Fpl/Aws. Liiufuuu/ Adviser I

THE IONIZATION CONSTANTS AND ESTERIFICATION RATES OF SUBSTITUTED PHENYLPROPIOLIC ACIDS DISSERTATION Presented In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By STEWART HENRY MERRILL, B.S. The Ohio State University 1953 Approved by: /fPl/AwS. LiiUfUUU/ Adviser i ACKNOWLEDGEMENT To Dr. Melvin S. Neman -the author wishes to express his gratitude for the suggestion and guidance of this work and for inspiration which -will serve throughout his career in chemistry. \ s : : r ? 8 0 li TABLE OP CONTENTS Page I. INTRODUCTION 1 A. P u r p o s e .................................... 1 B. Plan ......... 1 C. Background ........... 1 D. Present W o r k ....................... 8 II. EXPERIMENTAL 11 A. Preparation of A d d s ....................... 11 (1) Introduction ............ 11 (2) Phenylpropiolic A c i d ................. 11 (3) p-Chlorophenylpropiolic A c i d ......... 14 (A) nr*Chlorophenylpropiolic A c i d ......... 15 (5) o-Chlorophenylpropiolic Acid 17 (6) p-Nitro^.ienylpropiolic A d d ........... 18 (7) »-Nitrophenylpropiolic A c i d .......... 21 (8) o-Nitrophenylpropiolic Acid ........... 23 (9) p-Methoxyphenylpropiolic A c i d ........ 25 (10) m-Methoxyphenylpropioli c Acid ...... 26 (11) o-Methoxyphenylpropioli c A d d ......... 28 B. Purity of the Adds ................. 30 C. Preparation of Esters ....................... 31 D. Ionisation Constants ••.••.....••• 32 E. Kinetic R u n s .............................. 33 (1) Apparatus .......................... 33 (2) Catalyst Solution ............... 34 (3) The Experimental R u n ................. 34 F. Spectra ......... 35 III. CALCULATIONS AND RESULTS 36 A. Ionization Constants ................. 36 B. Rates of Esterlflcation .............. 39 (1) Rate Constants....................... 39 (2) Activation Energies and Entropies of Activation ...................... 42 IV. DISCUSSION 45 A. Ionization Constants 45 B. Esterlflcation Rates ............. 48 C. Energies and Entropies of Activation • • • * • 54 D. Spectra ............................. 55 ill TABLE OP CONTENTS (CCNT.) V. S U M M A R Y ........................................ 60 APPENDIX ........................................... 61 AUTOBIOGRAPHY....................................... 93 iv TABLES Pag* Table I Berger*a Parameter of Sterlc Effect ................... 3 Table II Size of Sterlc Effect According to Kindler ......... • • 4 Table III Polar and Sterlc Substituent Constants for Ortho Substituents 7 Table IV Melting Points and Neutral Equivalents of Substituted Phsnylpropiollc Acids 31 Table V Physical Constants of Ethyl Esters of Substituted Phenylpropiollc Adds 32 Table VI Ionization Constants of Substituted Phenylpropiollc Acids in 35)5 Dloxane (wt.) at 2 5 * ........................ 33 Table VII Esterlflcation Rate Constants of Substituted Phenyl- propiolic Acids In Methanol Catalyzed by Hydrogen Ions (ca. 0.01N) 42 Table VIII Energi®® and Entropies of Activation for the Add- Catalyzed Esterlflcation of Substituted Phenyl— propiolic Adds 25-35* ............................... 44 Table IX Comparison of Ionization Constants of Ortho— and Para- Substituted Adds in the Phenylpropiollc and Cinnamic Series ••••••• ••...*• .................. 4 6 Table X Ultraviolet Absorption Maxima of Ethyl Esters of Phenylpropiollc Adds . • .......... 57 FIGURES Pag* Figure I Hamnett Relationship between Log k and Log K/Kq for Substituted Phenylpropiolie A d d s ............. * • 50 Figure 2 Hamnett Relationship for the Esterlflcation of Substituted Benzoic Acids ........................... 51 Figure 3 Hamnett Relationship for the Alkaline Saponification of Substituted Ethyl Cinnamates ............ 51 Figure A Scale Diagram of o-Nitrophenylpropiolic Add • • • • • 53 Figure 5 Ultraviolet Spectra of Ethyl Esters of Phenyl propiollc A d d s 56 - 1- THE IONIZATION CONSTANTS AND ESTERIFICATION RATES OF SUBSTITUTED PHENYLPROPIOLIC ACIDS I. INTRODUCTION A. Purpose The purpose of the present work was to find a benzene side chain which would be free from steric effects^" of ordinary substituents in the ortho position. A method would then be available for correlating the influence of ortho substituents with meta and para substituents on the side-chain reactivity. B. Plan Phenylpropiolic acid appeared to be a satisfactory structure for this purpose. The plan was to determine the ionization constants and the esterification rate constants of ring-substituted derivatives of this acid. steric effects could be presumed absent if a logarithmic nlot of the rate data against the ionization constants In accordance with the Hammett equation showed that the data of the ortho—substi tuted acids as vrell as the meta- and para-substituted acids obey a straight-line relationship. C. Background Thotigh steric effects in organic reactions have been recognized and studied extensively for about sixty years, few attempts have been made to separate ouantitatively the steric contribution and the polar 1. The term ITsteric effect” as used throughout this work includes all effects such as bulk interference, field effects, etc. that are due to the proximity in space of two .groups on a molecule. - 2- or purely electrical contribution which combine to influence reactivity. This is true partly because only in the last fevr years has any great progress been made in the understanding of either of these effects and partly because the number of structural types for which a suitable scheme can be devised for observing one or both of the influences separately is limited. The effect of nuclear substituents on the reactivity of benzene derivatives has been widely studied. Meta and para substituents are generally recognized to exert solely electrical effects on the reacting side chain. Ortho substituents combine a steric effect, sometimes called ortho effect, with the electrical effect. Consequently, quanti tative knowledge of either the steric effect or the electrical effect of an ortho group is difficult to obtain. In the monoraolecular hydrolysis of benzyl chloride derivatives and the alkaline saponification of substituted ethyl benzoates Berger^" assumed the polar effect of an ortho substituent to be equal to the polar effect of the same substituent in the para position and attributed the difference in reaction rates to a steric effect. He calculated a steric effect factor u» from the expression K - K o P where Kq and Kp are the rate constants of the ortho- and para-sub stituted reactants respectively. The « values for halogen deriva- 1. G. Berger, Rec. trav. chim., 46. 541 (1927)* tives are listed in Table I. The conclusion was that the steric Table I Berger1s Parameter of Steric Effect Substituent to CO Benzoyl Chloride Ethyl Benzoate Hydrolysis Saponification o—Cl 1.75 1.33 o-3r 1.75 1.98 o-I 1.66 2.92 effect of the halogens was constant for the hydrolysis of benzyl chloride while the steric effect increased with the size of the substituent in ethyl benzoate saponification. However, the con clusions deoend upon the validity of the premise that the difference in rate of the ortho- and nara-substituted reactants is due only to a steric effect, and the truth of this assumption is not known. Only when steric effects have been shown to be absent can the elec trical influence of an ortho substituent be measured for correlation with the influence of the substituent in the nara position. Kindler^ compared the alkaline saponification of substituted ethyl benzoate with the saponification of substituted ethyl cinnamates. He considered the saponification of cinnamates to be unhindered by ortho substituents and calculated a value for the steric effect in the saponification of each ortho—substituted benzoate. He defined the size of the steric effect to be the ratio k^/kj^ where is the rate constant of saponification of a substituted aromatic ester and 1 Kindier, Ann.. A6A. 278 (1928) and is the rate constant the ester would give if no steric effect were oresent. The known rate gave k^, and k^ was calculated from an emnerical relationshin as being proportional to the square of the saponification rate constant of the correspondingly substituted cinnamate. The results, Table II, nive the size of the steric effect for ortho—substituted ethyl benzoate as the ratio v v Table II Size of Steric Effect According to Kindier Substituent kh km kh^km o-F 0.272 0 .2 6 8 1 o-Gl 0.194 0.094 2 o-Br 0.461 0.093 5 o-I 0.311 0.041 7.5 o-N02 3.085 0.280 11 Kindlerfs assumntion that the saponification of ortho—sub stituted cinnamates is not subject to an ortho effect is the foundation of this treatment. This assumption was undoubtedly use ful for oualitative comparison of ortho substituents, but for a quantitative correlation it should be soundly established. Later it will be shown that ortho effects are not absent in cinnamate saoonification. Hith the advent of the Hamnett eauation^ a method became available for distinguishing the existence of steric effects not 1. L. P. Hammett, tJ. Am. Chan. Soc.. 59, 96 (1937). See also L. P. Hammett, "Physical Organic Chemistry", McGraw- Hill Book Co., Inc., New York, N. Y., 1940, n. discemable by ordinary chemical observations. The eouilibrium and rate constants of nearly all the side—chain reactions of meta— and nara-substituted benzene derivatives obey the linear relationship log k » yo log k* + A in which k and k* are two rate or equilibrium constants, or one of each, and and A are constants. The form of the equation which is generally used is log k/kQ - yo <r in which

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