied by preparative gas chromatography, a Wilkens Aerograph Autoprep

Model A -700 preparative instrument was used.

Mass spectrometry (Consolidated Electrodynamics Corporation

Model 21-620 ) was the method used for the analysis of gaseous

products formed on the thermolysis of ^-butyl peracetate and photol

ysis of _t-butyl peroxyformate in protium and deuterated J:-butyl

mercaptan. In the case of the thermolysis of _t-butyl peracetate,

the instrument was calibrated to separate overlapping cracking

patterns of methane and methane-dx. This was accomplished by the

synthesis of pure methane-dx (See MATERIALS). The ratio of m/e

16 to m/e 17 was equal to 0.688 for pure methane-dx under the con

ditions of analysis. This correction factor was applied to the

m/e 16 peak for mixtures of methane and methane-dx.

In the case of the photolysis of jt-butyl peroxyformate, the

instrument was calibrated for different response sensitivities to

hydrogen and hydrogen-dx. A series of standard mixtures of hydrogen

and hydrogen-dx were prepared on a vacuum line and were analyzed to

give the following relationship (see Fig. I and Table III):

_ n 662 t-m/e / 2^ Hil ltbt3d [m/e 2] [1)

This relationship was found to be valid over a 25-200 mp, range of

gas inlet pressures and was constant over an indefinite period9 as

[HD]/[Ha] 0.2 0.6 2.2 IUEI Ms Setoee tnadzto [HD]/[H2] Standardization Spectrometer Mass I. FIGURE 0.2 vs. (m/e 3)/(m/e 2); slope = 1.662 = slope 2); 3)/(m/e (m/e vs. 0.6 (m/e 3)/(m/e 2) 3)/(m/e (m/e 1.0 1.4

Table III

Mass Spectrometric Measurement of Standard Mixtures of

Hydrogen and Hydrogen-dx

(m/e 3) Tube No. Pressure Ha Pressure HD [HD]/[Ha] (m/e 2) (mm Hg) (mm Hg)

1 150.0 30.0 0.200 0.123

2 61.5 30.0 0.488 0.311

3 29.8 30.0 1.006 0.635

4 20.2 30.0 1.485 O.89O

5 14.5 30.0 2.070 1.25

* These values were found to be identical at mass spec trometer gas inlet pressures of 25 mp< and 200 mp,.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. long as the same instrument conditions and technique of measurement

vrere employed. Using eq. (1), the relative amounts of hydrogen and

hydrogen-dx are easily obtainable from the mass spectroscopic data.

The extent deuteration of a sample of protium and deuterated _t-

butyl mercaptan was determined by recording and integrating the

nuclear magnetic resonance spectrum. All nmr spectra were recorded

using a Varian A-60 A nmr spectrometer and, unless otherwise noted,

were taken as about a 20$ sample solution in carbon tetrachloride

using tetramethylsilane as internal reference. The nmr spectrum of

t-butyl mercaptan consists of a singlet at -1 .4 ppm corresponding to

the nine _t-butyl protons and a singlet at -1.7 ppm corresponding to

the single S-H proton. The extent deuteration of a particular sample

of deuterated jt-butyl mercaptan was determined from the integrations

of the two signals, after accounting for the 9:1 ratio of _t-butyl

hydrogens to S-H hydrogen. The percent J:-BuSH in the sample may be ca

culated by eq. (2)

Such analyses were verified by mass spectroscopic measurements

recorded on a Varian-M66 Mass Spectrometer by Mrs. Cheryl White of

this Department. Low energy electron bombardment is a convenient way

to determine the molecular weight of a compound by mass spectrometric

techniques.10 Under these conditions of analysis, the molecular ion

Mt, which is the precursor of all other ions in the spectrum, is the

last to disappear because it requires the least energy for its forma

tion. Thus, when the spectrum of deuterated J:-butyl mercaptan is

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 50 measured at an electron energy of about 7 eV, the only signals

obtained are those corresponding to m/e 90 and m/e 91. The ratio of

the m/e 91 signal to the m/e 90 signal is therefore equal to the

[_t-BuSD]/[j:-BuSH] ratio under the conditions of bombardment by elec

trons of low energy. A comparison of the data obtained by nmr and

mass spectrometry for one sample of deuterated _t-butyl mercaptan is

shown in Table IV. The nmr method of analysis is preferred by this

author because much extreme care must be taken to avoid deuterium

exchange on sample introduction into the inlet of the mass

spectrometer.

Uncorrected melting points were determined using a Thomas-Hoover

Capillary melting point apparatus. Elemental analyses (carbon-

hydrogen) were done in this Department by Mr. Ralph Seab with the

Coleman Carbon-Hydrogen Analyzer, model 3 3 * Infrared spectra were

taken on either a Beckman IR-7 or Perkin-Elmer Infracord, model 137 *

B. Determination of Tritium Absolute Activities

1 . General Method

Tritium activity was determined with a Packard Tri-Carb Liquid

Scintillation Spectrometer, model 3365 * In order to determine the

absolute activity, A, of a sample, it is necessary that the effi

ciency of counting, E, be determined. Pulse-amplitude spectra are

analyzed electronically by division into one or more channels, with

lower amplitude limit X and upper limit Y set by signal

discriminators, so that only pulses of amplitude between X and Y

are recorded in a given channel. The instrument sensitivity to

energy pulses of different amplitude may be varied by adjusting

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table IV

Comparison of nmr and Mass Spectrometric Methods for

Determination of the Extent Deuteration of J:-butyl Mercaptan

A: Determination by nmr Peak Integrations

T r-7 a .. 1 b -1.7 ppm~ -1.4- ppm [-1.7 ppm]/[-1.4 ppm] t-BuSD- x 102

5.0 184 2.72 75-5 4.8 180 2.67 76.0 5.7 202 2.82 74.6 5-3 188 2.82 74.6

nmr Average 75-2 + 0.8

B: Determination by Mass Spectrometer

m/e 91 m/e 90 [m/e 9l]/[m/e 90] _t-BuSD

74.7 25.3 2.95 74.7 74.7 25.3 2.95 74.7 74.4 25.6 2.91 74.4 74.7 25.3 2.95 74.7 75.1 24.9 3.02 75.1

M.S. Average 74.7 + 0.4

— Signal from S-H proton

— Signal from J;-butyl protons

i t-BuSH = 9 * t-l-T Ppm P^k] 10Q ' — [-1.4 ppm peakj

io t-BuSD = 100 - ($ t-BuSH)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 52 what is known as the channel width. When only one isotope is

present, the net channel counting rate C (counts per minute, cpm)

is proportional to the absolute activity, with the efficiency of

counting equal to the proportionality constant. Thus, E = C/A. E

depends on the particular operating conditions, the isotope, the

channel, and the scintillator-photomultiplier combination.

The counting efficiency is reduced if quenchers are present.

Quenching molecules reduce the observed cpm by dissipating the TT-

electron excitation energy of the excited solvent or solute mole

cule via a collision deactivation process. Various methods may be

used for calibration of the channel counting efficiency and for

correction for the quenching factor. The method which has been

used exclusively here is the automatic external standard calibration.

In this method, a series of samples of known activity A, with various

known quenching factors, is counted (net count rate = EA), and then

recounted (net count rate = EA + C c) after bringing an external ES gamma-ray source into a well-defined position near the sample. This

is done automatically using the automatic external standardization

mode of the counter. A calibration curve is thus obtained of the

beta channel counting efficiency E versus the external count rate

C„c. Using this curve determined from the series of standards, the ES efficiency of counting of an unknown sample can now be determined

from the observed CL,0. This calibration is only valid for specimens ES of the same basic scintillator composition and volume vial dimensions,

isotope, and instrument channel settings.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A computer program (activity deck) has been written by Mr.

Henry R. Streiffer of this Department for handling the data obtained

in the calibration procedure described above. The first part of the

program consists of a least squares analysis of the data for the

efficiency plot obtained by counting the Packard quenched standards.

In order to place the activities on an absolute basis, a correction

is made for ageing of these standards knowing the halflife of

tritium is 1 2 .26 years. For the standards used, t0 = January 1 ,

1966, so the ageing factor (At/A0) at time t can be calculated; A fc

is the relative activity at time t and A 0 is 1 .0 at time to- The

second part of the program computes the absolute activity of the

sample using the least square efficiency plot data obtained in the

first part. Table V indicates the proper use of this program.

The dilution factor shown in Table V is equal to 1 .0 0 unless

the sample weight has been diluted before counting. For example,

if l.OOg of RSH( T) is dissolved in 50 ml toluene and 1 .0 0 ml of

this solution counted, the sample weight is 1.00 and dilution factor

equals 5 0 .0 0 . An equivalent way of expressing this data is to

denote the sample weight as 0.02 and the dilution factor as 1.00.

The data obtained as output from this program is given as abso

lute activity in dpm/g and dpm/mole, where dpm = disintegrations per

minute. The following relationship applies:

(dpm/mole) ( efficiency) = cpm/mole.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table V

Data Cards for Use of Activity Computer Program

Card # Description

1 Header card for efficiency plot data, columns 1-80

2 ageing factor A/A0, columns 1-10

3 -9 cpm for standard, columns 1-10 automatic external standard cpm, column 11-20

10 "1" in column 21

11 Header card for sanple counts data, columns 1-80

12 molecular weight of material being counted, columns 1-10

1 3 -(1 3 + n) 1) Sample number, columns 1-10 (n = number of samples 2 ) Sample weight, columns 11-20 counted) 3) cpm, columns 21-30 4) cpm, automatic external standardization mode, columns Jl-kO 5) dilution factor (see below) columns 41-50

14 + n "-1" in columns 9 an^ 10

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 55 2 . Liquid Scintillation Counting of Mercaptans

In work relating to the measurement of isotope effects for

abstraction from RSH( T) by a free radical, it is essential that

absolute activity of the mercaptan be precisely measurable. Propyl

mercaptan, thiophenol, and t^-butyl mercaptan have all been counted

in this regard. Considerable ambiguity was encountered in determi

ning the activity for all three compounds.

Ordinarily, counting time for most samples is one minute, pro

viding the sample is active enough and a large enough mass can be

incorporated into the fluor solution. (This is governed by solu

bility and quenching considerations). Counting of the quenched stan

dards and unknown samples is done at least three times, and the

average cpm taken to reduce: the statistical variance. Ordinarily,

both the absolute cpm and counting efficiency are observed to increase

up to several percent over this time period as the samples are cooled.

This is due to decreased solubility of dissolved oxygen in the fluor

solution at lower temperatures. (Temperature in this chamber is near

0°C - thus, the choice of toluene as solvent). When mercaptans are

counted, however, the absolute cpm is seen to decrease (up to 30$)

from the counting rate initially measured. This decrease occurs over

a time period of six to eight hours, after which time the cpm

measured appeared to level off. No such decrease in cpm is observed

when the samples are de-oxygenated (bubbling nitrogen) prior to

counting, and the counting rate is the same as the initial measure

ment in the samples in which oxygen was not removed. No satisfactory

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5 6 explanation can currently be offered to explain this unusual effect.

Nevertheless, the true activity for the mercaptan is necessary, and

several alternative methods of determining activity were attempted.

a. Effect of Scintillator Composition o

The counting of mercaptans discussed above was done using

Packard's "Permafluor" as the fluor solution and toluene as solvent.

Results identical to those measured in the de-oxygenated samples were

obtained when the activity was measured using New England Nuclear Cor

poration's "Aquasol", a xylene based solution. Removing oxygen from

the "Aquasol" samples appeared to have no effect on the measured

counting rate.

b. Activity of Mercaptan Measured by Proportional Tube

Flow Counter Techniques.

The flow counter can be used to detect and measure the activity

in several components of a complex mixture by separating the compo

nents by gas chromatography and flowing them separately from the

thermal conductivity detector of the chromatograph into the propor

tional tube where the activity is determined. A Nuclear-Chicago Cor

poration Model ^998 Gas Radiochromatography Counting System equipped

with digital integrator and Varian Aerograph chromatograph was

used.

In this system, the detector efficiency must be calibrated with

a sample of known activity before absolute activities of unknown

samples can be calculated. The absolute activity of a compound which

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 57 causes no difficulties in liquid scintillation counting (e.g., side

chain tritiated toluene or tritiated cyclohexane) is determined and

is used to calibrate the flow counter efficiency. This efficiency

is greatly dependent on the flow rate of carrier gas and purge gas

(propane) because the residence time of the component in the propor

tional tube is a function of flow rate. The following equation may

be used to calculate activities:11

B - A[¥ r h s ] <5> where N = net integrated count recorded

A = activity, cpm

V = sensitive volume of detector in ml (85 ml, in this case)

Fi = purge gas (propane) flow rate (ml/min)

F2 = chromatograph flow rate (ml/min)

Calibration of the instrument was performed at the following

flow condition: Fi = 95-25 ml/min

F2 = 26.21 ml/min

The flow rates are measured with a soap film flowmeter. Table VI

gives a sample of the data obtained for the determination of the flow

counter efficiency using tritiated toluene and cyclohexane.

If the efficiency of counting in the flow counter system is

assumed to be independent of sample composition,12 the absolute

activity of a mercaptan may now be determined. Table VII shows this

data, calculated from the efficiency data in Table VI.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table VI

Flow Counter Calibration Using Tritiated Toluene

and Cyclohexane

Compound Counts/ N (c/g) A (cpm/g) A*— 'ja Efficiency, Injection- x 10”5 x 10"5 (dpm/g) A fc/A* x 10_5 •

cyclohexane 1883 3.021 5 .971 6.582 90.7 cyclohexane 169 ^ 2.718 5.372 5-575 8 9 .9 cyclohexane 2760 it.lf28 8.7 5 1 9 .681 *- 90.it cyclohexane 1913 3.069 6.066 6.582 92.2 cyclohexane 1032 1.656 3.272 3 .7 1 6 88.1

cyclohexane 2722 i+.367 8.631 9 .681 * 89.1 toluene it 103 9.^65 18.706 2 0 .4 it it 91.5 toluene M 5 6 9.588 18.9^8 20. it it it 92.7

toluene if 137 9 . 5 H 18.862 2 0 .4 it it 92.3 toluene itll8 9-501 18.775 20. it it it 91.8

Average 90.9

■a Injection size for cyclohexane sample equals 8 p,l

Injection size for toluene sample equals 5 |J<1 b * ~ A represents activity determined by liquid scintillation counting.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table VII

Determination of Absolute Activity of Tritiated _t-butyl Mercaptan

Using Flow Counter

Counts |il N (c/g) A, A*— /3 * a fc Afc f " injection x 10_7 efficiency^ (cpm/g) (dpm/g) (dpm/g) x 10 8 x 10 8 x 10 8

190858 8.005 9 0 .9 1 .4 9 9 1 .6 4 9 1.670

190915 8.008 9 0 .9 1.500 1.650 1.670

191005 8.012 9 0 .9 1.500 1.650 1.670

191102 8.016 9 0 .9 1 .5 0 1 1 .651 1.670

Average 1.650 1.670

— determined in Table VI

— A^c( cpm/g)/A^c (dpm/g) = $ efficiency/100

- A* represents activity using de-oxygenated "Permafluor"

solutions in the liquid scintillation counter.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 60

Unfortunately, counting the mercaptan using the flow counter

is not as simple and straightforward as it may at first appear. The

hydrogen bound to sulfur is so labile to exchange (especially at

elevated temperatures in the chromatograph column and detector) that

a considerable portion of the activity must be "washed" into the

proportional tube with several water injections. This procedure was

necessary even when a short chromatograph column packed with glass

beads was used. The value for N in Table VII represents the total

count obtained after thorough "washing" of the column with water.

It is seen that the activity obtained with the flow counter

(Afc, dpm/g) agrees well with the activity determined using liquid

scintillation counting (A ) when care is taken to remove oxygen,

c. Combustion Analysis of t,-butyl mercaptan.

Organic tritium-containing compounds may also be analyzed by

burning the sample and measuring the activity of the resulting

water. A sample of £-Bu SH(t ) was submitted to New England Nuclear

Corporation13 for this analysis. Table VIII shows a comparison of

the activity obtained using deoxygenated "Permafluor" solutions

(liquid scintillation counting) with the activity obtained by com

bustion analysis. Combustion analysis is perhaps the most indepen

dent method for determining the activity since the water obtained

must be dissolved in a dioxane based fluor solution and the absolute

activity calculated from dioxane quenched standards.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table VIII

Activity of _t-butyl Mercaptan Determined

by Combustion Analysis

Activity (dpm/mole) Activity (dpm/mole) Deoxygenated "Permafluor" Combustion Analysis 3 LSC Technique (x IO"10) (x 10"10)

7^613

7.662 7 .5 1

7 .752 7 . ^

7.k-66 7.62

7.705

7.689

Average 7 .6 4 8 + 0 .1 7.52 + 0.1

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 62 d. Conclusion

The excellent agreement in determining the activity by liquid

scintillation techniques (using de-oxygenated "Permafluor" and

"Aquasol"), flow counter techniques, and combustion analysis would

seem to indicate that these methods all give a true and accurate

activity measurement.

C. Liquid Scintillation Counting of Nitrobenzene from NAT

It was impossible to determine the activity of the nitrobenzene

formed upon decomposition of NAT in tritiated J:-butyl mercaptan

either by liquid scintillation techniques or using the flow counter

because of excessive quenching of the count rate. In order to circum

vent this difficulty, the nitrobenzene was reduced to aniline using

granulated tin and concentrated hydrochloric acid as discussed by

Roberts and Caserio.14 To 2 .1 ml nitrobenzene in a 50 ml round-

bottom flask, k.$g granulated tin was added. The flask was equipped

with magnetic stirring bar and condenser. Concentrated hydrochloric

acid (10 ml) was added in small portions, accompanied by a vigorous

exothermic reaction. After addition of the acid was complete, the

mixture was refluxed in a boiling water bath for one hour, and then

allowed to cool to room temperature. Upon cooling, a yellow solid

(aniline hydrochloride and stannic chloride) separated. Sodium

hydroxide (7*5g) in 12*5 ml water was slowly added (exothermic reac

tion) and the mixture was steam distilled. The distillate was

collected until about 15 ml clear liquid was obtained after the

collection of ml turbid liquid. To decrease the solubility of

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 63 aniline in water, h.1} g sodium chloride was added, and the mixture

was extracted with ether. The ether washings were dried over

sodium carbonate, concentrated on a rotary evaporator and the

collected aniline (1 -2 ml) was dried over sodium carbonate. Further

purification of the aniline was effected by vacuum distillation from

a small amount of granulated zinc.

Aniline, however, also quenched during the counting procedure,

and therefore was converted, via a diazotization reaction,15 to

benzene, which could be counted by the liquid scintillation counter

without difficulty. To a solution of 15 ml concentrated sulfuric

acid and 75 ml water in a beaker, 13 g aniline was added with

stirring. Aniline sulfate was formed as a precipitate. The mixture

was cooled in ice and 120 g ice was gradually added. A solution of

10 g sodium nitrite in ^0 ml water was then slowly added from a

dropping funnel while the mixture was kept in ice and stirred

vigorously. The solution gradually became yellow in color and was

stirred in an ice bath until all of the aniline sulfate dissolved.

An alkaline stannite solution was prepared by adding a solution of

68 g sodium hydroxide in 85 ml water to the turbid mixture of 6^.8 g

stannous chloride dihydrate in 300 ml water. This stannite solution

was immediately cooled in ice and then added in small portions to

the diazonium salt solution prepared previously. Each addition was

made only after the evolution of nitrogen from the preceding

portion had ceased. The benzene which was formed was distilled

directly from the mixture, along with the passage of a considerable

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 64 portion of water. The aqueous distillate was extracted several

times with ether and the washings were dried over calcium chloride.

Following removal of the ether on a rotary evaporator, the benzene

was distilled.

OXIDATION OF AROMATIC HYDROCARBONS

To measure the degree of ambident character of the benzyl,

diphenylmethyl, and trityl radicals, the activity incorporated into

the ring positions of the resulting toluene, diphenylmethane, and

triphenylmethane was determined. In order to separate ring from

side chain activity, it was necessary to oxidize the hydrocarbon to

a derivative in which the side chain activity was removed. The

activity in the ring was determined by difference.

A. The Benzyl Radical

The benzyl radical, generated by the decomposition of J:-butyl

phenylperacetate in tritiated Jt-butyl mercaptan, led to tritiated

toluene as a major product. In order to separate ring from side

chain activity, the toluene was oxidized to benzoic acid in basic

potassium permanganate solution16 according to the following

equation:

PhCH3 + 2KMn04 — > PhCOO”K+ + KOH + H2O + 2Mn02 (4 )

Toluene (one ml) was added to a solution of 3 *1 8 potassium perman

ganate in 40 ml water to which one ml 10$ aqueous sodium hydroxide

had been added. The mixture was refluxed for 5 hours. Following

reaction, the mixture was cooled, filtered, and acidified with

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 65 dilute sulfuric acid. Upon further cooling, the benzoic acid

separated and was recrystallized from hot water/ethanol (90/10).

After drying under vacuum, the benzoic acid (M.P. 121-2°c) was

counted by the liquid scintillation counter, and was found to

contain no tritium. Thus, abstraction by the benzyl radical occurs

exclusively through the side chain.

B . The Diphenylmethyl Radical

The diphenylmethyl radical, generated by the decomposition of

t-butyl diphenylperacetate in tritiated J:-butyl mercaptan, led to

diphenylmethane as a major product. In order to separate ring from

side chain activity, the diphenylmethane was oxidized to benzo-

phenone. The methods of Wiberg and Evans17 and Mares and Rocek18

were attempted without success. The oxidation was achieved,

however, by heating the diphenylmethane with lead acetate and con

centrated nitric acid in the method described by Rivkin.19 A mixture

of 2.0 g diphenylmethane, 0.5 ml water and 0.3 g lead acetate tri

hydrate were heated to boiling and stirred during the dropwise addi

tion of ^ .0 ml concentrated nitric acid. The reaction is accompanied

by the evolution of nitric oxide. The mixture was stirred for an

additional 1.5 hours, then cooled to room temperature and neutralized

by the slow addition of saturated aqueous sodium carbonate. A yellow

oil separated and was taken up in ether. The ether extracts were

washed with saturated aqueous sodium chloride and concentrated on

a rotary evaporator to give an oil which crystallized on standing.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 66 Repeated crystallization from ethanol/water gave pure benzophenonc

(m.p. kr}-)\-7 °C) which was found to contain no tritium. Thus, abstrac

tion by the diphenylmethyl radical occurs exclusively outside of the

ring positions.

C. The Trityl Radical

The trityl radical, generated by the decomposition of NAT in

tritiated J:-butyl mercaptan, led to triphenylmethane as a major

product. In order to separate ring from side chain activity, the

triphenylmethane was oxidized to triphenylcarbinol by the method of

Law and Perkin. 20 Triphenylmethane (0.5 g) was dissolved in 4 ml

carbon tetrachloride and, while stirring, a solution of 2 g chromyl

chloride, Cr02Cl2> in 6 ml carbon tetrachloride was slowly added.

(Preparation of chromyl chloride is described below) A black tar

was obtained after allowing the mixture to stand for 6 hours. As

much of the carbon tetrachloride as possible was removed under a

stream of dry nitrogen, and the black tar was added in small pieces

to 40 ml water. After sitting overnight, white crystals separated

and the mixture was extracted with ether. The material obtained

upon extraction and concentration of the solution by removal of

the ether on a rotary evaporator was recrystallized from carbon tetra

chloride and dried in a vacuum. The pure triphenylcarbinol thus

obtained melted at l6 l-2°C.

Chromyl chloride used in this oxidation was prepared by the

method of Sisler. 21 Chromium trioxide (50 g) was dissolved in 55

ml water and 110 ml concentrated hydrochloric acid was added. The

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 67

mixture was cooled to 5°C and 150 ml concentrated sulfuric acid was

added dropwise with stirring, keeping the temperature between 10°

and 20°C. After addition was complete, the mixture was placed in a

separatory funnel and the lower layer of chromyl chloride was drawn

off. Yield was about 60 g. The material thus obtained was used

without further purification.

Counting the triphenylcarbinol in the liquid scintillation

counter indicated no tritium. Thus, abstraction by the trityl

radical also occurs exclusively outside of the ring positions.

KINETICS OF INITIATOR DECOMPOSITION

The rate constants and Arrhenius activation parameters for the

decomposition of all new initiators used in these studies have been

measured by the method described by Pryor and Smith. 2 2 ’ 23

The rate of disappearance of the initiator was measured by

following the rate of disappearance of the carbonyl absorption at

about I78O cm 1. A solution of the initiator, approximately 3 x 10 2

M in an n-alkane solvent, was distributed into one ml break-seal

ampoules, which were sealed at atmospheric pressure without degassing.

The entire lot was placed in a constant temperature bath. Samples

were removed from the bath at preselected time, quenched in dry ice-

acetone, and placed in cold storage. Two samples were reacted to

infinity time, at ’least ten half-lives of the initiator. After all

the samples had been removed, the optical density was determined

using a Beckman IR-7 using a cell thickness of 0 .5 mm.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 68

The transmittance vs. time data were fitted by the method of

least-squares to the equation

M A t - A j - kobsdt - fc, (Ao - AJ (5)

where is the absorbance at time J: and Ara is the experimentally

determined absorbance of the initiator after decomposition for ten

half-lives.

The hydrocarbon solvents were purchased from Phillips Petroleum

Company and were specially purified by stirring for several hours

with concentrated sulfuric acid, washing with 10$> sodium bicarbonate

and distilled water, and distilling. In some of the later experi

ments, the hydrocarbons were purified by the method of Murray and

Keller24 in which the hydrocarbon was passed through a 12 inch

column packed with silver nitrate on alumina.

MECHANISM OF INITIATOR DECOMPOSITION

Several methods have been developed for probing the mechanism

of perester decomposition. The following three methods have been

used in connection with some of the initiators discussed in this

dissertation: the viscosity dependence of bond homolysis, secondary

kinetic isotope effects in initiator decomposition, and the use of

scavengers in homolytic initiator decomposition. A complete discus

sion of these and other methods for investigating the mechanism of

perester decomposition will be presented in Part I of the APPENDIX.

The purpose of this section is to discuss the experimental details

of the methods and the procedures for preparing the compounds studied.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 69

The three methods outlined above have been applied to the

decomposition of the Jt-butyl peresters of cyclohexanecarboxylic

acid, cyclooctanecarboxylic acid, and Q'-deuteriocyclooctanecarboxylic

acid„ The first two peresters were prepared by the method of

Bartlett and Riichardt.3 The synthesis of _t-butyl cv-deuteriocyclo-

octaneperoxycarboxylate was effected as described below.

A. Synthesis of _t-butyl a-deuteriocyclooctaneperoxycarboxylate

1 . Preparation of a-deuteriocyclooctanecarboxylic acid

Cyclooctanecarboxylic acid25 (13-8 g), 2 7 .0 g deuterio sulfuric

acid (93 mole percent d2)25> and 3 S deuterium oxide (99*8 mole

percent d2 , International Chemical and Nuclear Corp.) were stirred

at about 90°C f°r nine days. The isotopic exchange was begun under

a slow stream of dry nitrogen, but after thermal equilibrium was

attained, the reaction mixture was simply heated under reflux.

After four days, an additional 2 ml deuterium oxide was added.

Following reaction, the contents of the flask were transferred to a

500 ml separatory funnel and diluted with 125 ml pentane and 100 ml

3M sodium chloride solution. Salt was added to facilitate breakup

of the gel-like emulsion which formed. Repeated washing with pentane

was continuted until the two layers were easily separated. The

pentane washings were extracted twice with 50 ml portions of 1 N

sodium hydroxide and the resulting aqueous basic carboxylate solution

was acidified with 1 N hydrochloric acid. The organic acid which

separated was taken up in five 50 ml pentane washings and this

pentane solution was dried over Drierite. The solvent was removed

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 70 on a rotary evaporator to yield 12.0 g partially deuterated a-

deuteriocyclooctanecarboxylic acid. A second deuteration was per

formed by stirring the acid from the first exchange with 2k g

deuterio sulfuric acid and 5 nil deuterium oxide at 90°C for seven

days. After two days, an additional 3 g deuterium oxide was

added. Following the same workup procedure described above, 1 1 .0

g o'-deuteriocyclooctanecarboxylic acid was isolated.

2 . Preparation of Q'-deuteriocyclooctanoyl chloride

The deuterated acid prepared above (1 1 .0 g) was added to 19-0

g thionyl chloride in a 100 ml round bottom flask equipped with

condenser and drying tube. The mixture was stirred for one hour

at room temperature and then heated under reflux for four hours

until no further evolution of hydrogen chloride was evident. The

excess thionyl chloride was distilled off and the product was dis

tilled (73 °C/2mmHg) to give 1 1 .0 g Q'-deuteriocyclooctanoyl chloride.

3 . Preparation of sodium J:-butylperoxide

To 150 ml of a 20 per cent sodium hydroxide solution at 5°C,

25 g 90 Per cent J;-butyl hydroperoxide (Lucidol Division, Wallace

and Tiernan, Inc.) was added dropwise with stirring keeping the tem

perature below 10°C. The crystals which precipitated were collected

by suction filtration and washed with cold acetone. The product

was dried over Drierite in a vacuum.

k. Preparation of the perester

The deuterated acid chloride (1 1 .0 g) prepared above was

dissolved in 10 ml dry ether and added over 1.5 hours to a slurry of

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 71

6.9 g sodium Jt-butylperoxide prepared above in JO ml dry ether.

The mixture was kept below 10°C until addition was complete and

then stirred for three hours at room temperature. Water (15 ml)

was added and stirring was continued for an additional JO minutes.

The aqueous layer was removed and the organic layer was extracted

twice with 2J ml aliquots of 1 M sulfuric acid, twice with 2J ml

aliquots of 1 N sodium hydroxide, and four times with 50 ml aliquots

of cold water. After drying over anhydrous magnesium sulfate, the

ether was removed on a rotary evaporator. The perester obtained

(8 g) was passed through a short Florisil column and placed in

cold storage.

B. Analysis of £-butyl o'-deuteriocyclooctaneperoxycarboxylate

1 . Analysis of cyclooctane carboxylic acid and a-deu-

teriocyclooctane carboxylic acid by nmr

The nmr spectrum of the undeuterated acid had signals at

-12.6 ppm (sharp singlet due to acid proton), -2.5 ppm (broad

absorption due to hydrogen in the a-position), -1.7 ppm (broad

absorption due to 3-hydrogens), and -1 .4 ppm (a broad strong absorp

tion due to the remaining methylene hydrogens of the ring).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 72 In the nmr spectrum of the undeuterated acid, the area due to the a -

hydrogen signal was found to be equivalent to the area due

to the acid proton. The nmr spectrum of the deuterated acid (i, =

D) was the same as that of the undeuterated acid except that the

absorption at -2.5 ppm was largely reduced and the signal at -1.7 ppm

was sharpened considerably due to reduced coupling to the hydrogen

in the 0-position. Both spectra were recorded as a 30 percent solu

tion of the acid in benzene with tetramethylsilane as internal

reference. Calculation of the extent deuteration of the deuterated

acid was made by comparing the areas of the signals due to discrete

portions of the molecule. The data in Table IX indicate that the a -

deuterated acid is 67.3 percent a'-deuteriocyclooctanecarboxylic acid.

2. Analysis of cyclooctanecarbonyl chloride and ce-

deuteriocyclooctanecarbonyl chloride by nmr.

The nmr spectrum of the undeuterated acid chloride had signals

at -2.7 ppm (broad absorption due to hydrogen in the or-position),

-1.7 ppm (broad absorption due to 0-hydrogens), and -1.3 ppm (a

broad strong absorption due to the remaining methylene hydrogens of

the ring).

C0C1

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. In the nmr spectrum of the undeuterated acid chloride, the area due to

the a-hydrogen to that due to the remaining ring hydrogens was found

to be 1:13.8 (theoretically, 1:1b-). The nmr spectrum of the deuter

ated acid chloride was the same as that of the undeuterated acid

chloride except that the absorption at -2.7 ppm was largely reduced

due to deuterium substitution at the a-position and the signal at -1.7

ppm was sharpened considerably due to reduced coupling to the hydrogen

in the 3-position. Both spectra were taken as a 30 percent solution

of the acid chloride in benzene. Calculation of the extent deuteration

of the deuterated acid chloride was made by comparing the area of the

signal due to the a-proton to the area due to the remainder of the

ring protons. The data in Table X indicate that the a-deuterated

acid chloride is 66.2 percent a-deuteriocyclooctanecarbonyl chloride.

3. Analysis of the j>-bromophenacyl ester26 of a -

deuteriocyclooctanecarboxylic acid by nmr.

One g of the deuterated acid was dissolved in enough 2N sodium

hydroxide so that the solution was barely basic to litmus. One g £-

bromophenacylbromide25 was added with 20 ml 95 percent ethanol and the

mixture was heated under reflux for one hour. The crystalline white

solid which separated on cooling »raa racryiataMizeid twice frrometfranol/

water, dried in a vacuum, and melted at 92“93°C. The nmr spectrum of

the £-bromophenacyl ester had signals at - J . 6 (a broad, unresolved

multiplet due to the aromatic protons), -5.2 ppm (a broad singlet due

to the hydrogens a to the carbonyl and ester linkages), -2.6 ppm (a

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table IX

NMR Analysis of cy-deuteriocyclooctanecarboxylic Acid

Calculation of Extent Deuteration

C02H

Signal Integration H Ha a Run H H °jo Qf-d- a a «p + Hx H Hq + H

1 1 2 .0 4.0 1 7 0 .0 0.333 66.7 0.024 67.1

2 1 1 .8 3-9 1 6 8 .0 0.331 6 6 .9 0.023 67.5

3 1 2 .0 4.0 1 7 5 .0 0.333 66.7 0.023 6 8 .0

it 1 2 .2 4.0 173.0 0.328 6 7 .2 0.023 6 7 .6

Average 6 6 .9 6 7 .6

— calculated by the following equation: io a -d = 100-[(H /H )100] (6) cy 3

— calculated by the following equation: $ a-d = 100-{l4 x 100 [Ha/(Hp + Hx)]} (?)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table X

NMR Analysis of Q'-deuteriocyclooctanecarbonyl Chloride

Calculation of Extent Deuteration

(Hv)a (Hq)2 \ / C 0 C l

(Hx ) 2 (Hx ) 2

H a Run H„ + H a -d — H* 0 x H_ + H 0 X

1 4.8 194 .0 0.025 65.4

2 k .9 198.0 0.025 65.4

3 5.0 209 .0 0.024 66.5

4 4.8 20 6 .0 0.023 67.4

Average 6 6 .2

— calculated by the following equation:

i a-d = 100-{l4 x 100 [Ha/(Hp+ Hx)]} (8 )

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 76 broad absorption due to the proton in the a-position of the eight-

membered ring), and -1.7 ppm (broad absorption due to the remaining

protons in the cyclooctyl ring).

Br >C-CHa-O-C III

Calculation of the extent deuteration in the a-position in the eight-

membered ring was made by comparing the area of the signal due to this

a-proton to the area of signal due to the two protons a to the carbonyl

and ester linkages. The data in Table XI indicate that the £-bromo-

phenacyl ester contained 66.3 percent deuterium in the a-position of

the cyclooctyl ring.

k . Conclusion

Direct measurement of the extent deuteration in a'-deuteriocyclo-

octaneperoxycarboxylate was impossible due to overlapping signals from

different portions of the molecule. Thus, the percent deuteration was

determined by the three analyses described above. The extent deutera

tion obtained by averaging the results of these measurements is 66.6

percent. The secondary deuterium isotope effect for decomposition of

this perester was corrected for the portion of a-H perester present

as discussed in Part I of the APPENDIX.

C. Viscosity Dependence of Bond Homolysis

The effect of solvent viscosity on the rate of perester decom

position was determined by measuring the rate of decomposition in a

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table XI

NMR Analysis of the £-bromophenacyl Ester of

a-deuteriocyclooctylcarboxylic Acid

Calculation of Extent Deuteration

H* Run H Hi °jo a-d— a Hi

1 5.2 3 1 .0 0.168 66.5

2 5.8 3 ^ .0 0.171 65.9

3 5-1 3 0 .0 0 .170 6 6 .0

k 5.8 3 5 .0 0.166 6 6 .9

Average 6 6.3

~calculated by the following equation:

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7 8 series of hydrocarbon solvents. The method was described in an earlier

section of this Chapter. The effect of solvent viscosity on the rate

of decomposition of the t-butyl peresters of cyclohexanecarboxylic

acid, cyclooctanecarboxylic acid, and Q'-deuteriocyclooctanecarboxylic

acid will be discussed in Part I of the APPENDIX.

SECONDARY ISOTOPE EFFECTS IN INITIATOR DECOMPOSITION

Secondary isotope effects were measured for the decomposition

of Q'-deuteriocyclooctaneperoxycarboxylate. This was accomplished by

measuring the rate of decomposition of the undeuterated perester as

described earlier and comparing that rate to the rate of the Q'-deuterio

perester. The secondary isotope effect measured was corrected for the

fact that the deuterated perester contained 33*^ percent or-H perester.

Thus, the observed rate for decomposition of the deuterated perester,

kp 0bS(j’ reflects the rate for the undeuterated portion of the mixture

as well. The rate constant for decomposition of the pure a-d perester,

kp, is calculated as follows:

A = total perester concentration

a = atom fraction of deuterium in the a-position

k = rate constant for decomposition of the pure a-H perester H Thus,

-dA/dt = kH(l-a)[A] + a [A] = (10)

V o b s d = kH(1‘a) + “d a

In the present case, a was found to equal 0.666.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 79 THIS USE OF SCAVENGERS IN HOMOLYTIC INITIATOR DECOMPOSITION

The amount of carbon dioxide produced on thermolysis of t-butyl

cyclooctaneperoxycarboxylate was measured in the presence and absence

of free radical scavengers. A 200 ml 5_neck round bottom flask was

equipped with condenser and fritted glass nitrogen bubbler. In all

cases, 80 ml of solvent was used and the solvent was degassed by

bubbling in nitrogen for several hours while equilibrating at the

desired temperature. The perester was weighed into a small aluminum

foil "boat" and dropped directly into the reaction flask. A slow,

steady stream of nitrogen was passed through a column of Ascarite

before entering the reaction vessel during the entire time of reaction

(10 half-lives of perester). The effluent gases were passed through

the condenser (cooled to -15°C), a dry ice-acetone trap, and, finally,

a U-tube filled with Ascarite to absorb the carbon dioxide produced

from the perester decomposition. The U-tube was allowed to reach

constant weight after the solvent was degassed with nitrogen and

before reaction was begun. The U-tube was weighed after reaching

constant weight following the decomposition, and the amount of carbon

dioxide trapped was determined by difference. This experimental

design is essentially identical to that described by Misra and

Mathur.27 The results are given in Part I of the APPENDIX.

RADIOLYTIC GENERATION OF FREE RADICALS

The isotope effect for abstraction from J:-butyl mercaptan by a

free radical was measured independently by generating the radical

radiolytically in mixtures of isotopically substituted mercaptan. In

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 80 two separate experiments, a radical was generated in the presence

of t-butyl mercaptan by the y-irradiation of a substrate in a

cobalt-60 pool reactor. In one case, the radical was allowed to

compete between hydrogen and tritium atoms from tritiated _t-butyl

mercaptan. In the other case, the mercaptan was extensively

deuterated (ca. 95 percent) at the S-H position and then tracer

labelled with tritium at the S-H position, so that competition during

abstraction was between deuterium and tritium atoms. The kinetic

analysis will be presented in detail in Part II of the APPENDIX.

Each of the solutions for radiolysis was degassed through three

freeze-pump-thaw cycles and sealed under vacuum in special ampoules

constructed from 25mm pyrex tubing with 10mm 0D pyrex necks. The

ampoules were constructed so that 10 ml of liquid occupied approximately

90 percent of the volume. In order to reproduce conditions of radiolysis

for each sample, each radiolysis was carried out individually so that

the same geometrical arrangement in the diving bell could be used. In

all cases, including the Fricke dosimetry, only the central vertical

position in the diving bell was used.

The dose delivery rate for this vertical position was determined by

Fricke dosimetry using the standard aqueous solution of 0.8 N sulfuric

acid, 0.001 M sodium chloride, and 0.001 M ferrous ammonium sulfate

hexahydrate. The dosimetry solutions were not sealed under vacuum

but were placed in the ampoules and capped with aluminum foil. The

times of radiation of the dosimetry solutions were 5> 10, 1 5 , and 20

minutes; the amount of ferric ion formed in each solution was measured

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission 81 at 305 mp< on a Beckman DU Spectrophotometer at 29.7°C using the

unirradiated solution as reference. This data is shown in Table

XII and Figure II.

The rate of formation of ferric ion in the dosimetry solution

has been standardized and is given28 by eq. (ll).

Dp = 2 .8 0 x 104 (absorbance) rads = absorbed dose (ll)

Eq. (ll) applies when the temperature of the absorbance measurement

is 2j5.7°C. The temperature correction is given by eq. (12).

_ absorbed dose (measured at tp°C) (io\ D, corr “ 1 + 0.0C7 (t2 - 23.7°C) { '

In the present case, t2 °C = 29-7> so the corrected absorbed dose

D^ for a ferric ion absorbance of unity is given by eq. (13)- D,corr J v

„ 2.80 X 104 (1.0) fCtr-r 1 r ^ A 1 / T D_ = T , r,"^7b I d XY = 2.687 X 10 rads (13) D,corr 1 + 0.007 (6.0) ' "

From the least-squares slope of Figure II, the time necessary to

produce a total dose which will give a ferric ion concentration of

unity may be calculated:

Absorbance (305 nip,) = 0.0^48 (time, min.) (1*0

(time, min.) = (1/0.0W-8) = 22.185 (15)

Thus, the radiation flux in the central axial position of the diving

bell is calculated by eq. (l6):

Dose rate = (2.687 x 104 rads/22.I85 min.) =

7 .2 6 7 x 104 rads/hr (l6)

Using this dose rate and the G value for formation of a particular

radical, the yield of product may be calculated. For example, if

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table XII

Fricke Dosimetry. Ferric Ion Absorbance as

a Function of the Time of Cobalt-60 Irradiation

Time of Irradiation Absorbance (minutes) (505 mp.)

0 .0 0.000

5.0 0.237

1 0 .0 0.461

1 5 .0 0.675

2 0 .0 0.900

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0.9

0.8

0.7

e 0.6 LT\ o

0«>" 0.5 nic Xi 1 0.4 < 0.3

0.2

0.0 15 20

Time (minutes) FIGURE II. Fricke Dosimetry Absorbance vs. Time. slope = 0.CM8

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 81* cyclohexane is the substrate, cyclohexyl radicals will be formed with

a G value of k . J .29

The G value for formation of a radical is defined as follows:

G (radical) = radicals produced/100 eV energy absorbed (1?)

1 rad = energy absorption of 100 erg/g (18)

Spinks and Woods28 have shown the relationship between the total

energy absorbed and the number of radicals formed, given in eq. (19)*

Energy absorbed = 1.602 x 10 12 [-rad.^-^ - | ^ S g y/-H-r^I?]rads (19)

Therefore, if the substrate is cyclohexane, the rate of formation of

cyclohexyl radicals may be calculated.

7.267 x 104 rads/hr =

1 ,^^1).^7 10 -- [radicals formed/gram] rads (20)

Radicals (g 1 hr x) = 2.13 x 1017 (21)

In 10.0 ml (7-79 g) cyclohexane, the cyclohexyl radical yield per

hour is:

Radicals (hr'1) = 7-79 x 2.13 x 1017 = 1.66 x 1018 (22)

The results of the radiolysis experiments will be presented

and discussed in Part II of the APPENDIX.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. REFERENCES - III

(1) R.E. Pincock, J. Amer. Chem. Soc., 8 6 , 1820 (196 ^).

(2) W.A. Pryor and R.W. Henderson, 3 , Amer* Chem. Soc., %2, 723k (1 9 1 0 ).

(3) P.D. Bartlett and R. Ruchardt, J. Amer. Chem. Soc., 82^, 1756

(I960 ).

(^) Obtained from Mr. R.W. Henderson.

(5) S.G. Cohen and C.H. Wang, J. Amer. Chem. Soc., J g , 550^ (1953).

(6) Obtained from Dr. K. Smith.

(7 ) L.F. Fieser, "Experiments in Organic Chemistry", D.C. Heath

and Company, Boston, 1957* p. 79«

(8) I. Wender, R.A. Friedel, and M. Orchin, J. Amer. Chem. Soc.,

7 1 , 11^0 (19^9).

(9) This has been verified by Dr. J.P. Stanley of this Department

who similarly measured relative yields of H2 , HD, and D2 in con

nection with another project.

(10) F.W. McLafferty, "Interpretation of Mass Spectra", W.A. Benjamin,

Inc., New York, 1967> P» 37.

(11) Nuclear-Chicago Corporation instruction manuals and literature

references cited therein.

(12) This is not strictly true if the sample exhibits extreme

electron capture ability. Some sulfur compounds are known to

have counter poisoning characteristics.11

(13) Dr. Wayne Harris, Head, Analytical Division, New England Nuclear

Corporation, 575 Albany Street, Boston, Massachusetts 02118.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 86 (1*0 J.D. Roberts and M.C. Caserio, "Basic Principles of Organic

Chemistry", W.A. Benjamin, Inc., New York, I965 , p. 867- 870.

(19) L. Gattermann, "Laboratory Methods of Organic Chemistry",

Macmillan and Co., Ltd., London, 1948, p. 281, 285.

(16) R.L. Shriner, R.C. Fuson, and D.Y. Curtin, "The Systematic

Identification of Organic Compounds", John Wiley and Sons,

Inc., New York, 1965j P- 285.

(17) K.B. Wiberg and R.J. Evans, Tetrahedron, 8, 313 (19^0).

(18) F. Mares and J. Rocek, Coll. Czech. Chem. Comm., 26, 2370

(1961 ).

(19) S.M. Rivkin, J. Appl. Chem. (USSR), IQ, 83 (1938); Chem.

Abstracts, 32, 4566 y (1938).

(20) H.D. Law and F.M. Perkin, J. Chem. Soc., 1637 (1908).

(21) H.H. Sisler, Inorg. Syn., 2, 205 (1946).

(22) W.A. Pryor and K. Smith, J. Amer. Chem. Soc., 92, 5403 (1970).

(23) K. Smith, Ph.D. Dissertation, Louisiana State University, 1969>

p. 4 .

(24) E.C. Murray and R.N. Keller, J. Org. Chem., 34^, 2234 (1969 ).

(25) Obtained from Professor J.G. Traynham and Dr. E.E. Green of this

Department.

(26) Ref. 16, p. 235.

(27) a) G.S. Misra and V.R.B. Mathur, Makromol. Chem., 1 0 0 , 5 (19^7)-

b) G.S. Misra and V.R.B. Mathur, Makromol. Chem., 10^, 1 64

(1967).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (28) J.W.T. Spinks and R.J. Woods, "An Introduction to Radiation

Chemistry, John Wiley and Sons, Inc., New York, 19$+, P* 108

(29) W.A. Pryor and U. Tonellato, J. Phys. Chem., 7j5> 850 (1969)*

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. RESULTS

ISOTOPE EFFECT DATA

This section summarizes the data collected for each free radical

in both table and graphic form. In all cases, except studies of DPPH,

the polystyryl, the trifluoromethyl, the methyl radicals, and the

hydrogen atom, the isotope effect was measured by competition between

hydrogen atom and tritium atom abstraction to give k^/k^. The data

for the polystyryl and trifluoromethyl radicals are taken from the

literature and are treated separately. The data for the methyl

radical and hydrogen atom involve competition between hydrogen atom

and deuterium atom abstraction, and these two cases are also treated

separately. The data for DPPH results from measurements of the rate

of disappearance of DPPH in benzene solutions of protiated and

deuterated t^-butyl mercaptan.

The deuterium isotope effect may be calculated from the corres

ponding tritium isotope effect by means of the Swain1 equation:

(1) rt *d

Each table contains temperature conditions for each run, the specific

activity of the mercaptan used, the resultant specific activity of the

product arising from hydrogen (tritium) atom abstraction (k^/k^), the

corresponding deuterium isotope effect (k^/k^) calculated using eq.

(l), log (kjj/kp), and the error limits which will be discussed in the

next section. Each of the Figures shows the temperature dependence

of the calculated deuterium isotope effect.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 89 In the cases of abstraction by the polystyryl radical and tri-

fluoromethyl radical, the data cited are taken from the literature.

Wall and Brown2 reported the isotope effect for abstraction by the

polystyryl radical. Their study was limited to only two temperatures.

The isotope effect for this radical is reported as the ratio of the

transfer constant for protiated n-butyl mercaptan to the transfer

constant for deuterated mercaptan. Since the transfer constant is

defined3 as the ratio of specific rate constant for transfer (abstrac

tion) divided by the propagation rate constant for polymerization,

the division of the transfer constant for protiated mercaptan by that

for deuterated mercaptan gives the isotope effect, k^/k^, or the ratio

of specific rate constants for abstraction. This is so because the

propagation rate constant for polymerization should be the same in

protiated and deuterated mercaptan.4

Arthur and Gray5 reported the isotope effect for abstraction by

the trifluoromethyl radical. The data shown in Table VII and Figure

VII are calculated using their Arrhenius activation parameters for

reaction of the trifluoromethyl radical with H2 S and D2 S.

The isotope effect for abstraction by the methyl radical was

determined by decomposition of t^-butyl peracetate in a mixture of

protiated and deuterated t^-butyl mercaptan. The extent deuteration of

the mercaptan was measured as described in the ANALYTICAL section of

Chapter III. The corrections applied to the mass spectrometric

analysis for methane and methane-di was also described in that section.

Table V and Figure V give the data thus obtained for the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 90

methyl radical. A kinetic analysis of this system will be presented

in the DISCUSSION chapter.

The isotope effect for abstraction by the hydrogen atom was deter

mined by photolysis of _t-butyl peroxyformate in mixtures of protium

and deuterated J:-butyl mercaptan. A kinetic analysis of this system

will be presented in the DISCUSSION chapter. The correction applied

to the mass spectrometric analysis for hydrogen and hydrogen-di was

described in the ANALYTICAL section of chapter III and is shown in

Figure I of that chapter. Table I and Figures I and XA-IC of this

section give the data obtained for the hydrogen atom.

Rate constants for the reaction of DPPH with _t-butyl mercaptan

were measured by following changes in DPPH absorbance over a reaction

time of several hours. Mercaptan was present in sufficient excess in

each run so that its concentration may be considered constant, and the

kinetic treatment may follow a pseudo-first order approach. The rate

constants are calculated from eq. (1A):

where B0 = initial mercaptan concentration in moles/liter (B=B0)

Aq = initial DPPH concentration in moles/liter (A0 « B0)

and B and A are the concentrations of mercaptan and DPPH after

reaction time t.

Tables XV-XVC and Figures XV and XYA-XVC of this section give the

data obtained for DPPH.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table I

Abstraction from jt-BuSH(D) by the Hydrogen Atom.

H0) p- H 3 o o T) VJ1 O O

o H ro VjJ VM O vji ro I NO VJ1 X H O 03

VjJ V_M CT\ H O O O H H O O O O H H O O & O • • • • • • • • v n v jj ro CO O VO vji p- ro o -P" ro H O i v o O vji ro v q ro vjj ro _\V>J ON 00 1 + M O O VO P" ON Vn O H MO VO — J NO O —J o

I IT I to c C/3 H O O O H O O O O O H O O O 32 p v n ro O h o n v n ro ro b h o i ro o H v n o CO — 3 ro v n v_n o 00 S v n o CD H H ON ON H VO H W ON ON H H ON ON I rr i W c C/3 o a ro vo 1+ o H H

o Vn Vn p- < T \ 32 ro VM O ro VJl 3 VM

00O H H ro VjJ !p- VJl VJl PI 3 r VJl CD o cT

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. .150

.145

.140

*P >.135-

00 r-Ho

.130

.125

.120 -

(1/T°K) x 103

FIGURE I. Abstraction from jt-BuSH (d ) by the hydrogen atom

log (kjj/1^) vs (1/ ^K) x 103

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2.0

i—i 3 1.0 i—i C\J i_Pi>

[t-BuSH]/[t-BuSD]

FIGURE IA. Abstraction from J:-BuSH(D) by the hydrogen atom at 10.5°C. Slope = (1.558 + 0.02^); intercept = (0.15^ + 0.015)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2.0

r—> i_i

CV) i_sat

0 2 4 6 .8 1.0 1.2

[t-BuSH]/[t-BuSD] FIGURE IB. Abstraction from t-BuSH(D) by the hydrogen atom at 40.0°C. Slope = (1.481 + 0.039); intercept = (0.128 + 0.024)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2.0

i—i i_j ,— ,

0 2 .4 1.0 1 2 [t-BuSH]/[t-BuSD] FIGURE IC. Abstraction from J:-BuSH( D) by the hydrogen atom at 70.0°C. Slope = (1.441 + 0.050); intercept = (0.104 + 0.053)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table II

Abstraction from _t-BuSH( T) by the Phenyl Radical

H X) < ro • ID oo cr\ a v 3 p* o o o X) H C O ro b o o O * rt o 01 o 7? ro 3 H I—1 l-h o 3 H OP O ro Vj J Vji o 3 oo o o -S- H Vjj O O w ro ro ro X jo H o ►n u H* II w 7T C ' l r t ro D- J H T3 tO 3 C CO k 3 » o ? o a J-* 01 1 + ov ov ro o • • — " rt o a Vjj Vji i H* • H- -P — ] X < ro to H H H* 4=- ro (—1 rt -J ro O •< " 3 i rt H X 01 O rr H H* O X I % 3 a 3 * X I I i— i 3 W H f O 3 o i HC o o O H- i—1 rt 7S W Vjj Vjj ro 3 * i__ i H* • • < 01 VJl Vji 1 H - i 3 -o ro X rt 01 VO -P " VJ H O tn O • rt i Vj J 01 P OO rt O ro ro 1 + c3 3 H H ro < o H* • • • < - P V Q O X* o ro O O — 3 O H —1 3 VjJ ro H* rt H H H VS V CT\ O V vo o ro c ? H i-* o 0Q o o o h o ro oro VJJ - p - 0 0 ro H —5 P

9 6

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. erdcd ih emsin fte oyih onr Frhrrpouto poiie ihu permission. without prohibited reproduction Further owner. copyright the of permission with Reproduced

log (kjj/kjj) .200 .205 .185 .190 .180 .195 .175 .170 kalue tkn rmTR Fse P.. israin Louisiana Dissertation, Ph.D. Fiske, T.R. from taken e u l Akva mercaptan State University, University, State IUEI. btato fo tBS() y h pey radical phenyl the by _t-BuSH(D) from Abstraction II. FIGURE 2.8 log (kjj/kp) vs (1/ (1/ (kjj/kp) vs log 98 p 100; p. 1968, (l/T°K) x 103 x (l/T°K) 97 2.9 abstraction from n-heptyl from abstraction T°K) x x 3 0 1 3.0

Table III

Abstraction from t-BuSH( T) by the 1-AdamantyL Radic

hT(D VO —] VJI 3 VO VO VO T) 0 0 0 0 -Nl O n

H o m ro ro vn ••• crv 00 o 03 vn o pT o vn vn X H O CO

'"Hrra i T3 td 3 C \ 0 3 £ p- I A ■P" p- p- M t-3 1+ • •• ro — o o o o crv crv ov ro ro ro ro X o H H rrH- 5 O I H*< H rt X Hv; H O ai ro T3a ro a. —3 3ro 3 O o rt ro ro I - 1 QJ ro d vn h — ro ro co & co ro o X ro o OH H- rr i < H P vn H*C rr 1+ ro o « —] 0 3 o VJl OV —j X* £ H 00

XlF p- vn crv —J p- crv cT

o Otj H H ro H ^ s? pT crv ro

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. .22

> 00 r-Ho

.16

2.6 2.7 2.8 2.9 3.0

(1/T°K) X 103 FIGURE III. Abstraction from t-BuSH(D) by the 1- adamantyl radical log (kjj/kjj) vs (1/T°K) x 103

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table IV

Abstraction from £-Bu SH(t ) by the Cyclohexyl Radical

H fD I—1 3 Vn o T3 O o o O O

.H o rt (TO ro ro ro o • • • K -p- ON VO 00 CO H H O CTV X H t ? o o II CHrr (X I Vjj -o bd 3 C p- C/3 H 3 PC H o •p- 4=- 4=- H-* 03 1 + • • • 0) o VJI VJl VJl — rr O VJl VJl VJl H* o o o X < ■b H‘ H l- 1 rr O •<; ON i P O

O o ro CL VJ >13 O 3 i-> o 3 or H ro o (0 o h-* x b\ ON fD 03 VN >8 O ^ PJ o o ro x 03 H O O rt i p H< O H- ro rr v; 1+ H ro ro o pcT ■b oo VN oo VN VN ro PiT ■P- oo o on ro on

00o H ro VN ON H £ o VN PC -F- H vo

100

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. .32

.30

.28

^ . 2 6 £ 'oo o I—I .2^

.22

.20

2.4 2.6 2.8 3.0

(X/T°k ) x 103 FIGURE IV. Abstraction from t:-BuSH(D) by the cyclohexyl radical log (kjj/kjj) vs (l/T°K) x 103

101

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table V

Abstraction from t:-BuSH(D) by the Methyl Radical

t-T CD H H H 3 ro h o T> o o o o o

o H 09 H ro ••ro ro O VJI cr\ cr\ ■2S ■p- CD ro H o X H O63 H o JP H OM VjJ 1 + o H O crv VJl * 00 VJl -J] o 03 3 o X I—I H IlfI O w ro c cn o OO a ••• ONON ON O H I - 1 H V/J VjJ VjJ IrtI tfl c U3

ro ro co HH ro pT 1+ 00 VO O 0 0 Vj j O o o H CD o O o 00 00ro VOji •p- 1 pT ro VJlON or— tT

102

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. .31

.30

> 'co .29 o

.28

.27

J______L______I------L 2.5 2.6 2.7 (1/T°K) X 103 FIGURE V. Abstraction from t-BuSH(D) by the methyl radical

log (kjj/kjj) vs (1/T°K) x 103

103

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table VI

Abstraction from _t-BuSH(T) by the jo-Nitropheny 1 Radic

Hro O v n 3 O v o VO T3 • • • O v o O0 VO VM o OO

H H ro ro vm O OV 0 3 o CO VM o H VM VM X H OW Jt-BuSH activity (dpm/mole) x

H H H • • • p - p - p - -p- -P- -P" —■>] —V] 0 1 1 10 (dpm/mole) x benzene activity

p - p - p " • • • v n crv v m O ro v n 0 1 1 10

ro v m vm < VO H VM v n v m ro H 1 2 . 2 1 3 . 2 1 1 . 2 bF

h-4 O 0 0 0 OQ • • • VM VM VM ro p - crv P " P - VM z F VM P - OV

10li

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. .37

.36

.35

o00 - .34

.33

2.6 2.72.8 2.9 3.0

(l/T°K) X 103 FIGURE VI. Abstraction from t-BuSH(D) by the £-nitrophenyl radical log (k^/kp) vs (l/T°K) x 103

105

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table VII

Abstraction from H2 S/D2S by the Trifluoromethyl Radical , C ° ., p m e T

h 100.0

CD o 8 6 0 . 0 Hi 03 0 . 0 I-1 o

c 2cr\ 25 . » t-* ro p- * o- VN > P" H H rr o c 3 H & ro ro vn VO 01 i r 3 00 b \ oo o a. H 00 VN o X o ro ro ►u H 0 O X 03 o T) H 1 0) I P- ro ro ro < o ro vn p- 1+ O VN oo * ►n 01 % H 03 & H* 0* o 03 o o o m K V n Vn Vjj ca o £ $ ^ a T o -P P~ vn t ?

106

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. .40

.39

.38

.37

60O

.36

.35

.34 X 2.6 2.7 2.8 2.9 3.0 (1/T°K) x 103 FIGURE VII. Abstraction from H2S/D2S by the trifluoromethyl radical log (kjj/k^) vs (1/T^) x 103 ref. N.L. Arthur and P. Gray, Trans. Faraday Soc., 65 , k 3 k (1969 )

107

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table VIII

Abstraction from J:-BuSH( T) by the 1-Nonyl Radical

H fD H 1—1 H 3 ro H o o o o Vjj o ro oo F1 H •••ro ro ro o VJI ov •p- H -0 ro H 0 0 X H O0) ^CL rti T3 W

VJl VJl VJl I W • •• I-* 0) H H H n> o -p- •p- p - — rt OO O F- X H* < H rf o Iv: F o ^ 3 Tlp. o 3 w S' m I-*o QJ VjJ VjJ ro (D ("3 Vjj O cn —' rt vji VJI ro X F-< H rt O i•< OF

VjJ Vjj ■p- ■^1 r f VO

ro ro ro • • • VJl VJI Ov ro vo v ji

t-* o o o o 0Q • • • -p- -p- -p- o h ro h ro V jj h ? V Jl 0 0 O

108

i°g ( y y .400 .405 .420 .425 .410 .415 .430 IUEVI. Abstractionfrom _t-BuSH(D)FIGURE VIII. bythe . 26 2.7 2.6 2.5 log(ky/kp) vs(1/T°K) x 10 1 -nonyl radical (3/T 109 0 K) x 103 3 Table IX

Abstraction from j:-BuSH( T) by the ^“Heptyl Radical

t-3 I- 1 ro O VO -V] 3 VO VO VO "O oo 00 VO VN -•1 o o

o r o r o r o H OQ O b \ CXI H s VN H H VN pT X

H £ O U II ' 'Irt CL I r o no CO 3 C o c/i so 3 PC v n —a o (_■ tu ro o 1 + £ £ £ — rt o 0 0 OO CD H* X c F* g > H rt CSV O v< I P X O

H p Cw CL O T33 Pro P 3 ro H H H o O •• (-* ro v j i -p - r o ro o H VN — rt v n c r \ p * X < H - OM vsrt i 0 P vn O VO 1 + VJl VJl VJl ••• pT O o VjJ CD VJl Vjj VO o Vr on VN VN VN PC H b * <3 VO r o

o O o o 00 b- VJl VJl 0 0 o VN - a VN -P" pr* crv — J H cT

110

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. .54

.53

.52

> 51

o00

.50

.49

.48

2.6 2.7 2.8 2.9 (1/T°K) X 103 FIGURE IX. Abstraction from _t-BuSH(D) by the j5-heptyl radical log (kjj/k^) vs (1/T°K) x 103

111

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table X

Abstraction from n-BuSH(D) by the Polystyryl Radical 0 . 0 7 CD 6 ro

f-h 0 . 0 3 X) vji H1 • o VfJ > o o 0Q H z: VO 01 v /i t- i ■P" < oi H P c£ H3 p. ro vjj O • • II VO o 5 - H O ro ov ro X v n H w O >i NOVjj U O S X P OH |C-4 ro Vrt P " s T 00 o o o

o 75

C/3 i C3 i - 1 H* O o I o o 0Q ig v /l CT\ cr\ -5 o v o ro £ a > H «=T

112

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. .61

.60

.58

.57 2.90 2.94 2.98 (1/T°k) X 103 FIGURE X. Abstraction from n-BuSH(D) by the polystyryl radical log (kjj/kjj) vs (1 /T°k) x 103

ref. L.A. Wall and D.W. Brown, J. Polym. Sci.,

14,/V s / 513 (195*0

115

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table XI

Abstraction f rom J:-BuSH( T) by the Triethylmethyl Radical

H 0) O - a va 3 O VO VO na O CD v a IV) 'is oo

H ro ro va H o ••• o OQ crv 00 o CD va o o va ro X H o «=r 0) ( dpra/mo t-BuSH

—j CT\ -F--F-•F- H •• • I—* 03 00 OO OO CD (0 1 + ro ro ro rr VO VO VO H- o X < H- o H rr ov o < < o\ i H O H O a i ro TJ fD 3 v

-F" •F- pT Va 3 va $ t?

OQo OV ov OV o va cr\ £ pT

H it-

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. .67

.66

.65

J 3 > .63

.62

.60

2.6 2.7 2.8 3.0 (1/T°K) x 103 FIGURE XI. Abstraction from t-BuSH(D) by the triethylmethyl radical log (ky/1^) vs (1/T°k) x 103

115

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table XII

Abstraction from Jt-BuSH( T) by the Benzyl Radical

H(D H 3 H MO TO MO MO MO 03 P" Mn o no

H CTO ro ro ro O /— S v_n ov 03 P~ oo MM ec p - p - ro X H O U ii ^ 1 r r CL 1 ro T ) Cd 3 C CO -p- 3 EC MO O o\ CTN (-* 03 1 + •• ro o Mm MM M m rr o HH H H- H ro ro ro X < H CO °.S H X O ( dpm/mo H toluene %

H O 00 OO 3-1 • ro or —J 00 — o —^ o rr 00 o X H- < H H* O IMSrr O (0 Mn ro 03 VO 1+ —oro H ooro o ro H o MM ro MM p - CT\ MO

o o O o (N Mn ov CT\ M m —J O H Mn ov O c?

116

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. .68

.67

.66

.65

^ .64 J* 60 O .63 »-l

.62

.60

.59

2.5 2.6 2.7 2.8 2.9 (l/T°K) x 103 FIGURE XII. Abstraction from t-BuSH(D) by the benzyl radical log (1^/kp) vs (l/^K) x 103

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table XIII

Abstraction from _t-BuSH(T) by the Diphenylmethyl Radical

kroT 3 VJ1 VJl VrJ VM T) v o O NO o •••• v o o CO o v_n ro o vn O n

H V>J V^ V_N V_M o o o H ro o VO ro vn x H O 03 (dpm/mo t-BuSH

VOVOVO VO ni • • ro o ON ONONON — ' rr H HMH H* HHH H X H-< H rr O i H O

T3CL >0 H- 3 ET — ro 3 e> ON ON I—* ••• ro ON ro CD VJ1 ^ ^ ro V>1 v o ■F" rr ■p- ro H v o X Er H Dto o i ro (0 n0j rr H- < H* rr vj i- 1 ro VjJ v* -p- EC VJ3 ro vo ro VO On 03

VJ1 ON ON ON • EC^ b O ro ■f - H ro -p-

o o o o o 00 b b b CD ON vo o I- 1 V>l * r ro V 0 0 'rS

118

log ( y ^ ) .80 .79 .78 .76 .77 FIGURE XIII. Abstraction from from Abstraction XIII. FIGURE 3.0 o v (1/I°K) vs x 10 log dipheny lmethy dipheny 1radical 3.1 (l/T°K)x 10 3.2 £-B 3 u SH( d 3 ) y the by 3.3

T a b l e X IV

Abstraction from Ji-Bu SH(t ) by the Trityl Radical

r-3 (D 3 —a c r\ v a T) VO VO VO v o O0 v o ro H v a o o

H H ro ro v a o • • • a . OO v o O V » H O ro ok ro X o I - 1 09 o u

pT ' - ' I r t p . 1 T> W t? 3 C C/5 3 a 0 0 0 0 oo o II • • • I—. OJ o o o (D O ro Va Va Va — rr H H H H* fo X < va H* -p- H rr O •

k ' ' r r —I D - i-t XI H- 3 T ) X c r 3 ro H - J OV CT\ O P O • • • ro ro - J v a (D I—1 ro -p * o v 3 H v a o (D X r r c r H H 03 O O P IfD CD 03 O r r H* < r r

1 + H H H O h h ro < H VO OK X* oro ro h v a H ro v_n v a v a • • • p T V a V a oo ro —j h I?

o(-■ o o o <—*09 S H V X" ro -p- ov a V a ON Va v a H 00 t £

120

iog ( y k p ) .72 .73 .74 .75 .76 .77 IUEXV Asrcinfo :BS()b the by J:-BuSH(D) from Abstraction XIV. FIGURE 2.8 o (W s (1/T°K)vs x 10 W log ( trityl radical trityl (1/T°K)x 10 . 3.0 2.9 121 3 3

3.1 Table XV

Abstraction from t-BuSH and t-BuSD by DPFH

|D3 h o (D o Vn VN H 3 i-t O v n o X3 n • • • • n> O v_n vn ss o rt o 0) o P* rtO o H TO 0 H VN VN VN • • • |rt o ro v n Q 1 v o -p" ro tfl -F - o v n CTO X o H O U ro -q H vo H ON H x • • • •P" O H s' (-■ —q v n -F" o Vn v n -F" oro cn H H H n> O O O o i i i 1 , U iF >F

■F- H ro • • • lo ro o -F" -F" ON VQ ON s' o n ro ^ o n o XXX H H H cn | oj O O O fl> i i i o if iF in

vn -P" P"

& $ ■ $

I-* o o o o TO__ • • • vn ON ON ON O & PT VN ON H f v o ro

122

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. . 6 8

.66

.64

> .62 60 HO

.60

.58

.56

3.0 3.2 3.4 3.6 (l/T°K) x lo3 FIGURE XV. Abstraction from t-BuSH(D) by DPPH log vs (1/T0K) x 103

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table XVA

The Disappearance of DPPH in a Benzene Solution of _t-butyl Mercaptan at 10.5°C

DPPH + jt-BuSH— DPPH + _t-BuSD~

A t( sec) conc. DPPH log(conc.) A t( sec) conc. DPPH log(conc.) M x 105 M x 105

0.665 0 5.835 -4.2341 0.922 0 8.088 -4.0922 0.590 1800 5.175 -4.2861 0.875 720 7.675 -4.1150 0.492 3600 4.316 -4.3649 0.831 1440 7.289 -4.1374 0.405 5400 5-553 -4.4494 0.793 2160 6.956 -4.1576 0.335 7200 2.959 -4.5318 0.752 2880 6.596 -4.1807 0.274 9000 2.404 -4.6191 0.715 3600 6.272 -4.2026 0.230 10800 2.018 -4.6951 O.676 4320 5.950 -4.2269 0.195 12600 1.711 -4.7668 0.639 5040 5.605 -4.2514 0.601 5760 5.272 -4.2780 0.567 6480 4.97^ -4.3032 0.536 7200 4.702 -4.3277

a conc. of £-butyl mercaptan is 0,8811 moles/liter. b conc. of deuterated t-butyl mercaptan is 5*287 moles/liter. jt-BuSD contained 4.6^ £-BuSH as impurity; determined by nmr. £ log CDppH = -4.3778 x 10 5 (time, sec) - 4.2176

d log °DPPH = _5*2^39 x 10 5 (time, sec) - 4.0891 Reproduced Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. -4.0

-4.2

g PM OQ 60O i-H -4 .4

-4.5

-4 .7

2000 6 000 1 0 0 0 0 time, sec. FIGURE XVA. The Disappearance of DPPH in a Benzene Solution of J:-butyl Mercaptan at 10.5°C. A: J:-BuSD B: t-BuSH

125

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table XVB

The Disappearance of DPPH in a Benzene Solution of _t-butyl Mercaptan at 35*5°C

b, d DPPH + £-BuSH— DPPH + _t-BuSD— —

A t( sec) conc. DPPH log(conc.) A t( sec) conc. DPPH log(conc.) M x 105 M x 105

0.665 0 5-335 -4.2731 0.855 0 7.500 -4.1249 0.495 3600 4.342 -4.3623 0.763 7560 6.693 -4.1744 0.335 7200 2.939 -4.5318 0.647 15120 5.675 -4.2460 0.215 10800 1 .886 -4.7244 0.530 22680 4.649 -4.3527 0.140 14400 1.228 -4.9108 0.418 30240 5.367 -4.4357 0.095 18000 0.833 -5.079*+ 0.338 37800 2.956 -4.5295 0.068 21600 0.596 -5.2248 0.265 ^5360 2.325 -^.6 3 5 6 a conc. of t-butyl mercaptan is 0.1762 moles/liter. b _t-BuSD contained 4.6$ _t-BuSH as impurity; determined by nmr. £ log 0 = -4.6306 x 10 5 (time, sec) - 4.2293 d loge C pppjj , = -1.1460 x 10 5 (time, \ sec)• - 4.0939 Reproduced Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. without prohibited reproduction Further owner. copyright the of permission with Reproduced U DPPH 4.0 -4 -4.2 -4.4 -5.0 -4.6 -5.2 -4.8 _t-BuSD :A : t-BuSH B: IUEXB TheDisappearance ofinDPPHa BenzeneFIGUREXVB. Solutionoft^-butyl Mercaptan at 35.5°C. 0 0 0 0 1 127 time, sec. 30000 50000 icr |03 1 o o o o a In- o ro i o Vm VjJ p" p- VJl ON -Q n- w a l\) H IM vo. ON ro O “'I

CD c o vji Vjj P" P" O ON O m The Disappearance of DPPH in a Benzene Solution of £-butyl MercaptaiP at 50.0°C h cn • 3 O H- rt 3 n i CD o a* a 0 d v>j ro ro h n rt Vjj Oo p - vn P"!p- vn P" cn o' 03 »

«■< \ vn in w in vo O H o Table XVC 3-* o ro p ro o n in ooav H* r t CD n o o o o o o o o o o • ••••• | a ON ON S H H —5 oo go eg go p- —q o in on C? o ro o o •p- ro vn o o P" ON VJl p- vo OQ OQ

a a •x) ►d O’ -p- v>j v>j ro ro H h v>j go Voj os P vn P" cn ro -p on oo o ro p s CD o o o o o o o o o o o hio I i ro H fe CT\ In - I i VJl VJl a\ OV 0\ ON H -v) --3 —3 X O fcd • •••••• C H H QN 03 P P ON 0 0 Q rro o P ON cn O O P* vn o o n —^ —n vjjQ.ro i i vg ^ p — —3 o P VjJ ui P a |a

~~cn cn CD n> o o o i i I i ■ i i i i I OQ p p •P" 4=r P p- P- P P 4^ • • • ■ i ro ro H H H H H p ro VO ON VJl P " V jl p- p 0 0 8 i n o n r o o o s • o H v n o o n On i n & oo H VJl ON~~

~~128~~

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. without prohibited reproduction Further owner. copyright the of permission with Reproduced o DPPH -4.3 -4.4 -4.2 -4.6 -4. -4.5 : J:-BuSDA: : t-BuSH B: Solution of J:-butyl Mercaptanat 50*0°C. IUE V. TheDisappearance ofin DPPH a XVC.BenzeneFIGURE 1000 30 00 30 1000 time, sec. 129 130 ANALYSIS OF THE DATA

~~Plots of log (k^/k^) vs. (1/T0K) are linear, and the best~~

~~straight line was obtained by the method of least squares. 6 8~~

~~The values of the slope and intercept for the best straight line~~

~~were calculated in each case, and the error limits were determined~~

~~by calculating the standard deviation of the slope and intercept.~~

~~The least squares equation and error limits for the plot of~~

~~vs (l/T0K) for each radical is given at the bottom~~

~~of Table I through Table XV of this section.~~

~~All of the data were calculated by an IBM 70^-0 or IBM 360~~

~~Computer using a program developed by Mr. Henry R. Streiffer of this~~

~~Department. Options were available in the program to allow for input~~

~~data involving logarithmic, reciprocal, or square root functions.~~

~~Table XVI indicates the proper use of this program. Output data~~

~~included values for the least squares slope and intercept and standard~~

~~deviations for the slope, intercept, and value of log (k^/kp).~~

~~Isotope effects for each radical studied were calculated at the~~

~~common temperature of 60°C, and the results are shown in Table XVII.~~

~~These data were calculated using the least squares equation of the~~

~~form log (k^/k^) vs. (l/T°K) and the error limits cited are obtained~~

~~from the standard deviation derived in the computer treatment.~~

~~A discussion of the significance of these results will be~~

~~presented in Chapter V.~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table XVI~~

~~Data Cards for Use of Least Squares Computer Program~~

~~Card # Description~~

~~1 - 2 80 column header cards~~

~~3 option card for input data~~

~~y option in column 1 ; x option in column 2~~

~~options: 0 (blank): z = z~~

~~1 : z = Q/n ( z)~~

~~2 : z = log10 (z)~~

~~3 : z = 1 /z~~

~~k : z = (z)^ if - (^t+n) input data~~

~~(n = number of sample) y values in columns 1-10~~

~~x values in columns 11-20~~

~~5+n "1" in column 21~~

~~151~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table XVII~~

~~Kinetic Isotope Effects on Hydrogen Atom Abstraction~~

~~from t-butyl Mercaptan at 60°C~~

~~Radical V 1^~~

~~hydrogen atom 1.35 ± .003 phenyl 1.61 + .009 1-adamantyl 1.65 + .010 cyclohexyl 2.25 + .082 £-nitrophenyl 2 .29 + .001~~

~~methyl 2.31 ± .003 trifluoromethyl 2.5a 1-nonyl 2.99 + .003~~

~~3-heptyl 3.71 + .009 polystyryl 4.0- triethylmethyl 4. 6b + . Oil benzyl 5.17 + .020 diphenylmethyl 5.79 + -018 trityl 5.81 + .011~~

~~DPPH 3.47 ± .010~~

~~-ref. 5~~

~~~ref. 2~~

~~132~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 133~~

~~ARRHENIUS ACTIVATION PARAMETERS~~

~~The Arrhenius law was originally an empirical and macroscopic~~

~~law relating reaction rate constant to temperature. Considering~~

~~the following reactions, k R- + QH — > RH + Q- (2)~~

~~R- + QD — > RD + Q- , (3)~~

~~the corresponding Arrhenius equations may be written:~~

~~-E„/RT kH= AHe <“>~~

~~-E /RT (5)~~

~~Dividing (if) by (5):~~

~~kH AH 6XP (“EH/RT) k^ = A^ exp ( -Ejj/RT) (6)~~

~~which may be rearranged to give:~~

~~k A = -M eXp -[(e r -Ed )/RT]. (7)~~

~~Therefore,~~

~~log (kjj/k^) = log ( V V ' t(EH -ED )/RT] log e (8)~~

~~Thus, a plot of log ( ^/^j) vs« 1/T°K should give a linear relation~~

~~ship as seen in Figures I-XV .~~

~~It was shown in Chapter II that the major portion of the~~

~~kinetic isotope effect arises from contributions to the activation~~

~~energy from changes in zero-point energy which occur when the~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 13 h reactants are converted to an activated complex. The stretching~~

~~vibration of QH (eq. 2) is quantized with frequency ct and an asso-~~

~~ciated zero-point vibration energy gh-y. Since ve for protium t- H o “H butyl mercaptan is 2587 cm 1,9 the corresponding zero-point energy~~

~~is equal to 3*55 kcal/mole. The stretching frequency \7Cb ~ D for deuterated _t-butyl mercaptan is 1875 cm 1,9 and the zero-point~~

~~energy is 2.57 kcal/mole. The difference in zero-point energy given~~

~~by the following:~~

~~h hv - g hV = -(E -E ) = 0 .9 8 kcal/mole H D H D~~

~~and is shown by the potential energy diagram in Figure XV. On the basis~~

~~of the approximations in deriving expressions for the kinetic isotope~~

~~effect as discussed in Chapter II, the ratio of pre-exponential terms,~~

~~A^/Ap, eq. (j) should be very nearly unity. Westheimer10 has shown~~

~~that, to a first approximation, the isotope effect may be given by~~

~~zero-point energy considerations, and that the pre-exponential ratio~~

~~A^/Ap is near unity.~~

~~By measuring the kinetic isotope effect over a range of tempera~~

~~tures, the experimental value of -(E -E ) may be calculated. From H D eq. (8), it follows that a plot of log vs (l/T°k) will have~~

~~slope equal to -(E„-E_) ( log e)/R. Thus, the values for -(E -E ), the HD HD zero-point energy difference, may be calculated from the slopes of~~

~~Figures I-X V . The limit of error in -(E -E^) is determined by the H D standard deviation of the slope calculated using the computer~~

~~treatment. The data are shown in Table XVIII.~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. C-H~~

~~ZPE levels~~

~~•H •H •H o •H C-H~~

~~ZPE levels C-D~~

~~initial state~~

~~Reaction Coordinate~~

~~FIGURE XVI. Potential energy vs. Reaction Coor dinate. Effect of deuterium substi tution on zero-point energy levels.~~

~~135~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table XVIII~~

~~Zero-point Energy Differences and Pre-exponential~~

~~Ratios for Hydrogen Atom Abstraction from~~

~~_t-butyl Mercaptan at 60°C~~

~~Radical - < w a h /a d kcal/mole~~

hydrogen atom 0.154 + 0.032 1.07 ± 0.05 phenyl 0.897 ± 0.113 0.42 + 0.07 1-adamantyl 0.752 + 0.077 0.53 + 0.06 cyclohexyl 1.561 + 0.328 0.21 + 0.08 £-nitrophenyl 0.458 + 0.040 1.11 ± 0.03 methyl 0.902 + 0.031 0.59 + 0.03 trifluoromethyl 0.740 + 0 .190- 0.81- 1-nonyl 0.732 + 0.027 0.99 + o.o4 3-heptyl 0.958 + 0.039 0.87 + 0.05 polystyryl 1 .170 - 0 .68 - triethylmethyl 0.806 + 0.030 1.14 + 0.01 benzyl 1.166 + 0.054 0 .8 9 + 0.07 diphenylmethyl 0.730 + 0.003 1.92 + 0.01 trityl 1.031 ± 0.035 1.12 + 0.01

~~DPPH 1.245 ± O .033 0.53 + 0.01~~

~~-ref. 5~~

~~“ ref. 2~~

~~136~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1:57 The pre-exponential ratios may now be calculated from cq. (8 ).~~

~~The results are shown in Table XVIII. These error limits are also~~

~~calculated from the standard deviation of the slope of the plot of~~

~~log (kn/kp) vs (1/T°K).~~

~~It is seen that the data in Table XVIH are in good agreement~~

~~with the expected10 magnitudes of -(E -E ) and A H D H~~

~~KINETICS OF INITIATOR DECOMPOSITION~~

~~Kinetic data for a number of perester and peroxide initiators~~

~~have been tabulated by several workers. 11 15 In addition, a number~~

~~of new free radical initiators were prepared in the course of the~~

~~present work and the kinetics of their decomposition was studied as~~

~~described in Chapter III. Table XIX shows the compiled data.~~

~~The activation parameters were calculated according to the Eyring~~

~~equation. 14~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table XIX~~

~~Collected Kinetic Data on Peresters~~

~~R in RC03j:-Bu AH* AS* Ref. tx at 60 ° kcal/mole cal/deg O~~

~~1 -adamantyl 27.9 d4 .9 16 300 “ 2 7 .6 +3-7 17, 18~~

~~cyclohexyl 31.3 +8 .6 19 7500§~~

~~1 -nonyl 3 2 .6 +7.0 a 119200~~

~~3-heptyl 33.6 +9-5 a 153^00~~

~~triethylmethyl 31.9 +12.8 a 29200~~

~~“ this work~~

~~138~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. REFERENCES - IV~~

~~1. a) C.G. Swain, E.C. Stivers, J.F. Reuwer, Jr., and L.J. Schaad,~~

~~J. Amer. Chem. Soc., 5885 (1958).~~

~~b) L. Melander, "Isotope Effects on Reaction Rates", Ronald~~

~~Press Co., New York, i960, p. 2 3 .~~

~~c) J. Bigeleisen, "Tritium in the Physical and Biological~~

~~Sciences", Vol. I, International Atomic Energy Agency, Vienna,~~

~~I962, p. l6l.~~

~~2. L.A. Wall and D.W. Brown, J. Polym. Sci., 14, 513 (1954).~~

~~3. W.A. Pryor, "Free Radicals", McGraw-Hill Book Co., 1966,~~

~~p. 205.~~

~~4. a) G.M. Burnett, F.L. Ross, and J.N. Hay, J. Polym. Sci.,~~

~~A^l, 1467 (1963).~~

~~b) C.H. Bamford and S. Brumby, Makromol. Chem., lOJj,, 122~~

~~(1967).~~

~~c) D.B. Anderson, G.M. Burnett, and A.C. Gowan, J. Polym. Sci.,~~

~~Part A . 1, 1465 (1963).~~

~~5- N.L. Arthur and P. Gray, Trans. Faraday Soc., 6 5 , 434 (1969).~~

~~6. J. Mandel, "The Statistical Analysis of Experimental Data",~~

John Wiley and Sons, New York, 1964, pp. 131-159*

~~7. H. Margenau and G.M. Murphy, "Ihe Mathematics of Physics and~~

~~Chemistry", D. Van Nostrand Co., Inc., New York, 1965» P* 506-~~

~~519.~~

~~8 . R.T. Birge, Phya. Rev., 40, 207 (1932).~~

~~139~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 140 9. See Chapter II.~~

~~10. F.H. Westheimer, Chem. Rev., 61, 265 (I96 I).~~

~~11. J.P. Lorand, Ph.D. Dissertation, Harvard University, 19$+.~~

12. K. Smith, Ph.D. Dissertation, Louisiana State University, 1969 *

~~p. 6 0 .~~

~~1 3 . P.D. Bartlett and C. Ruchardt, J. Amer. Chem. Soc., 82^ 1756~~

~~(I960 ) .~~

~~14. P.D. Bartlett and R.R. Hiatt, J. Amer. Chem. Soc., 80, I398~~

~~(1958).~~

~~15.^ P.D. Bartlett and R.E. Pincock, J. Amer. Chem. 1 Soc., 82, I769 ( I960) . 16. J.P. Lorand, S.D. Chodroff, and R.W. Wallace, J. Amer. Chem.~~

~~Soc., £0, 5266 (1968 ).~~

~~17. R.C. Fort, Jr. and R.E. Franklin, J. Amer. Chem. Soc., ^0,~~

~~5267 (1968 ).~~

~~18. R.C. Fort, Jr., R.E. Franklin, and J. Smith, 159th A.C.S.~~

~~National Meeting, February, 19T0, Houston, Texas, Division~~

~~of Petroleum Chemistry, abstract 6 9 .~~

~~19. P.D. Bartlett and R.E. Pincock, J. Amer. Chem. Soc., 84,~~

~~2445 (1962 ).~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. DISCUSSION~~

~~The isotope effects for abstaction by each radical studied,~~

~~except the hydrogen atom and the methyl radical, involved abstraction~~

~~from tritiated t-butyl mercaptan. The kinetic analysis for the hydrogen~~

~~atom and methyl radical system will be treated separately.~~

~~ABSTRACTION FROM TRITIATED t-BUTYL MERCAPTAN BY FREE RADICALS~~

~~The reactions under consideration here are given by eq. (l) - (2).~~

~~k R- + t-BuSH -£— > RH + t-BuS- ( l)~~

~~k R- + t-BuST — — > RT + t-BuS* (2)~~

~~The rate of production of labelled substrate, d(RT)/dt, is~~

~~related to the total rate of reaction of R* with t^-BuSH, the unlabelled~~

~~mercaptan, as shown in eq. (3 ) " 1~~

~~d(RT) d(RH) l > BuST] kT v ( ) dt dt [t-BuSH] Vk^~~

~~Rearranging:~~

~~a n ^ : B1 S H 1 . h . w [RH] [t-BuST] kH ' }~~

~~Thus, the isotope effect for abstraction is given by~~

~~kH_ rEHl OlSuS1] kT “ [RT] Lt-BuSHJ~~

~~Molar specific activities, A° may be substituted into eq. (5) since~~

~~[t-BuST] ASH = C U-BuSH] ^~~

~~I k l~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Ih2~~

~~and ^ * c { S (7)~~

~~where c is a constant of proportionality. Thus, the isotope effect~~

~~for abstraction may be given by eq. (8 ).~~

~~kH ASH jr = — (8 ) T ^~~

~~The experiments were carried out at low concentrations of~~

~~initiator so that the mercaptan molar specific activity, A° , remained oil essentially constant. The isotope effects reported in Chapter IV for~~

~~all radicals except the hydrogen atom and methyl radical were calculated~~

~~using eq. (8 ). For an isotope effect of unity, the molar specific~~

~~activity of the substrate RH formed will be equal to the mercaptan~~

~~activity. For all isotope effects greater than unity, the activity of~~

~~RH will be less than the activity of the mercaptan from which abstrac~~

~~tion occurs.~~

~~ABSTRACTION FROM DEUTERATED t-BUTYL MERCAPTAN BY THE METHYL RADICAL~~

~~The reactions of interest here are given by eq. (9) - (10) .~~

~~kH CH3* + t-BuSH — — > CH4 + t-BuS* (9) ]r CH3 * + _t-BuSD — — > CH3D + t-BuS* (10)~~

~~The rates of production of methane and methane —di may be calculated.~~

~~= kH [CH3-][ t-BuSH] (11)~~

~~d(,CgaP.).. = i^CcHa'lCt-BuSDl (12)~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Ihl~~

~~Dividing (11) by (12):~~

~~[CH.l ^ - BUSH1 ICH3D] k [t-BuSD] ' ^ D~~

~~which may be rearranged to give:~~

~~kjj [CH4][ t-BuSD] = LCH3Dj[ t-BuSH] ^~~

~~The isotope effect for abstraction by the methyl radical was calculated~~

~~using eq. (lk). The results are given in Table V and Figure V of~~

~~Chapter IV.~~

~~ABSTRACTION FROM DEUTERATED Jt-BUTYL MERCAPTAN BY THE HYDROGEN ATOM~~

~~Solutions of £-butyl peroxyformate in partially deuterated _t-~~

~~butyl mercaptan were photolyzed under conditions where the thiol did~~

~~not photolyze but the peroxyformate was a convenient source of~~

~~hydrogen atoms. The reactions were performed in pyrex at 36 OO k.~~

~~The reaction scheme is shown below. k. HC020-_t-Bu — > H* + C02 + -O-t-Bu (15)~~

~~k H H* + t-BuSH -2-> H2 + t-BuS- (16)~~

~~k) H- + _t-BuSD — > HD + t-BuS* (17)~~

~~H* + t-BuSH Ife + *R-SH (18)~~

~~H- + t-BuSD ^- > H2 + -R-SD (19)~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. m The rate of formation of Hs is given by:~~

~~= kH [H-]C t-BuSH] + k4 [H-][ t-BuSH]~~

~~+ k5[H-][t-BuSD] (20)~~

~~The rate of formation of HD is given by:~~

~~= kjjCHOC t-BuSD] (21)~~

~~Dividing (20) by (21):~~

~~r i k^[t-BuSH] + k4[t-BuSH] + k5[t-BuSD] [HD] " kpCt-BuSD]~~

~~which may be rearranged to give:~~

~~kR + k4 t-BuSH XSal = (23) t-BuSD [HD] *D D~~

~~Plotting [Hs]/[HD] v s . [jt-BuSH]/[_t-BuSD] for the runs at different~~

~~temperatures gave straight lines as shown in Figures IA-IC in Chapter~~

~~IV. The intercept obtained should be equal to [k5/k^] and the slope~~

~~equal to [k^ + k4 ]/k^. The results of the least squares treatment of~~

~~the data at three temperature are shown in Table I.~~

~~If one makes the reasonable assumption that k5 = k4> eq. (23) may~~

~~be rewritten as follows:~~

~~t-BuSH "kH + ks (2il) LHD. t-BuSD > *d~~

~~Since [ks/k^] is determined as the intercept in each plot of [Hs]/~~

~~[HD] v s [j:-BuSH]/[.t-BuSD], the isotope effect for abstraction,~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 145 kH^kD ’ ma^ ke ca-*-cu^a tec* ^y substracting the measured intercept from the slope. Thus,~~

~~—kH = — kH------+ kg ks (25) c ;~~

~~The results are shown in Table I of Chapter IV.~~

~~ISOTOPE EFFECT FOR PHOTOLYSIS OF DEUTERATED t-BUTYL MERCAPTAN~~

~~The photolysis of J:-butyl mercaptan was shown not to occur 0 when the reaction was performed in Pyrex at JoOO A. Considerable~~

~~quantities of hydrogen were produced, however, when neat mercaptan O was photolyzed in quartz at 3000 A and 35 C.~~

~~This system is somewhat more complex than that described pre~~

~~viously since both hydrogen and deuterium atoms are present at steady-~~

~~state concentrations. The reaction scheme is shown below.~~

~~k i H RSH RS- -F H* (26)~~

~~K H- + RSH -iL-> H2 + RS- (27)~~

~~k H- + RSD — > HD + RS- (28)~~

~~H. + RSH ^ - >Ha + -R-SH (29)~~

~~H- + RSD ^ - >Hs + -R-SD (30)~~

~~k RSD - ± = - > RS- + D- (31)~~

~~D- + RSH HD + RS* (32)~~

~~D- + RSD D2 + RS- (35)~~

~~D- + RSH HD + -R-SH (34)~~

~~D- + RSD ^ La-> HD + -R-SD (35)~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table I~~

~~Photolysis of Jt-butyl Peroxyformate. Evaluation~~

~~of Isotope Effect for Abstraction by the Hydrogen Atom.~~

~~kjj + k* t-BuSH D k l = 4* (23) [HD] Jt-BuSD _ *d~~

~~k + k4 H. kH Temperature, °C ks. L ^ S ( s lope] (intercept)~~

~~10.5 1.557 0.154 1.403~~

~~40.0 1.493 0.126 1.367~~

~~7 0 .0 1.440 0.105 1.335~~

~~146~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. From the previous discussion on the photolysis of J;-butyl~~

~~peroxyformate, it was shown (Table i) that~~

~~^ = ^ 0 .13 (36 ) *D *0~~

~~The value for at 35°C is 1.372, as calculated from the least-~~

~~squares equation relating log (k^/k^ vs (l/T°K) for abstraction by~~

~~the hydrogen atom. This equation is given in Table I of Chapter~~

~~IV. Thus,~~

~~= “ 1 ^ 7 “ °'°9 (5T)~~

~~If the assumption is made that the amount of H2 produced by k4 and k~~

~~is negligible relative to the amount formed by k , and also that the~~

~~amount of HD produced by kg and kio is negligible relative to the~~

~~amount formed by k7 and k^, i.e.,~~

~~k4 = k5 = k9 = kio = 0 (38)~~

~~the following expressions may be derived. If the rate of formation~~

~~hydrogen atoms equals the rate of their disappearance, the steady-~~

~~state concentration of H* applies, and~~

~~k.H[RSH] = kgCH-][RSH] + kjjCH-ltRSD] (39)~~

~~[H-] = {k.^RSrn/tk^RSH] + kD[RSH]} (40)~~

~~1/[H*] = {^[RSH] + kjjCRSDlJ/Ck^CRSH]} (41)~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 148 Similarly, if the rate of formation of deuterium atoms equals the~~

~~rate of their disappearance,~~

~~k [RSD] = ky[D-][RSH] + ka[D-][RSD] (),2)~~

~~CD-3 = [k.D[RSD]}/{k7 [RSH] + ks[RSD]} (4})~~

~~Multiplying (4l) by (43):~~

~~[p.] k.D[RSD] kH[RSH]kHlRSH] +H- k^RSD]I^LRSD] [ H - j " k 7[RSH] + kB[RSP] k. [RSH] '~~

~~The rate of formation of H2 and HD may be given as follows:~~

~~= ^[H-][R^H] (45)~~

~~= kjj[H*][RSD] + k7 [D-][RSH] (46)~~

~~Dividing (46) by (45):~~

~~m i V R S D ] Ha] k^RSH] + k ^ (kr) H H * ]~~

~~The reactivities of the hydrogen atom and deuterium atom are expected~~

~~to be nearly identical2 so that k^ = k7 and k^ = k8. Eq. (44) may be~~

~~simplified as follows:~~

~~[DO _ kiP[RSDj (kM [ H O " k.H[RSH] ( ^~~

~~Substituting the expression for [D*3/[H*] given in eq. (48) into eq.~~

~~(47), it follows that~~

~~i s o . V ™ ? , kiD[BSD] (k9) [Ha] k^RSHj k 1H [ R S H ] 1 9)~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. lk9 Rearranging, k [RSD] k [RSD] [HD [h2] kH[RSH] = kiR[RSH]~~

~~k [RSH] r k [RSD] k.D[RSDj " 1 /tf c ] ' kH[RSH]^~~

~~The value for k /k may be calculated from eq. (51) by measuring 1H lD the relative amounts of H2 and HD at known ratios of [t-BuSH]/-~~

~~[_t-BuSD]. The results are shown in Table II. "h d Inspection of eq. (51) indicates that a plot of l/{r„ ■ L k [RSD] [RSH] k [RSH]^ VS‘ [RSD] should a straight line with slope kiR/kiD and H~~

~~zero intercept. The results are shown in Figure I and Table II. The~~

least squares slope of the line obtained is 1 .0 3 *

~~The isotope effect for photolysis, k£jj/kj_j)> should be dependent~~

~~on the wavelength of the light used. Several attempts were made to~~

~~o measure k. /k using 2557 A lamps, but reproducible results were not iH iD obtained. Since significant amounts of H2S, HDS, and D2S may be~~

~~formed3 using the higher energy lamps, the proposed kinetic scheme~~

o would presumably be invalidated in the photolysis at 2537 A*

~~ABSTRACTION ISOTOPE EFFECT DATA~~

~~On the basis of the usually observed pattern of reactivity or~~

~~stability versus selectivity, 4 it would be expected that the most reac~~

~~tive (least stable) radical is the least selective, i.e., exhibit the~~

~~smallest isotope effect for abstraction.~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table II~~

~~Isotope Effect for Photolysis— of J:-butyl Mercaptan~~

~~in Quartz at j35°C~~

In lo- |Q1 1— 1 P Irr 0 T) H H VP 1 1 03 PC cr • 03 03 H-* **■*«%. 0 O VJ1 0 c C O rr O O H COCO c O OV CSV OV 3 3 0 h-* I-* 1__1 1_1 03 II >< rf CO (D H H* a* co VP c — 3 rr CO ro H- H* 3 t—1 ro a 01 CD • P p 0Q rt —i o H i i s: VP VO W 03 CD VP 01 VP VO c C vn CO co CO O 33 a n ro VJ1 sr O 0 c l-t CO H ro VJl 1— 1 1— 1 H* • •• 33 3 —4 VJ1 H 10 0 O v n H 1__11__1 01 -3 VO 4^ t-1 I—* 0 01 CO CD CO

~~ro © VPp- -P- VP VJ1 H i—i 33 s CO 3 3 a~~

~~HHH • « • H- 0 OO a VP VPVP |o~~

~~150~~

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I______L I l______I______L ______1______I------1------1------1------0.0 0.2 0.4 0.6 0.8 1.0 [t-BuSH]/[t-BuSD] FIGURE I Isotope Effect for Photolysis of J:-butyl mercaptan in quartz at 35°C. Slope = 1 .0 3 ; intercept = 0.0.

~~151~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 152 The relative stability of free radicals parallels that of~~

~~carbonium ions. The more stable the radical, the easier it is to~~

~~form, and the less energy is required to dissociate the alkane pre~~

~~cursor. Bond dissociation energies for the reaction~~

~~R-H D(£rH)..> R* + H- ( 52)~~

~~indicate that more energy is required to form a primary radical than~~

~~a secondary radical, which, in turn, requires more energy than the~~

~~formation of a tertiary radical. Kerr4 has compiled bond dissocia~~

~~tion energy data for a number of compounds. The data in Table III~~

~~indicate that the stability of free radicals follows the pattern 3° >~~

~~2° > 1° > CH3 .~~

~~The similar pattern of carbonium ion stabilities may be deduced~~

~~by measuring the corresponding ionization potential, the amount of~~

~~energy required to remove an electron from a molecule or atom. Ioni~~

~~zation potentials for the reaction~~

~~R- — > R© + e" (53)~~

~~indicate that more energy is required to form a primary carbonium ion~~

~~than a secondary carbonium ion, which, in turn, requires more energy~~

~~than the formation of a tertiary carbonium ion. The data in Table IV~~

~~indicate that the stability of carbonium ions follows the pattern 3°~~

~~> 2° > 1° > CHd®.~~

~~Differences in stability between carbonium ions are much larger~~

~~than between free radicals. The t-butyl free radical, for example, is~~

~~13 kcal (1 0 ^ .0 - 91.0) more stable than the methyl free radical; the~~

~~t-butyl cation is 69 kcal (2 3 0 - 171 ) more stable than the~~

~~methyl cation.~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. T a b le I I I~~

~~Bond Dissociation Energies.4 Relative Stability~~

~~of Free Radicals~~

~~Bond D(R-H), 298°K, kcal/mole~~

~~c h 3-h 10k . 0 + 1~~

~~CH3CH2-H 9 8 .0 + 1~~

~~ch3ch2ch2-h 9 8 .0 + 2~~

~~(ch3)2c h -h 9k.5 ± 1~~

~~(ch3)3c -h 91.O + I~~

~~153~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table IV~~

~~Ionization Potentials.5 Relative Stability of~~

~~Carbonium Ions.~~

~~Radical AH, kcal/mole~~

~~c h 3- 230~~

~~CH3CH2* 202~~

~~(CH3)^CH* 182~~

~~(ch 3)3c . 171~~

~~15^~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 155 The data in Table XVEE of Chapter IV indicate the expected~~

~~trend of isotope effects in the order 3° -> 2° > 1° > CH3 . The~~

~~tertiary radical, triethylmethy1, is most stable, least reactive,~~

~~and has the highest isotope effect compared to the other alkyl~~

~~radicals studied.~~

~~The benzyl, diphenylmethyl, and trityl radicals form another~~

~~homologous series. In the case of these aryl radicals, contributing~~

~~resonance structures may be drawn in which the odd electron may be~~

~~placed on the ortho and para positions of the ring. Scheme I shows~~

~~the contributing structures for the benzyl radical.~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Lrj(' In the diphenylmethyl radical case, six contributing structures may~~

~~be drawn, and, in the trityl radical case, nine structures are possible.~~

~~In Chapter III it was shown that when these radicals were generated in~~

~~tritiated t-butyl mercaptan, no tritium was incorporated into the~~

~~rings. The data in Table XVII of Chapter IV indicate the expected~~

~~trend of isotope effect for abstraction by these radicals is in the~~

~~order trityl S: diphenylmethyl > benzyl.~~

~~The magnitude of the isotope effect can be related to the posi~~

~~tion of the transition state along the reaction coordinate. The corres~~

~~pondence of the highest isotope effect to the most symmetrical transi~~

~~tion state was pointed out by Westheimer6 and discussed in Chapter II.~~

~~Also, the fact that the most symmetrical transition state occurs in~~

~~the case of the most nearly thermoneutral reaction can be deduced from~~

~~the Hammond7 postulate (Figure II, Chapter II). Thus, the highest~~

~~isotope effect for abstraction is expected when the dissociation~~

~~energy of the bond being broken most nearly equals the dissociation~~

~~energy of the bond being formed.~~

~~In the radical attack on J:-butyl mercaptan, the dissociation~~

~~energy of the S-H bond is 88 kcal/mole.4 Table V tabulates isotope~~

~~effect and bond dissociation energy data for all of the cases which~~

~~were investigated. Figure II shows the data in graphical form. The~~

~~dissociation energies reported in most cases are taken from the review~~

~~by Kerr.4 Several values used are estimated by the author on the~~

~~basis of analogous cases considered by Cottrell.8 The value for ada-~~

~~mantane was estimated on the basis of discussions by Fort and~~

~~Schleyer.9 10 Some of the values are also calculated by Benson23.~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table V~~

~~Dissociation Energy of Bond Formed Versus Measured Isotope Effect~~

~~R* + t-BuSH(D) -- > R-H(D) + t-BuS-~~

~~R* Dissociation Energy kR/kD(6 o0c) R-H (kcal/mole)~~

~~phenyl 112.-’- 1 .6 1~~

~~£-nitrophenyl 108.- 2 .2 9~~

~~trifluoromethyl 106.4-*- 2 .5~~

~~hydrogen atom 104.2-'- 1.35~~

~~methyl 103.9s*£ 2 .3 1 1-nonyl 98. £ 2 .9 9~~

~~3-heptyl 94.6 ^ 3.71~~

~~cyclohexyl 94.4^’^ 2.25~~

~~1-adamantyl 92. - 1.65~~

~~triethylmethyl 91.0s- 4.64~~

~~benzyl 8 5.1- ’- 5.17~~

~~diphenylmethyl 84. 5-79~~

~~trityl 8 3. 5 .8 1~~

~~DPPH 7 0 . - 3.45~~

~~a calculated from thermochemical data in ref. 2 3 - b estimated by the author. c ref. 4. d ref. 9 , ref. 1 0 . e ref. 8 , p. I8 9. f ref. 24.~~

~~157~~

Dissociation Energy (RH), kcal/mole 100 0 8 0 9 o n 70 IUEI. Dissociation Energy ofBond FIGURE FormedII.vs. Isotope 1.0 a # - n CaH Effect. 2.0 indicatesestimated bond dissociation Q energy. -adamantyl* 3.0 W 158 DPPH 5 600(3 -heptyl• 4.0 5.0 6.0 159 There are some discrepancies in the literature concerning

~~values for some of the dissociation energies reported. The disso~~

~~ciation energy of an S-H bond in an alkyl mercaptan has been reported~~

~~by Porter11 as 85 + 5 kcal/mole. Kerr4 has considered the best~~

~~value to be 88 kcal/mole. The dissociation energy of the C-H bond in~~

~~the methyl group of toluene has also been the subject of some contro~~

~~versy. The value of 77*5 + 3 kcal/mole has been reported,12 but a~~

~~much higher value, 89 kcal/mole, has also been obtained. 13 The~~

~~value preferred by Kerr4 is 88 + 1 kcal/mole. Considering the ambi~~

~~guities discussed here, it appears that the reactions of benzyl,~~

~~diphenylmethyl, and trityl radicals with _t-butyl mercaptan are approxi~~

~~mately thermoneutral. The data in Table V indicate that these cases~~

~~exhibit the highest isotope effect for abstraction.~~

~~It would be expected that for radicals which form an R-H bond~~

~~with dissociation energy considerably less than that for triphenyl-~~

~~methane, the abstraction reaction would be endothermic, the transition~~

~~state unsymmetrical, and the isotope effect lowered relative to that~~

~~for the trityl radical. In an attempt to test this prediction, the~~

~~isotope effect for abstraction from tritiated t-butyl mercaptan by the~~

~~diphenylpicrylhydrazyl (DPPH) radical was studied. 14 The reaction to~~

~~give diphenylpicrylhydrazine (DPPH-H) is shown in eq. (5^-) •~~

~~N02 NO2 H(D) ‘ t .N-N-(Ph); N-N-(Ph) 2 _t-BuSH(D) (5*0 02N ' N02 02N~~

~~(d p p h ) (d p p h -h )~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 160 The activation energy for abstraction by DPPH from a number of~~

~~mercaptans has been reported as 15 kcal/mole15. For this endothermic~~

~~reaction, the difference between the S-H dissociation energy and the~~

~~DPPH-H dissociation energy, AH, is probably about 10-13 kcal/mole.~~

~~See Figure III. Since the S-H dissociation energy in t:-butyl mercap~~

~~tan is 88 kcal/mole, the bond dissociation energy for the DPPH-H~~

~~bond is expected to be less than 80 kcal/mole. On the basis of the~~

~~j k kcal/mole value for the N-H bond dissociation energy in N-methyl~~

~~aniline reported by Kerr4, an approximate value of 70 kcal/mole is~~

~~estimated for the DPPH-H bond. Thus, the abstraction reaction from~~

~~J:-butyl mercaptan would be endothermic, the (DPPH H) transition~~

~~state would be unsymmetrical and would closely resemble products~~

~~(see Figure III), and the measured isotope effect should be less~~

~~than that observed for the most symmetrical transition state. The~~

~~data in Table V and Figure II indicate that such a reduced isotope~~

~~effect is, in fact, observed. Attempts to measure the tritium~~

~~isotope effect for abstraction from tritiated j:-butyl mercaptan and~~

~~cumene were unsuccessful due to exchange of the DPPH-H upon purifica~~

~~tion of the reaction mixture.~~

~~The graphical representation of the correlation between bond~~

~~dissociation energy and isotope effect shown in Figure II shows~~

~~that only the cyclohexyl and 1 -adamantyl radicals do not closely~~

~~parellel the relation observed by the remainder of the radicals.~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. — A ~ ~ DPPH-H Moo Q) + t-BuS* d w~~

~~■Hra 4J 4J c 0) ■u o fM IT~~

~~DPPH + t-BuSH~~

~~Reaction Coordina te FIGURE III. Potential Energy Diagram for the Endothermic Abstraction of a hydrogen atom from t_-butyl mercaptan by DPPH.~~

~~161~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 162~~

~~Both the cyclohexyl and 1-adamantyl radicals have lower isotope~~

~~effects for abstraction than would be predicted on the basis of~~

~~the bond dissociation energies of the respective R-H bonds. In~~

~~the case of these two radicals, a consideration of the most~~

~~favorable geometrical conformation of the radical must be~~

~~examined.~~

~~The cyclohexyl radical has been generated in the cavity of~~

~~an electron spin resonance (esr) instrument by irradiating liquid~~

~~cyclohexane with high energy electrons. 16 The esr spectrum of~~

~~the cyclohexyl radical shows that the pair of f3-hydrogens on the~~

~~carbon next to the radical center are not equivalent. The struc~~

~~tures in Figure IV show that these axial and equatorial 0-hydrogens~~

~~would not be equivalent regardless of whether the radical center~~

~~were planar or pyramidal. Thus, the splitting constants for the~~

~~interaction of the odd electron with the axial and equatorial 13-~~

~~hydrogens do not provide conclusive evidence regarding the~~

~~geometrical conformation of the cyclohexyl radical. 17~~

~~The question of the conformation of the cyclohexyl radical was~~

~~examined by Greene and his coworkersla. They studied the decomposi~~

~~tion of the cis- and trans-4-t-butyl hypochlorites shown in eq. (55).~~

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. FIGURE IV. The Conformation of the Cyclohexyl Radical-A, planar; B, pyramidal with odd electron equatorial; C, pyramidal with odd electron axial.

~~165~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 164 The ratio of cis-chloride to trans-chloride is exactly the same for~~

~~both hypochlorites when the reaction was performed in carbon tetra~~

~~chloride at 80°C, 0°C, and -j50°C. (see Table Vi)~~

~~(55)~~

~~This exact correspondence of products at all temperatures is strong~~

~~evidence for the intermediacy of the 4-_t-butylcyclohexyl radical in~~

~~the two decompositions. If the cyclohexyl radical is represented by~~

~~the planar form (structure A in Figure IV) , the major product formed~~

~~would be trans-4-t-butylcyclohexy1 chloride. This product is thermo~~

~~dynamically more stable and would result from attack at the more~~

~~accessible equatorial position. The fact that cis-4-t-butylcyclohexy1~~

~~chloride is formed preferentially implies that the cyclohexyl radical~~

~~is better represented by the non-planar forms (structures B and C in~~

~~Figure IV).~~

~~Fort and Schleyer10 have shown that carbon free radicals prefer a~~

~~planar geometry, as do carbonium ions, but that the force constants~~

~~for distortion of the radicals are much smaller than those for distor~~

~~tion of the cations. Thus, the stability of the non-planar,~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. T a b le V I~~

~~Decomposition of cis- and trans-^-t-butylcyclohexy1 10 Hypochlorites in Carbon Tetrachloride. Formation~~

~~of cis- and trans-4-t-butylcyclohexyl Chloride (eq. 55)-~~

~~Percent Product Temperature, °C cis-chloride trans-chloride~~

~~80 67 33~~

~~0 65 35~~

~~-30 62 38~~

~~165~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 166 pyramidal cyclohexyl radical would be less than the stability that~~

~~would be predicted for a strain-free secondary radical. A comparison~~

~~of the measured isotope effects for abstraction by the cyclohexyl~~

~~radical and the 3“heptyl radical (Figure II) indicates an enhanced~~

~~reactivity for the cyclohexyl radical compared to an ordinary strain-~~

~~free secondary radical which may assume a planar conformation. This~~

~~lowered isotope effect for the cyclohexyl radical is consistent with~~

~~the notion of some degree of geometrical destabilization for the~~

~~non-planar, pyramidal radical.~~

~~The question of stability and the most favorable geometric con~~

~~formation of the bridgehead 1 -adamantyl radical has been examined by~~

~~Lorand, Chodroff, and Wallace19 and Fort and Franklin20. The rate~~

~~constants for the decomposition of a series of _t-butyl peresters of~~

~~bridgehead and ordinary tertiary carboxylic acids were determined, and~~

~~were taken to reflect the stability of the radicals formed. The~~

~~results are shown in Table VII. The corrected relative rate ratios are~~

~~obtained by accounting for the inductive contribution of the carbon~~

~~skeleton in the bridged systems.20 22 Thus, the corrected relative~~

~~rate ratios are considered to reflect the destabilizing influence due~~

~~to constrained geometry in the bridgehead free radicals. On the basis~~

~~of the data presented in Table VII, Lorand19 concludes that the 1-ada-~~

~~mantyl radical exhibits "considerable" geometrical destabilization and~~

~~that alkyl radicals "tend toward planar geometry." Fort20 22, on the~~

~~other hand, interprets the data to indicate the there is only "weak~~

~~preference for planar geometry" in alkyl radicals, and that the 1-~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 6 7 adamantyl radical is an "ordinary tertiary radical with slight~~

~~geometrical stabilization.~~

~~The isotope effect for abstraction by the 1-adamantyl radical~~

~~compared to the triethylmethyl radical indicates that the bridgehead~~

~~radical is not of "ordinary" stability or reactivity. The lowered~~

~~isotope effect for the 1 -adamantyl radical is consistent with the idea~~

~~of geometrical destabilization due to non-planarity of the radical~~

~~center.~~

~~An interesting and useful relationship between radical stability~~

~~or reactivity and the kinetic isotope effect for abstraction of hydrogen~~

~~from a donor molecule has been developed. This relationship should be~~

~~helpful in systems where free radical reactivities are difficult to~~

~~assess. An application of this relationship to radicals produced in~~

~~protein radiolysis is discussed in Part II of the APPENDIX.~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.~~

~~H o 1 i 01 (D CL 01 s? ro 3 H 3 3 tfl H* EE O o 0 to 03 Irt~~

ui I I ui 01 (0 rt o ro H (0 H po M po o pd ro ro ro ro Hi X on EE |oi< 01 01 rj rj 01 01 ro h- o o h- rr rt ro ro pd 1—1 pd*~4 1—1 o o H o o 00 O EE O vn NO 03 03 -1 l b h 00 £ • £ON • ON ON oo 01 CL 01 01 3 H 3 rt v n - 3 NO NcJ H P" H o o ON CD O o O -3 NO 03 03 l H P" ro p- P- -3 & 01 O ON 01 a. 01 H 3 3 rt NO -3 v n v n v; H o o p- p- o o 00 o EE o NO vn VO 168 03 1 H H & P" CD ON ON 01 00 H 01 3 - i-1 3 01 3 rt vo -3 - 3 v n v n v; T a b le V I I ro o o o H o o ON s vn vn vn ---- 03 0) I ro o p- ON o P* O H CL 01 ON 01 n ON H 01 3 3 rr -3 v n -3 P" vn v n •C 1 i

IV) o o H ro ro o o o rt ON H o n c r H- I—* n H* o vn —3 VJ1 vn — __ v ; 1 i I I i 1 03 03 ro o P- X H H P- H ro O ro o o ON ON o ON ro ro i-* O ro or u * rr t-* H O l-» O i - 1 •• __ —1 — vn vn i l N

~~Bridgehead and Ordinary Tertiary Carboxylic Acids Carboxylic Tertiary Ordinary and Bridgehead~~

~~20-22 ref. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. REFERENCES - V~~

~~(1) L. Melander, Arklv. Kemi., 7» 287 (1954).~~

~~(2) W.A. Pryor, J.P. Stanley, and M.G. Griffith, Science, l6£,~~

~~181 (1970 ).~~

~~(3) a) J.K.S. Wan, Chem. Commun., 429 (I967 ).~~

~~b) W.E. Haines, G.L. Cook, and J.S. Ball, J. Amer.Chem. Soc.,~~

~~78, 5213 (1956).~~

~~(4) J.A. Kerr, Chem. Rev., 66, 465 (1966).~~

~~(5) R.T. Morrison and R.N. Boyd, "Organic Chemistry", Allyn and~~

~~Bacon, Inc., Boston, 1966 , p. 1 6 5 .~~

~~(6) F.H. Westheimer, Chem. Rev., 6l, 265 (I96 I) .~~

(7 ) G.S. Hammond, J. Amer. Chem. Soc., JJJ 334 (1955)*

~~(8) T.L. Cottrell, "The Strenths of Chemical Bonds", Butterworths~~

~~Scientific Publications, London, 1954.~~

~~(9) R.C. Fort, Jr., and P. von R. Schleyer, Chem. Rev., 64, 277~~

~~(1964).~~

~~(10) R.C. Fort, Jr., and P. Von R. Schleyer, Adv. Alicyclic Chem.,~~

~~1, 283 (1966). n-* (11) G. Porter, Disc. Faraday Soc., £, 60 (1950).~~

~~(12) D.O. Schissler and D.P. Stevenson, J. Chem. Phys., 22, 151 (1954).~~

~~(13) H.R. Anderson, H.A. Scheraga, and E.R. Van Artsdalen, J. Chem.~~

~~Phys., 21, 1258 (1953).~~

~~(14) Unpublished results in collaboration with L.D. Lasswell.~~

~~(15) K.E. Russell, J. Phys. Chem., 58^ 437 (1954).~~

~~169~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 170~~

~~(16 ) R.W. Fessenden and R.H. Schuler, J. Chem. Phys., 3^, 2l4'f~~

~~(196^).~~

~~(17) S. Ogawa and R.W. Fessenden, J. Chem. Phys., 4jL, 994 (1964).~~

~~(18) F.D. Greene, C. Chu, and J. Walia, J. Amer. Chem. Soc., 84^,~~

~~2463 (1962 ).~~

~~(19) J.P. Lorand, S.D. Chodroff, and R.W. Wallace, J. Amer. Chem.~~

~~Soc., 90, 5266 (1968). ——— (20) R.C. Fort, Jr., and R.E. Franklin, J. Amer. Chem. Soc., gO,~~

~~5267 (1968 ).~~

~~(21) R.C. Fort, Jr., and R.E. Franklin, 154th A.C.S. National~~

~~Meeting, September, 1970, Chicago, Illinois, Abstract S-I65 .~~

~~(22) R.C. Fort, Jr., R.E. Franklin, and J. Smith, 159th A.C.S.~~

~~National Meeting, February, 1970, Houston, Texas, Division~~

~~of Petroleum Chemistry, Abstract 6 9 .~~

~~(23) S.W. Benson, "Thermochemical Kinetics", John Wiley and Sons,~~

~~Inc., New York, I968 .~~

~~(24) A. S. Rodgers, D. M. Golden, and S. W. Benson, J. Amer. Chem.~~

~~Soc., 8g, 4578 (1967 ).~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX~~

~~PART I - MECHANISM OF PERESTER DECOMPOSITION~~

~~Thermal decomposition of J:-butyl peresters may be envisioned to~~

~~occur by either a one-bond or multiple-bond homolytic scission path~~

~~way. In the homolysis of a perester which occurs by scission of one~~

~~bond, an acyloxy and an alkoxy radical are formed as geminate pair,~~

~~as shown in Scheme I.~~

~~RC02-0-t-Bu homo^ .ls ■> [RC02 - -O-t-Bu] dl££usi°n > RCo2. + -o-t-Bu~~

~~recombination I decarboxylation I i V RC02-0-t-Bu R. + C02 + •O-t-Bu~~

~~(Scheme i)~~

~~In the homolysis of a perester which occurs by multiple-bond scission,~~

~~an alkyl radical, an alkoxy radical, and a C02 molecule are formed, as~~

~~shown in Scheme II.~~

~~RC02-0-t-Bu h°mol^SLS > [R- C02 -O-t-Bu] dl££usi°n > R. + -O-t-Bu~~

~~recombination V R-0-_t-Bu~~

~~(Scheme II)~~

~~The mechanism of decomposition is determined by the nature of~~

~~the R group in the perester shown in the preceding schemes. Several~~

~~tests have been used in the study of perester thermolyses to determine~~

~~if one-bond scission occurs or if the mode of decomposition is the~~

~~concerted homolysis of more than one bond.~~

~~171~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 172 A. Activation Parameters as a Test for Concerted Decomposition~~

~~It was originally proposed by Bartlett and Hiatt1 that the magni-~~

~~tudes of AH and AS allowed for the classification of initiators into~~

~~categories which assign the number of bonds which may rotate freely in~~

~~the transition state. This relationship between activation parameters~~

~~and the concept of restricted rotations in the transition state has~~

~~been extended by some workers to a definition of the number of bonds~~

~~breaking in the transition state. Martin2 proposed that one-bond~~

~~homolysis is associated with a AH greater than 35 kcal/mole and a AS~~

~~greater than 13 cal/deg. On the other hand, concerted decomposition~~

~~was associated with values of AH between 30 and 35 kcal/mole and AS~~

~~between 6 and 13 cal/deg.~~

~~A number of workers have extended this original Bartlett-Hiatt~~

~~concept to the correlation between activation parameters and concerted~~

~~decomposition in order to justify either a one-bond or concerted path~~

~~way of decomposition. Lorand3 and Fort4 have taken the decomposition~~

~~of the _t-butyl perester of 1-adamantanecarboxylic acid to be concerted~~

~~solely by measuring the activation parameters AH* and AS * (see/ Table~~

~~XIX, Chapter IV). A more questionable case was proposed by Neuman5’6~~

~~in his studies of the decomposition of the jt-butyl perester of cyclo-~~

~~hexanecarboxylic acid. This perester was also concluded to decompose~~

~~via a concerted mechanism. The activation parameters for this~~

~~perester are shown in Table XIX, Chapter IV.~~

~~Pryor and Smith7 ’8 have questioned this extension of the original~~

~~Bartlett-Hiatt test to include the correlation between concerted~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 173 decomposition and activation parameters. Their conclusion is that the~~

~~use of activation parameters to determine the number of bonds breaking~~

~~in the transition state is unjustified and unreliable. Rather, it is~~

~~concluded that initiators may not be unequivocally compartmentalized~~

~~into distinct classes on the basis of measured activation parameters.~~

~~It seems more likely that there is a broad continuum of initiator type~~

~~with those that decompose always by one-bond homolysis and those that~~

~~decompose always by synchronous multiple-bond homolysis forming the~~

~~extreme limits. Such a continuous spectrum could include some ini~~

~~tiators whose mode of decomposition partitions itself between the two~~

~~mechanisms. Pryor and Smith8*3 conclude that probably the most res trie $ tive classification that can be made is that peresters with AH above~~

~~33 kcal/mole probably decompose by simple one-bond homolysis with no~~

~~noteworthy restricted rotations in the transition state, and that $ those with AH below about 27 kcal/mole probably decompose by a con~~

~~certed mechanism with several bonds restricted in the transition~~

~~state.~~

~~B. Activation Volumes for Homolytic Scission Reactions~~

~~In addition to the conclusions based on activation parameter~~

~~measurements, Neuman5 ’6 has further proposed that the cyclohexyl~~

~~perester decomposes via a concerted mechanism on the basis of his~~

~~measurements of the effect of pressure on its rate of decomposition~~

~~in the solution phase. Studies of this so-called "activation~~

~~volume" for homolysis are relevant to the question of internal~~

~~return of free radicals within the solvent cage. For a single-step~~

~~reaction, the activation volume is related to the pressure dependence~~

~~of the rate constant. Positive activation volumes signify a decrease~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Vfk in reaction rate with an increase in pressure, as shown in eq. (l).~~

~~= -av7st (1) ■Xr where Av is the activation volume. A tabulation of some activation~~

~~volumes for homolytic scission reactions are given in Table I.~~

* These data have led to the generalization that values of AV for a

~~homolytic scission reaction in which cage recombination of the~~

~~primary radical products can occur to regenerate starting material~~

~~are in the region of + 10 cc/mole.14~~

~~However, the data in Table I point out two reasons why activa~~

~~tion volume studies may be inconclusive in assigning an unequivocal~~

~~reaction mechanism for perester decomposition. First, the value for * Av is seen to be dependent on the temperature of the reaction.~~

~~Secondly, there appears to be a substantial solvent effect.~~

~~Comparison of the value for AV for t>butyl peroxide, which is~~

~~presumed to undergo one-bond cleavage at the transition state,15 17 •x* to the AV value for the cyclohexyl perester, which is argued5’6 to~~

~~undergo concerted decomposition, serves to indicate that activation~~

~~volume data alone may be insufficient evidence in assigning a~~

~~definite reaction pathway, except, perhaps, in the extreme cases of~~

~~t-butyl perbenzoate and _t-butyl diphenylperacetate.~~

~~C. The Viscosity Dependence of Bond Homolysis~~

~~Pryor and Smith7 ’8’15 have demonstrated that the viscosity depen~~

~~dence of the observed rate constant for bond homolysis can be used to~~

~~determine whether a molecule decomposes by the initial scission of~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table I~~

~~Activation Volumes for Homolytic Scission Reactions~~

* Compound Solvent Temp.,°C AV Ref. cc/mole

Benzoyl peroxide styrene 30 +10 9 CCI4 60 10 9 CCI4 70 9 10 acetophenone 80 5 11 t-butyl peroxide PhH 120 13 11 CCI4 120 13 11 cyclohexane 120 7 11 PhCH3 120 5 11 pentaphenyle thane PhCH3 70 13 12 AIBNS PhCH3 70 4 12 t-butyl perbenzoate cumene 80 10-11 6,13 PhCl 80 13 13 carbo-t:-butylperoxycyclohexane cumene 80 j.k-k-.k- 5, 6 t-butyl phenylperacetate cumene 80 1-3 6,13 PhCl 80 1.5 13

~~a azobisisobutyronitrile~~

~~175~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 176 more than one bond. This test is based on the following~~

~~postulates10 I~~

~~1) Any molecule that decomposes by the scission of only one~~

~~bond can and will give cage return in solution.~~

~~2) For such a molecule, the observed rate constant for~~

~~decomposition will decrease as the viscosity of the solvent~~

~~increases, solvation forces being kept constant.~~

~~3) Any molecule that decomposes by the synchronous scission~~

~~of more than one bond cannot give cage return, and will have a~~

~~rate constant for decomposition that is independent of solvent~~

~~viscosity.~~

~~Clearly, this method allows for the detection of a one-bond homol-~~

~~ytic mode even in the case of a perester whose pathway for decomposition~~

~~is partitioned between the two alternative modes.~~

~~The viscosity test was applied to the case of the _t-butyl perester~~

~~of cyclohexanecarboxylic acid. The data are shown in Table II and~~

~~indicate the dependence of observed rate of decomposition on solvent~~

~~viscosity. These results indicate that this perester decomposes at~~

~~least partially by the one-bond scission pathway.~~

~~The viscosity test was also applied to the case of the J:-butyl~~

~~perester of cyclooctanecarboxylic acid and its a-deuterated analogue.~~

~~These data are shown in Table III; the viscosity dependence here also~~

~~reflects at least partial one-bond scission.~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table II~~

~~Observed Rate Constant for Decomposition of~~

~~Carbo-t^-Butylperoxycyclohexane in Hydrocarbon~~

~~Solvents at 79.31°C~~

~~Solvent Viscosity^’19 k x 105 sec 1~~

~~nonane O.368T 1.373~~

~~decane 0M j 6 1.362~~

~~dodecane 0.6393 1.3^5~~

~~tetradecane 0.8798 1.316~~

~~nujol 10.5 1.271~~

~~£ centipoises, 80°C~~

~~177~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. T a b l e I I I~~

~~Observed Rate Constant for Decomposition of~~

~~_t-butyl Cyclooctaneperoxycarboxylate and Jt-butyl-a-~~

~~deuteriocyclooctaneperoxycarboxylate in Hydrocarbon~~

~~Solvents at 79«^5°C~~

~~, ^ a Solvent a-H Perester a - a Perester- kH/kD k x 105 sec 1 k x 105 sec 1~~

~~octane 5.552 5-^ 11 1.026~~

~~nonane 5.516 5.512 1.058~~

~~decane 5.kk6 5-555 1.021~~

~~dodecane 5.^07 5-5^8 1.011~~

~~tetradecane 5.568 5-500 1.015~~

~~£ corrected to 100$> a-deuteration; see Chapter III.~~

~~178~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 7 9 D. Secondary Isotope Effects in Initiator Decomposition~~

~~The studies of Seltzer20 22 on the a-isotope effects in azo~~

~~compound decomposition have indicated that these a-effects are of the~~

~~order of 1 .10-1.15 per deuterium atom for these radical-forming reac~~

~~tions. Additionally, Koenig and his coworkers have investigated secon~~

~~dary deuterium isotope effects on perester decompositions. Prelimi- 23 nary reports on the decomposition of t^-butyl phenylperacetate and~~

~~_t-butyl perhydratropate gave isotope effect of 1.17 and 1.14 respec~~

~~tively. These observed isotope effects are very similar to those~~

~~reported by Seltzer, and, therefore, it was concluded that the two~~

~~peresters decomposed via a concerted process. More recently, however,~~

~~the measurement of the isotope effects on decomposition of these~~

~~peresters has been reinvestigated24 and the new Ic^/k^ value for J:-~~

~~butyl phenylperacetate is 1.05-1.06. Koenig24 still argues that the~~

~~decomposition of t-butyl phenylperacetate is completely concerted.~~

~~However, an alternative explanation for this reduced isotope effect is~~

~~that partial one-bond scission occurs, and, in the case of J:-butyl~~

~~phenylperacetate, Pryor and Smith7 ’8 find a slight dependence of the~~

~~rate of decomposition on solvent viscosity.~~

~~The kinetic isotope effect measurements can detect two-bond~~

~~scission, but would give no evidence about simultaneous one-bond~~

~~scission. The viscosity test, on the other hand, can detect the~~

~~presence of one-bond scission, but it gives no evidence for the exis~~

~~tence of simultaneous two-bond homolysis. The data presented in~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 180 Tabic III showing both the viscosity test and isotope effect results~~

~~for t-butyl cyclooctaneperoxycarboxylate and its a-deuterated analogue~~

~~probably indicates that this perester is partitioning its mode of~~

~~decomposition between one-bond and two-bond homolysis.~~

~~E. The Use of Scavengers in Homolytic Initiator Decomposition~~

~~Free radical scavengers have been used in perester thermolyses~~

~~in an attempt to trap the intermediate acyloxy radical which would be~~

~~formed upon one-bond scission of the initiator. This would reduce~~

~~the amount of CO2 formed during the reaction. Shine, Waters, and~~

~~Hoffman25 report the scavenging of acetoxy radicals by galvinoxyl26~~

~~2 7 and DPPH by noting a decrease in CO2 evolution when acetyl peroxide~~

~~is decomposed in the presence of these scavengers. Martin, Taylor,~~

~~and Drew2 Q pointed out that this observation may reflect induced decom~~

~~position of the peroxide by, e.g., DPPH in addition to (or instead of)~~

~~trapping of the acetoxy radical. Thus, attempts to scavenge inter~~

~~mediate acetoxy radicals have not been totally unambigous. Shine and~~

~~2 9 2 3 Slagle and Martin, et. a l . , have scavenged acetoxy radicals with~~

~~cyclohexene to give cyclohexyl acetate. Martin and Drew3 0 initially~~

~~proposed that the cyclohexyl acetate was formed in a molecule-induced 20 decomposition, but later invoked a mechanism involving the rapid~~

~~formation of a pi-complex between the acetoxy radical and olefin. The~~

~~final outcome of the complex is unimportant here, but its formation~~

~~serves to indicate that scavenging of an acetoxy radical can compete~~

~~) with decarboxylation even when the latter process has a rate constant~~

~~of 1.6 x 10s sec x.~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 181 Misra and Mathur32 have reported the scavenging of benzoyloxy~~

~~radicals from j:-butyl perbenzoate. Although the CO2 yield was not~~

~~completely eliminated, this was cited as evidence for one-bond~~

~~homolysis. Incomplete scavenging could be explained by either reduced~~

~~scavenger efficiency or by competing two-bond homolysis.~~

~~Attempts have been made to scavenge the intermediate acyloxy~~

~~radical which would be present if _t-butyl cyclooctaneperoxycarboxylate~~

~~decomposes at least partially by a one-bond scission pathway. The~~

~~data obtained are shown in Table IV. Using 3-methylcyclohexene, the~~

~~yield was reduced from over $0 percent to about 60 percent. These~~

~~are the results at 79*3 °C, the same temperature as the previous~~

~~viscosity test studies. Failure to reduce the yield of CO2 further is~~

~~due to either an operable two-bond scission mechanism for part of the~~

~~perester decomposition or incomplete scavenging of the first-formed~~

~~acyloxy radical. Similar results were obtained at 99*6 °C. It was~~

~~found that neither DPFH nor 1-octene had any appreciable effect on~~

~~C02 yield at 9 9 .6 °C.~~

~~F. Conclusion~~

~~The results of all the methods used for determining the mechanism~~

~~of perester decomposition are compiled in Table V. The data for J:-~~

~~butyl perbenzoate and J:-butyl peracetate have been included for~~

~~completeness.~~

~~The data for _t-butyl perbenzoate clearly indicate that this is~~

~~a one-bond scission initiator. The cases of the cyclohexyl and~~

~~cyclooctyl peresters are more ambiguous. The data for both of these~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table IV~~

~~Effect of Scavenger on Yield of Carbon Dioxide~~

~~in the Decomposition of J:-butyl Cyclooctaneperoxycarboxylate~~

~~Solvent Temp., °C Percent C02 Yield~~

~~n-decane 99.6 72.9 76.1 7^.3 7^.6~~

~~3-me thylcyclohexene 99.6 67.7 68.5 6 9 .0~~

~~n-decane 79-3 95-^ 90.0 92.8~~

~~3-me thylcyclohexene 79-3 61.k 59.^ 6 3 .2~~

~~1-octene 99-6 75.1t- 74.8~~

~~n-decane— 99.6 j k .6 75.0~~

~~a DPPH added 182~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. peresters is of a decidedly borderline nature and serves to indicate~~

~~that the decomposition of these compounds probably is partitioning~~

~~between competing one-bond and two-bond modes. The mechanism of~~

~~decomposition of Jt-butyl phenylperacetate is also somewhat questionable.~~

~~The activation parameter and activation volume data have been taken~~

~~to indicate concerted two-bond scission. On the other hand, the~~

~~effect of solvent viscosity or the rate of decomposition of t^-butyl~~

~~phenylperacetate and the magnitude of the secondary isotope effect are~~

~~indicative of one-bond scission. These seemingly conflicting results~~

~~can be reconciled by postulating that this perester is also decom~~

~~posing by both a one- and a two-bond mechanism.~~

~~Each of the methods described for determining the mechanism of~~

~~perester decomposition is somewhat inadequate when used as a sole~~

~~criterion. They are, however, complementary, when all of the data~~

~~are viewed together. The secondary kinetic isotope effect can detect~~

~~two-bond scission, but would give no evidence about simultaneous one-~~

~~bond scission. The viscosity test and the scavenger experiments can~~

~~detect the presence of one-bond scission, even if this is not the~~

~~main mechanistic pathway, but gives no direct evidence for the exis~~

~~tence of two-bond homolysis in the presence of one-bond homolysis.~~

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table V Collected Data on the Mechanism of Perester Decomposition j:-butyl J:-butyl inphenyl— ether, ref. 33; in chlorobenzene,“ 80°C, ref. 13; ref.35; — —ref. 32; -Table •^TableIV, APPENDIX, Part I; chlorobenzene, ^an ref. ref- 7, 24. 8b, 3^; ref. — p. 7> 30; Compound perbenzoate carboxylate aneperoxy- cyclohex- XVIII,II, Chapter APPENDIX, IV; Part -ref. 5, I; 6; -Table III,%able APPENDIX, Part I; phenyl oxycarboxy- octaneper- cyclo- peracetate t-butyl late sl t-butyl • rt i • •

~~6 . 8 = * s a AS*=U.A AS*-2.0 Activation Parameters r p ^ deg deg »o?e r £ S deg~~

c 6 d c b AV Av Av AV * cc— * cc— * Activation 1 , =13 — =3.4- =1.5 . 7 4.4 ^ Volume mole mole mole Decreasing^ Decreasing^ viscosity decomposition viscosity . h . Decreasing1- _ rate with decomposition viscosity increasing decomposition increasing rate with viscosity decreasing— increasing decomposition rate with increasing rate with k "Slight" Viscosity Effectof Solvent Secondary ' V = D ^ V 1.05 - 1 Effects Isotope 1.02^ . 06 d ^-

~~decreases with YieldC02 concentration^- increasing DPPH YieldC02less cyclohexene in 3-methy1- thanin decane— Experiments Scavenger~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX - REFERENCES - PART I~~

~~(1) P.D. Bartlett and R.R. Hiatt, J. Amer. Chem. Soc., jj3(D, 1398~~

~~(1958).~~

~~(2) M.M. Martin and D.C. Dejough, J . Amer. Chem. Soc., j^, 3526 (1962).~~

~~(3) Lorand, S.D. Chodroff, and R.W. Wallace, J. Amer. Chem.~~

~~Soc.. 5266 (1968).~~

~~(4) R.C. Fort, Jr., and R.E. Franklin, J. Amer. Chem. Soc..~~

~~5267 (1 9 6 8 ).~~

~~(5) R.C. Neuman, Jr., and J.V. Behar, Tetrahedron Lett., 328I (1968 ).~~

~~(6) R.C. Neuman, Jr., and J.V. Behar, J. Amer. Chem. Soc.. $L, 6024~~

~~(1969). (7) K. Smith, Ph.D. Dissertation, Louisiana State University,~~

~~1969 , p. 58-65.~~

~~(8) a) W.A. Pryor and K. Smith, J. Amer. Chem. Soc.. 5^05 (W).~~

~~b) W.A. Pryor and K. Smith, Int. S. Chem. Kinetics, in press.~~

~~(9) A.E. Nicholson and R.G.W. Norrish, Discussions Faraday Soc.,~~

~~io4 (195 6 ).~~

(10) C. Walling and J. Pellon, J. Amer. Chem. Soc., 4786 (1957)*

(11) C. Walling and G. Metzger, J. Amer. Chem. Soc., 5365 (1959)*

~~(12) A.H. Ewald, Discussions Faraday Soc., 138 (1956 ).~~

~~(13) R.C. Neuman, Jr., and J.V. Behar, J. Amer. Chem. Soc..~~

~~W 9 (1967).~~

~~185~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 186 (14) See ref. 4, footnote 13 •~~

~~(13) W.A. Pryor and K. Smith, J. Amer. Chem. Soc., 8^), 1741 (1967 ) •~~

~~(16) H. Kiefer and T.G. Traylor, J. Amer. Chem. Soc., 8j9, 6667~~

~~(1967).~~

~~(17) C. Walling and H. Waits, J. Phys. Chem., 71, 2361 (1967 ).~~

~~(18) Ref. 7, p. 20.~~

~~(19) F.D. Rossini,"Selected Values of Physical and Thermodynamic~~

~~Properties of Hydrocarbons and Related Compounds", American~~

Petroleum Institute Research Project 44, Carnegie Press, 1953*

~~(20) S. Seltzer, J. Amer. Chem. Soc., 8 ^, 2625 (1961 ).~~

~~(21) S. Seltzer, J. Amer. Chem. Soc., 8^, 14 (1963 ).~~

~~(22) S. Seltzer and F.T. Dunne, J. Amer. Chem. Soc., 8^, 2628 (1965 ).~~

~~(23) T. Koenig and W. Brewer, Tetrahedron Lett., 2773 (19^5)•~~

~~(24) T. Koenig and R. Wolf, J. Amer. Chem. Soc., £1, 2574 (1969).~~

~~(25) H.J. Shine, J.A. Waters, and D.M. Hoffman, J. Amer. Chem. Soc.,~~

~~8£, 3613 (1963). ( 26) [2,6-Di-_t-butyl-a-( 3>5-di-_t-butyl-4-oxo-2,5-cyclohexadien-l-~~

~~ylidine) -p-tolyloxy]31.~~

~~(27 ) diphenylpicrylhydrazyl.~~

~~(28) J.C. Martin, J.W. Taylor, and E.H. Drew, J. Amer. Chem. Soc.,~~

~~8£, 129 (1967).~~

~~(29) H.J. Shine and J.R. Slagle, J. Amer. Chem. Soc., &L, 6309~~

~~(1959).~~

~~(30) J.C. Martin and E.H. Drew, J. Amer. Chem. Soc., 8£, 1232 (1961 ).~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 18 7~~

~~(31) P.D. Bartlett and T. Funahashi, J. Amer. Chem. Soc., m , 2569~~

~~(1962 ).~~

~~(32) a) G.S. Misra and V.R.B. Mathur, Makromol. Chem., 100, 5^~~

~~(1967).~~

~~b) G.S. Misra and V.R.B. Mathur, Makromol. Chem., 10^> l6*t~~

~~(1967). (33) A.T. Blomquist and I. A. Bernstein, J. Amer. Chem. Soc., Tj^,~~

~~55^6 (1951).~~

~~(3*0 P.D. Bartlett and C. Ruchardt, J. Amer. Chem. Soc., §£, 1756,~~

~~(I960 ).~~

~~(35) Mr. H.T. Bickley, Louisiana State University, unpublished~~

~~results (1$70).~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. PART II - RADIOLYTIC GENERATION OF FREE RADICALS~~

~~The destruction of organic materials by radiolysis occurs~~

~~largely by free radical and excited molecule reactions. 1 6 The~~

~~isotope effect for abstraction from J:-butyl mercaptan by a free~~

~~radical can be measured by generating the radical radiolytically~~

~~in mixtures of isotopically substituted mercaptan. In two separate~~

~~experiments, a radical was generated in the presence of ^-butyl mer~~

~~captan by the y-irradiation of a substrate in a cobalt-6 0 pool~~

~~reactor. In one case, the radical was allowed to compete between~~

~~abstraction of hydrogen and tritium atoms from tritiated j:-butyl mer~~

~~captan. In the other case, the mercaptan was extensively deuterated~~

~~( ca. 95 percent) at the S-H position and then tracer labelled with~~

~~tritium at the S-H position, so that competition during abstraction~~

~~was primarily between deuterium and tritium atoms. In order to test~~

~~the feasibility of such a system and examine its utility for the cal~~

~~culation of isotope effects for abstraction, the substrate initially~~

~~studied was cyclohexane. In this case, radiolysis to form the cyclo-~~

~~hexyl radical is unambiguous and the kinetics of the abstraction step~~

~~will not be affected even if this cyclohexyl radical attacks the~~

~~substrate. The experimental details for this system are described~~

~~in Chapter III. The kinetic analysis of the reactions are shown~~

~~below.~~

~~In the following kinetic scheme, Q* is the cyclohexyl radical,~~

~~QH is unreacted cyclohexane, QH* is unlabelled cyclohexane formed~~

~~188~~

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. i8q by Q- abstracting from J:-BuSH, QT is tritiated cyclohexane formed H in competition with hydrogen atom abstraction, and QT^ is tritiated

~~cyclohexane formed in competition with deuterium atom abstraction.~~

~~Experiment 1 Experiment 2~~

~~H' vs. T- Abstraction D* vs. T- Abstraction~~

~~iH iD QH -> Q ‘ (1) QH -> Q- (*0 Y-ray Y-ray~~

~~k. Q- + RSH -S— > Q H / + RS- (2) Q- + RSD -=— > QD + RS- (5)~~

~~k„ kT Q- + RST - L —> QT + RS- (3) Q- + RST — — > QT + RS- (6) H ^ %~~

~~If it is assumed that the isotopic composition of the small amount~~

~~of mercaptan present has no effect on the rate of radiolytic~~

~~destruction of the cyclohexane, then k =k , and the following i H XL) relationships may be derived.~~

~~In the case of hydrogen vs. tritium atom abstraction (experiment~~

~~1):~~

~~= k/ Q '^ (T)~~

~~d(QT„) dt - = kTCQ-l [RST] (8)~~

~~Dividing (7) by (8):~~

~~d(QH') = _H [RSH] (9) H) kT LRST^~~

~~In the case of deuterium vs. tritium atom abstraction (experiment~~

~~= ^[Q-ltRSD] (10)~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 190 d( Qtd) dt— = k_rQ-irRST] T' (n)~~

~~Dividing (10) by (ll):~~

~~d t Q D L = ^ [ r S D ] ( 1 2 ) dfOTp" = [RST]~~

~~The ratio [RSH]/[RST] may be given as 1/A° , where A° is the o n o n absolute activity of Jt-butyl mercaptan with hydrogen or tritium at~~

~~the S-H position (experiment 1). Similarly, the ratio [RSD]/[RST]~~

~~may be given as 1/A° , where A° is the absolute activity of £-butyl D JJ oJJ "" mercaptan with deuterium or tritium at the S-H position (experiment~~

~~2). Thus, eq. (9) becomes:~~

~~= Ni _i. (15) d(QTj K AO .0 h 1~~

~~and eq. (12) becomes:~~

~~4 s e l „ ^ _ i . (1m kT a o d (~~

~~Dividing (lj) by (1^):~~

~~[QH ]/[QTh1 kg kTAsp . . Tw Cq^ T = kT a o h fcj,~~

~~The ratio [QH/]/[QT ] may be given as 1/A°T> where A^T is the~~

~~absolute activity of the cyclohexane formed by the abstraction of~~

~~either hydrogen or tritium atoms from J:-BuSH(T) in experiment 1.~~

~~Similarly, the ratio [QD]/[QTd] may be given as where A^T~~

~~is the absolute activity of the cyclohexane formed by the abstrac~~

~~tion of either deuterium or tritium atoms from J:-BuSD(T) in~~

~~experiment 2. Thus, eq. (15) becomes:~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 191 ^dt _ Si ^sd~~

~~a £h~~

~~It is, of course, impossible to distinguish between unreacted cyclo~~

~~hexane and that formed by hydrogen atom abstraction by Q* from _t-~~

~~BuSH. Therefore, Aj^, and A^, are not directly measurable, and it is~~

~~desirable to substitute the absolute activity of the cyclohexane~~

~~isolated following radiolysis (combining that which is unreacted~~

~~and that formed by abstraction from t-butyl mercaptan) into eq. (16 ).~~

~~This substitution can be made if two conditions for the experiment~~

~~are met. First, the concentration of RSH( T) in QH (experiment l)~~

~~must be the same as the concentration of RSD(t) in QH (experiment~~

~~2). Secondly, the extent of radiolysis in the two experiments must~~

~~be identical. Once these two conditions are met, eq. (16) becomes:~~

~~A ° k A0 QD = _H _SD A° N d A° a q h SH~~

~~where A°^ is the final activity of the cyclohexane isolated in~~

~~experiment 2 where competition between deuterium and tritium atoms~~

~~occurs, A° is the final activity of the cyclohexane isolated in QH experiment 1 where competition between hydrogen and tritium atoms~~

~~occurs, and is the isotope effect for abstraction at 3^°C, the~~

~~temperature of radiolysis.~~

~~In the above scheme, it is assumed that radiolytic destruction~~

~~of the mercaptan does not occur. Normally, the extent of radiolysis~~

~~of a solute is not significant when its concentration is kept below~~

~~about 10 2 M .7 If appropriately labelled mercaptan is the solute in~~

~~the Y“radiolysis of cyclohexane and is present in about 10 2 M~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. concentration, it is assumed that all of the energy is absorbed by the~~

~~cyclohexane. In order to test the validity of this assumption, the~~

~~radiolysis was performed on samples which contained mercaptan in~~

~~varying concentrations. Table I shows the results of this studyj as~~

~~well as the results of studies in which the radiolysis time was varied.~~

~~The measured value for k^/k^ determined using eq. (17) must be~~

~~corrected for the small amount of J:-BuSH present in the case where~~

~~abstraction from J:-BuSD(t) occurs. Thus, the reaction sequence for~~

~~experiment 2 must be expanded to include eq. (18) in addition to eq.~~

~~(4) - eq. (6 ) noted previously.~~

~~Q- + RSH -2— > QH + RS- ( 18)~~

~~On the basis of this expanded reaction scheme, the absolute activity~~

~~of the cyclohexane following radiolysis, A may be given as follows~~

~~kT[RST] (20)~~

~~Rearranging eq. (20):~~

~~kT[RST] (21) QD~~

~~kx[RST] (22)~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 193 Thus, the measured value of A°R is less than if protium mercaptan is~~

~~not present as an impurity. In order to get a corrected value for~~

~~A°d , it is necessary to multiply the measured A°R by the factor~~

~~k , [RSH] H 1 + Therefore, eq. (17) may be corrected as follows: kQ [RSd ]~~

~~k [RSH] _H H ( A ° ) ( AgH ) 1 + kp [RSD] rs~ (23)~~

~~kR kR [RSH]~~

~~[RSH] (A° )(A° ) _H = (AU rAu T (25) CD 1 - 1™>J < $ < € > ' QH ' SD'~~

~~( a o d ^ a s i P _(a q h ) ( a | d )_ “ h (26 ) [ R S H 3 ( A “ d )(A“ h )-] T i , I 1 1LKSD-J ( A y ( A “D)J~~

~~The isotope effect data for abstraction by the cyclohexyl radical~~

~~as measured previously8 indicate that~~

~~K ^ = 2.75 ± 0 .1 2 (27)~~

~~at 3k 0 C, in excellent agreement with the average value of 2.70 shown~~

~~in Table I. The value for [t:-BuSH]/[t-BuSD] is experimentally deter~~

~~mined in each case. 9~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table I~~

~~Y-Radiolysis of Cyclohexane. Isotope Effect for Abstraction by the Cyclohexyl Radical at 5k°C.~~

Icr |o o n o n X > H* — 3 0 0 0 0 0 0 ro ro -'J -3 CO o 3 • •• • •• H O — 3 O 0 0 p- p" -PI o . n |c7 Vrt ro ro ro 0 0 ON ON I 3 3* w 3 VO H HH H (D 3 c o *3 CO rt 33 (D p - p- p- X > H H' H vn vn vn — -3 -3 H H CO o 3 •• • • • H 33 H ro H H H CO CO H H O , Irt ro P- p - P" VO v o I | 3 i OOO H W O cn3 O H X > vn p - ro H VO V n M 0 o • H O 0 0 p - o n P* CO VO o — 3 O . 3 IO VO o n o \ 1 13 cn 8 . - t j 0 0 o P ft) (3 H* f-1 X > 3 vn o n Vn O o n -3 P" 0 O • • • • • • H 33 ft) P) ON vn 00 o n ro h O X O P- SO ro H CD H 1 3 T) s Vn o n CO ON U1 fD H H- 3 h* cr ft) H 3 3 vn p- vn ro p- vn vn ro >-( • rt • •• « • •• • 3 O (3 o ro 0 0 0 0 0 0 0 • H)

~~a . ro 3 3 |rt Ir t rt I I ft) X w w 3 3 § H CO CO H* 00 Q a 33 3 ft) 00 -3 P) co oo CD CO H H a. o n ON o n OO oo CD CD O' o o V3 o H 3 o~~

3 3 3 3 t-1 TO CO VJ 00 Vn V* V n vo VO crv ON 3 I M H 33 H- -3 p- p- p- Vn VO *00 bo H* I o' cn V3r t 3 cn a. H* ro ro ro ro ro ro to 3 T S 3 • • •• H o p- P] —J 00 vn CT\ - 3 3 o o VO o ro ON OO 3 313

~~194~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 195 The data in Table I point out two significant features about~~

~~the radiolysis experiments. First, a large variation in the y-ray~~

~~dose delivery apparently does not influence the calculated isotope~~

~~effect. This indicates that the radiolysis time period of two to~~

~~four hours does not generate so many cyclohexyl radicals that a~~

~~significant amount of mercaptan is reacted. Secondly, varying the~~

~~J:-butyl mercaptan concentration over the range 0.88 x 10 2 M to~~

~~1.76 x 10 2 M does not change the calculated value of the isotope~~

~~effect. Hiis indicates that over this mercaptan concentration~~

~~range, the direct radiodestruction of the solute mercaptan occurs~~

~~only to a negligible extent. It is interesting to note, however,~~

~~that a ten-fold increase in mercaptan concentration does cause a~~

~~quite significant change in the value calculated for the isotope~~

~~effect, perhaps because of significant radiolysis of the mercaptan~~

~~at the higher concentration.~~

~~A logical extention of this radiolytic method for determining~~

~~isotope effects for abstraction and the subsequent assessment of~~

~~radical reactivity (as discussed in Chapter V), is the application~~

~~of the method to the radiation induced destruction of molecules of~~

~~biological significance. The examination of the effect of~~

~~ionizing radiation on dry proteins, for example, has been impor~~

~~tant in explaining the mechanism through which the absorption of~~

~~radiation energy leads to chemical changes in proteins.10 13~~

~~From investigations with quantitative esr spectroscopy, it has~~

~~been shown that a close relationship exists between protein~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 196 damage and the behavior of protein radicals. 14 19 One observation~~

~~of outstanding importance in this regard is that certain sulfur~~

~~containing compounds, particularly those containing a sulfur-~~

~~hydrogen bond, can partly protect proteins against radiation inac~~

~~tivation. A number of workers have shown that radiation can result~~

~~in the removal of a hydrogen atom from a biological polymer MH~~

~~leaving a radical M-. The mercaptan can then repair this damage~~

~~by a hydrogen atom transfer reaction, donating a hydrogen atom to~~

~~the polymer radical and producing a thiyl radical, as shown in eq.~~

~~( 28) and ( 29) .~~

~~MH -'W-> M* + H- (28)~~

~~M- + RSH --- > MH + RS- (29)~~

~~Many compounds containing S-H or S-S bonds act as protective agents~~

~~in laboratory systems13’ 20 24 when, for example, proteins are irra~~

~~diated in the dry state. Additionally, it has been well established~~

~~that small amounts of certain chemicals taken a short interval~~

~~before exposure to irradiation can provide a significant measure of~~

~~protection for living animals from the effects of high energy~~

~~ionizing radiation. 19’ 2 3 ’ 25 In studies of living systems, however,~~

~~problems of solubility, diffusion, and toxicity arise so that only~~

~~certain compounds are effective radiation protection drugs. 19’ 2 3 ’ 26~~

~~The effects of ionizing radiation on dry proteins have been~~

~~examined in some detail. When dry proteins are radiolyzed, trapped~~

~~radical sites are generated along the polypeptide chain. 27 The~~

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 197 question of whether these free radicals are randomly distributed or whether they are preferentially located on certain amino acid

~~residues has been studied using esr spectroscopy and isotopic~~

~~labelling techniques with a highly reactive scavenger. Riesz,~~

~~White, and Kon27 have irradiated dry, lyophilized ribonuclease in~~

~~the absence of air and subsequently exposed the radiolyzed sample~~

~~to tritiated hydrogen sulfide. This procedure allows the highly~~

~~reactive scavenger to tritium label the radicals produced by~~

~~radiation, forming new carbon-tritium bonds. The distribution of~~

~~tritium among the various amino acids was determined by hydrolysis~~

~~of the protein, separation of the amino acids, followed by their~~

~~analysis using scintillation counting techniques. The fact that~~

~~carbon radicals are formed during the radiolysis of native ribo~~

~~nuclease has been confirmed by esr measurements by these same inves~~

~~tigators.27 It was also noted that a marked decrease of the carbon~~

~~radical signal occurred when the sample was exposed to hydrogen~~

~~sulfide.~~

~~Several important findings have resulted from the yradiolysis~~

~~of dry ribonuclease. It was observed27 that some amino acid~~

~~residues contain much more tritium than others. No correlation~~

~~between tritium content and the number of primary, secondary, and~~

~~tertiary carbon-hydrogen bonds was observed, nor was there any~~

~~correlation between tritium content and the polar character of the~~

~~amino acid. Location of the amino acid residue on the surface or~~

~~in the interior of the protein was observed to be at least~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 9 8 partially responsible for the magnitude of tritium content. In~~

~~addition, a different order of tritium content was found when ribo~~

~~nuclease was compared to carboxymethylated reduced ribonuclease.~~

~~Reduction of native ribonuclease leads to enzymatically inactive~~

~~material with different physical properties, indicating extensive~~

~~destruction of the protein secondary and tertiary28 structure. It~~

~~was concluded27 that the free radical distribution in Y-irradiated~~

~~protein depends markedly on the conformation of the molecule.~~

~~More recently, Riesz and his coworkers29 reported on the~~

~~radical distributions in several dry proteins which were Y"irra“~~

~~diated at 195°K and room temperature. It was found that below~~

~~195°K, the formation of carbon radicals in native proteins is inde~~

~~pendent of amino acid sequence, conformation, and the presence of~~

~~disulfide bridges, whereas above this temperature, the radical~~

~~distribution is influenced by the specific conformation of each~~

~~native protein. These results are taken to indicate that at tem~~

~~peratures above 195°K, the primary radicals formed are quenched~~

~~rapidly by inter- or intramolecular transfer processes to produce~~

~~secondary radicals which are observed either by esr or by exposure~~

~~to tritiated radical scavengers. While the exact nature of these~~

~~radical migration processes in not known, it may be noted that~~

~~similar hydrogen abstraction reactions have been proposed for the~~

~~conversion of primary to secondary radicals in other systems. 30’ 31~~

~~It has further been recently reported32 that both Y-radiolysis~~

~~and hydrogen atom exposure of a protein produce very similar~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 199 distributions of radicals along the polypeptide chain. Thus, the~~

~~radicals formed initially by hydrogen atom attack may react to~~

~~form the secondary radical distribution observed. Alternatively,~~

~~hydrogen atoms may react like primary radicals to produce the same~~

~~secondary radical distribution.~~

~~Because of the importance of radicals of the type -NH-CR-CO-~~

~~in the radiochemistry of peptides33’ 34 and proteins35 as well as~~

~~in their photochemistry36, it is of interest to assess the reac~~

~~tivity profile of these radicals. Unfortunately, no common~~

~~initiators are available for the generation of radicals of this~~

~~structure. However, such radicals may be generated by the Y-radio-~~

~~lysis of a suitable substrate, and the isotope effect for abstrac~~

~~tion from J:-butyl mercaptan may be determined by the same procedure~~

~~as previously described for the radiolysis of cyclohexane. Two con~~

~~ditions must be met in order for such a system to be applicable to~~

~~the isotope effect determination. First, the substrate to be radio-~~

~~lyzed must be a liquid. Secondly, jt-butyl mercaptan must be suffi~~

~~ciently soluble in the substrate to give a solution which is 10 2 M~~

~~in mercaptan.~~

~~The substrate chosen in this study is the ethyl ester of N,N-~~

~~dimethylglycine37, shown as structure I. This material is a liquid~~

~~h 3cn ° n -ch2-c -oc h 2c h 3 h 3c/ (I)~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 200 and may be easily purified by distillation (B.P. 40°/7mmHg). Deter~~

~~mination of the purity and characterization of this compound is con~~

~~veniently achieved by nmr analysis. The nmr spectrum of this compound~~

~~shows a quartet centered at -4.1 ppm corresponding to the two~~

~~methylene protons of the ethyl group, a singlet at -3 -1 ppm corres~~

~~ponding to the two protons of the central methylene group, a singlet~~

~~at -2 .3 ppm corresponding to the six protons of the methyl groups~~

~~bound to nitrogen, and a triplet centered at -1 .3 ppm corresponding~~

~~to the three remaining protons of the ethyl group.~~

~~In order for the radiolysis of solutions of _t-BuSH(T) and _t-~~

~~BuSD(T) in the ethyl ester of N,N-dimethylglycine to be useful in~~

~~determining the isotope effect for hydrogen atom abstraction, it is~~

~~necessary that the two components do not undergo isotopic exchange.~~

~~The fact that such exchange does not occur here was demonstrated by~~

~~mixing approximately equal volumes of the ester and deuterated~~

~~(about 9 5 S-D) £-butyl mercaptan in an nmr tube. The mixture was~~

~~heated to 60°C and shaken for about ten minutes, and the nmr spectrum~~

~~was recorded. The relative integrations of the signals due to the~~

~~ester were observed not to change from the expected magnitudes, indi~~

~~cating that ordinary chemical exchange does not occur to a measurable~~

~~extent under the conditions of radiolysis.~~

~~The radiolysis of N,N-dimethylglycine ethyl ester is somewhat~~

~~more complex than the radiolysis of cyclohexane discussed previously.~~

~~With cyclohexane as the substrate, only one radical is present, and~~

~~attack of this radical on the substrate leads to no net reaction.~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. In the case of the glycine ester, however, radical formation is possibl~~

~~at two secondary carbon atoms, as shown in structures II and III.~~

~~0 0 n ii (CH3)2N-CH-C-0-CH2CH3 (CH3)2N-CHs-C-0-CHCH3~~

~~(II) (III) If both radicals II and III are formed during the radiolysis of N,N-~~

~~dimethylglycine ethyl ester, tritium would be incorporated into two~~

~~positions in the substrate. On the basis of the bond dissociation~~

~~energy difference between an ordinary primary carbon-hydrogen bond~~

~~and a secondary carbon-hydrogen bond, 38 and due to the presence of~~

~~the labile t,-butyl mercaptan, it is assumed here that radicals are~~

~~not formed at primary carbon atoms in the substrate either by direct~~

~~radiolysis or attack of II or III on the substrate.~~

~~In order to determine the fraction of tritium activity located~~

~~in the ester function, it is necessary to degrade the substrate to~~

~~give a product which does not contain that functional group. Compari~~

~~son of the activity of the purified N,N-dimethylglycine ethyl ester~~

~~to that of the degradation product will give information regarding~~

~~the location of tritium following radiolysis. The procedure used is~~

~~described below.~~

~~Following radiolysis, the ester substrate was purified by distil~~

~~lation at room temperature on a vacuum line. The purified ester was~~

~~collected at T7 °K, and it was found that three such vacuum transfers~~

~~achieved constant activity in the substrate. The activity was deter~~

~~mined by dissolving the purified product in the toluene based fluor~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 0 2 solution and counting by ordinary liquid scintillation techniques.~~

~~The method of degradation was basic saponification, as shown in~~

~~e1 * (30)» 0ne gram sodium hydroxide in 10 ml water was added to three~~

~~0 0 (CH3)2N-CH2 -C-0C2H5 + NaOH ^ - > (CH3)2N-CH2 -C-O0 Na® + C2H50H (30)~~

~~g of the purified substrate and the mixture was shaken vigorously.~~

~~After allowing the mixture to sit for 2k hours, it was evaporated to~~

~~dryness at room temperature under vacuum, leaving white crystals as a~~

~~product. These crystals were dissolved in the minimum amount of water~~

~~and 20 ml concentrated hydrochloric acid were added slowly to give~~

~~N,N-dimethylglycine hydrochloride, according to eq. (31)• Upon cooling~~

~~the solution, the hydrochloride crystallized and was separated by~~

~~vacuum filtration. Purification of N,N-dimethylglycine hydrochloride~~

~~was achieved by sublimation at 100-110°C. The observed melting point~~

~~of 185-7°C is the same as that reported in the literature. 39 The~~

~~results of carbon, hydrogen, and nitrogen elemental analysis are shown~~

~~in Table II. 40~~

~~The activity of N,N-dimethylglycine hydrochloride was determined~~

~~by dissolving the sample in one ml distilled water and adding 15 ml~~

~~Aquasol, a ready-to-use xylene based liquid scintillation counting~~

~~solution 41 to give ahomogeneous mixture. Absolute activities of~~

~~samples may be obtained from Aquasol solutions using the ordinary~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table II~~

~~Results of Elemental Analysis of N,H-Dimethylglycine~~

~~Hydrochloride40~~

~~H 0 f II (CH3)2N-CH2-C-0H~~

~~Cl®~~

~~Percent Calculated Product formed Product formed using J:-BuSH(T) using J:-Bu SD(t ) as solute as solute~~

~~carbon 34.42 34.40 34.37~~

~~hydrogen 7 .2 2 7.42 7.44~~

~~nitrogen 10.03 10.07 10.03~~

~~203~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 20)f tritium standards in toluene.The results of the activity determina~~

~~tion of samples N,N-dimethylglycine hydrochloride obtained from the~~

~~degradation of the radiolyzed ethyl ester of N,N-dimethylglycine are~~

~~shown in Table III. These data indicate that only about five percent~~

~~of the total tritium content is located in the ester function of the~~

~~radiolyzed ethyl ester. The fact that such a small fraction of the~~

~~total tritium is located in the ester function indicates that radical~~

~~II shown previously is responsible for almost all of the tritium incor~~

~~poration into the substrate. Also, this observation of only a small~~

~~fraction of the total tritium at the secondary carbon atom in the ester~~

~~function lends credence to the assumption that radicals are not formed~~

~~very extensively at primary carbon atoms in the substrate either by~~

~~direct radiolysis or by attack of any radical on the substrate.~~

~~The isotope effect for abstraction by radical II from labelled _t-~~

~~butyl mercaptan may therefore be approximated by the same calculation~~

~~procedure described for the radiolysis of cyclohexane. These calcula~~

~~tions are made using eq. (26), taking into account the small jt-BuSH~~

~~impurity in the abstraction from J:-BuSD(t ). The data and final~~

~~corrected isotope effect at ^k°C are shown in Table IV.~~

~~In order to compare the isotope effect for abstraction by (CH3 )aN- . fl CH-C-O-CH2CH3 to the data obtained for other radicals, it is necessary~~

~~to calculate the previous isotope effects at 3^°C. This may be done~~

~~conveniently by using eq. (32), the values for k^/k^ at 60°C given~~

~~in Table XVEH of Chapter IV, and the values for (E -E.J given in Table H D XVEH of Chapter IV.~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table III~~

~~Determination of Position of Tritium Labelling in the~~

~~ethyl ester of N,N-Dimethylglycine. Ester Activity~~

~~Compared to Activity of N,N-Dimethylglycine Hydrochloride~~

~~Activity of N,N- Activity of N,N- Dimethylglycine Dimethylglycine ethyl ester — Hydrochloride —~~

~~Abstraction from t-BuSH( T) 7 .3 1 1 x 10s 6.982 x 10s~~

~~Abstraction from t-BuSD(T) 3.148 x 105 2.975 x 105~~

~~a dpm/mole~~

~~205~~

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. y-Radiolysis of N,N-Dimethylglycine ethyl ester. '"m VW < .a ja 0 u-i14-1 CJ •H a • 0 r-M JP o z a z H w 4J 4-) 4-1 4J to U n o a tfl U to a) ai O o o o P. 01 01 1 01 i o -4- N"\ CJ 4-J tO A x 1.023 1.023 1.023 1.023 5 SD 0 $ $ J:-BuSHin t-BuSD(T) usedinexperiment 2, determined by nmranalysis as dpm/mole discussedin Chapter III. “ “ "9 0 1 a A° ~ x SH 5.058 5.058 5.058 5.058 10 a _1° qd a A° ” 4.722 x 5.192 .586 8 9-102 10 . a

5 A x 19 .0 2 11.91 11.42 QH .8 .0 1 6.986 0 6.238 “ “ "6 0 1 a r ofhr. irrad. .0 1 .0 2 t-BuSD(T) conc. t-BuSH(T) or M x M 1.056 2.112 1.056 2.112 102 Average # S-H impurity . 4.73 5.3 5.3 5.5 5.5 b 4.63 4.69 4.71 4.80 ' V corr. d s cvi

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 207 -(E -E ) log e (E -E ) log e~~

~~= — H f e ----- + “ “ “ H I ------(52) In eq. (32), the following values are used:~~

~~log e = 0.43^3~~

~~R = 1.9872 cal deg 1 mole 1~~

~~T2 = 333°K~~

~~Ti = 307°K~~

~~An identical set of values for k^/k^ at 3^°G may be calculated~~

~~from the least squares equations relating log (k^/k^) and ( 1/T°k )~~

~~given in Tables I - XV of Chapter IV. The data are shown in Table~~

~~V. The value of b .7 for k /le. at 3b0 for abstraction by (CH3)2N-CCH- 0 C-0-CH2 CH3 indicates that this radical is somewhat more stable than~~

~~an ordinary secondary alkyl radical. This increased stability of~~

~~radical II is consistent with the observation that radical II is~~

~~formed during the radiolysis to a much greater extent than radical~~

~~III.~~

~~There is considerable precedent in the literature in support~~

~~of enhanced stability for radicals of type II, with the unpaired elec~~

~~tron in the cv-position to the nitrogen atom. Garrison44 has shown~~

~~that OH radicals from the radiolysis of acetylglycine in aqueous~~

~~solutions attack at the peptide methylene group in preference to the~~

~~end methyl group, _i-£., k33 > > k34:~~

~~oh- + ch3conhch2coo0 £aa_> ch3conhchcoo® + h2o (33)~~

~~OH- + ch3conhch2coo® ^ L - > • CH2C0NHCH2C00® + H20 (3b)~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE V~~

~~Kinetic Isotope Effects on Hydrogen Atom~~

~~Abstraction from J:-butyl Mercaptan at 3k°C—~~

~~Radical V' d~~

~~hydrogen atom 1.38~~

~~phenyl 1 .8 1~~

~~1 -adamantyl 1 .8 2~~

~~jv-nitrophenyl 2.43~~

~~methyl 2.59~~

~~cyclohexyl 2.75~~

~~t ri fluorome thy1 2 .8 -~~

~~1 -nonyl 3 .2 8~~

~~3 -heptyl 4.19~~

~~triethylmethyl 5.1^~~

~~benzyl 6 .0 0~~

~~diphenylmethyl 6 .3 6~~

~~trityl 6 .6 2 DPPH 4.07~~

~~see Table XVH, Chapter IV for data at 60°C.~~

~~b ref. 43.~~

~~208~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 209 Similarly, it has been shown45 from results obtained in the determina~~

~~tion of the site of attack of OH radicals on simple amides, that the~~

~~amide nitrogen activates N-methyl groups, such that hydrogen abstrac~~

~~tion from an N-methyl group occurs much more readily than attack on~~

~~an a-methyl group, ji.e,., k35 » k36:~~

~~OH- + CH3C0NHCH3 CH3C0NHCH2 + H2O (35)~~

~~OH- + CH3C0NH2 •CHsC0NH2 + IfeO (56 )~~

~~Similar results were obtained by Simic, Neta, and Hayon46 upon aqueous~~

~~pulse radiolysis of a number of simple peptides indicating that the~~

~~odd electron is located cn to the nitrogen atom, even in cases where~~

~~other secondary hydrogens were present.~~

~~The results on the radiolysis of peptides, the formation of radical~~

~~II in preference to radical III upon radiolysis of N,N-dimethylglycine~~

~~ethyl ester, and the measured isotope effect for abstraction by~~

~~radical II all are consistent with the notion that (CH3 ,)2N-CH-C-0-~~

~~CH2 CH3 is somewhat more stable than an ordinary secondary alkyl~~

~~radical.~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX - REFERENCES - PART II~~

~~(1) J.W.T. Spinks and R.J. Woods, "Introduction to Radiation~~

~~Chemistry", John Wiley and Sons, Inc., New York, 196 k, Chapter~~

~~6 .~~

~~(2) F.S. Dainton, Pure Appl. Chem., 10, 395 (1965).~~

~~(3) R.A. Holroyd in "Aspects of Hydrocarbon Radiolysis", T.~~

~~Gaiimann and J. Hoigne, Ed., Academic Press, New York, 1968 ,~~

~~PP. 1-52.~~

~~(4) R.H. Schuler and R.W. Fessenden in "Radiation Research", North~~

~~Holland Publishing Co., Amsterdam, 1967 » PP* 99~112.~~

~~(5) P. Ausloos, Ann. Rev. Phys. Chem., !£, 205 (1966).~~

~~(6 ) W.A. Pryor and U. Tonellato, J. Phys. Chem., 850 (1969 ).~~

~~(7) Professor R.C. Mcllhenny, Nuclear Science Center, Louisiana~~

~~State University, personal communication, 1970.~~

~~(8) Table IV and Figure IV, Chapter IV.~~

~~(9) Analytical method1 isddesctiibdd in Chapter III.~~

(10) E.C. Pollard, Rev. Modern Phys., 273 (1959)*

~~(11) L. Augenstine, Adv. Enzymol., 2h, 259 (1962 ).~~

~~(12) F. Hutchinson, Science, 535 (1961).~~

~~(13) R. Braams, Nature, 200, 752 (1963 ).~~

~~(1*0 A. Muller, Inter. J. Radiat. Biol., £, 199 (1962).~~

~~(15) T. Henriksen, J. Chem. Phys., 2189 (1962).~~

~~(16) T. Henriksen, T. Sanner, and A. Pitil, Radiat. Res., IB, 1^7~~

~~(1963).~~

~~210~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 1 1~~

~~( I f ) T. Henriksen, T. Sanner, and A. Pihl, Radiat, Res., 18, 165~~

~~(1963)-~~

~~(18) W. Kohnlein and A. Muller, Phys. Med. Biol., 6, 599 (19^2).~~

~~(19) W. Gordy and I. Miyagawa, Radiat. Res., 12, 211 (i960 ).~~

~~(20) R. Braams, F. Hutchinson, and D. Ray, Nature, lB2, 1506~~

~~(1958).~~

~~(21) R. Braams, Radiat. Res., ^ , 113 (i960 ).~~

~~(22) A. Norman and W. Ginoza, Radiat. Res., 77 (1958).~~

~~(23) W.A. Pryor, Sci. Amer., 22^, 2, 70 (1970).~~

~~(2k) J.A.V. Butler and A.B. Robins, Radiat. Res., 1£, 63 (I962 ).~~

~~(25) H.A.S. van den Brenk and K. Elliott, Nature, ^1^2, 1506~~

~~(1958).~~

~~(26) D. Harman, in "Science and the Citizen", Sci. Amer., 22Q,,~~

~~2, 50 (1969).~~

~~(27) P. Riesz, F.H. White, Jr., and H. Kon, J. Amer. Chem. Soc.,~~

~~8&, 872 (1966 ).~~

~~(28) J.D. Roberts and M.C. Caserio, "Basic Principles of Organic~~

~~Chemistry", W.A. Benjamin, Inc., New York, 1965 , P* 72k.~~

~~(29) a) P. Riesz and F.H. White, Jr., Radiat. Res., kk, 2k~~

~~(1970).~~

~~b) C.D. Scher and P. Riesz, Radiat. Res., kk,, 35 (1970).~~

~~(30) H.C. Box, H.G. Freund, and E.E. Budzinski, J. Chem. Phys.,~~

~~k6, kk 70 ( 1967).~~

~~(31) A. Meybeck and J.J. Windle, Radiat. Res., kO,, 263 (1989).~~

~~(32) R.A. Holroyd, J. Glass, and P. Riesz, Radiat. Res., k^,~~

~~59 (1970).~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 212 (33) H.C. Box, H.G. Freund, and K. Lilga in "Free Radicals in~~

~~Biological Systems", M.S. Blois, Ed., Academic Press, New York,~~

~~1967, p. 239.~~

~~(3^) W.M. Garrison and B.M. Weeks, Radiat. Res., 1£, 3^-1 (1982).~~

~~(35) W.M. Garrison, M.E. Jayco, B.M. Weeks, H.A. Sokol, and W.~~

~~Bennett-Comiea, J. Phys. Chem., Jl, 1546 (1967 ).~~

~~(36 ) W.M. Garrison, M.E. Jayco, and W. Bennett, Radiat. Res., l£,~~

~~483 (1962 ).~~

~~(37) Obtained from Eastman Organic Chemicals.~~

~~(38) J.A. Kerr, Chem. Rev., 465 (1986).~~

~~(39) D.E. Pearson and J.D. Bruton, J. Amer. Chem. Soc., 864~~

~~(1951). (40) Measured by Mr. Ralph Seab, Department of Chemistry, Louisiana~~

~~State University, using a Perkin Elmer Model 240 Elemental~~

~~Analyzer.~~

~~(41) Obtained from the Pilot Chemicals Division, New England Nuclear~~

~~Corporation.~~

~~(42) a) Dr. Wayne Harris, New England Nuclear Corporation, personal~~

~~communication, 1970 .~~

~~b) See Chapter III.~~

~~(43) N.L. Arthur and P. Gray, Trans. Faraday Soc., 6 ^, 434 (I969 ).~~

~~(44) W.M. Garrison, Curr. Top. Radiat. Res., 4, 43 (1968 ).~~

~~(45) E. Hayon, T. Ibata, N.N. Lichtin, and M. Simic, J. Amer. Chem.~~

~~Soc., Sfc, 3898 (1970).~~

~~(46) M. Simic, P. Neta, and E. Hayon, J. Amer. Chem. Soc., £ 2 , 4763~~

~~(1570).~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. VITA~~

~~Kenneth George Kneipp~~

~~PLACE AND DATE OF BIRTH~~

~~New Orleans, Louisiana, July 6 , 1944~~

~~EDUCATION~~

~~Public School System, New Orleans, Louisiana, 1949-1962~~

~~Tulane University, I962 -I966 (B.S., 1966 )~~

~~Louisiana State University, 1966-19T1 (Ph.D., 1971)~~

~~PROFESSIONAL SOCIETIES~~

~~Alpha Chi Sigma Fraternity~~

~~American Chemical Society~~

~~American Association for the Advancement of Science~~

~~POSITIONS~~

~~National Science Foundation Undergraduate Research Participant, Louisiana State University, 1964~~

~~University Undergraduate Research Participant, The Florida State University, 1965~~

~~Teaching Assistant, Tulane University, I965 -I966~~

~~National Science Foundation Undergraduate Research Participant, Tulane University, I965 -I966~~

~~Research Participant, Ethyl Corporation, 1966~~

~~Teaching Assistant, Louisiana State University, I966 -I968~~

~~Research Participant, 3M Company, 1967~~

~~National Institutes of Health Research Assistant, Louisiana State University, 1968-1971~~

~~213~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. EXAMINATION AND THESIS REPORT~~

~~Candidate: Kenneth George Kneipp~~

~~Major Field: Chemi s try~~

~~Title of Thesis: Kinetic Isotope Effects in FreeRadical Chemistry~~

~~Approved:~~

~~Major Professor ana'Chairman~~

~~Dean of the Graduate School~~

~~EXAMINING COMMITTEE:~~

~~4 L~~

~~Date of Examination:~~

~~May It, 1971~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.~~