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Kinetics and Mechanisms of Displacement Reactions Involving Trithionate Ion

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Kinetics and Mechanisms of Displacement Reactions Involving Trithionate Ion AN ABSTRACT OF THE THESIS OF RONALD DALE RITTER for the DOCTOR OF PHILOSOPHY (Name) (Degree) in CHEMISTRY (INORGANIC) presented onLit c. 16/69 (Major) Date) Title:KINETICS AND MECHANISMS OF DISPLACEMENT REACTIONS INVOLVING TRITHIOIVATE ION Abstract approved:Redacted for Privacy J.Krilegei: The kinetics of reactions between various nucleophiles and tri- thionate ion ( O3SSSO3) have been studied spectrophotometrically in 50 wt % methanol-water to determine quantitative reactivity toward divalent sulfur. The reaction of thiophenoxide with trithionate was studied in the presence of S032-and excess trithionate. SO2- 2- 3 - 2- C6H5S + S306 CHSSO + S203 65 3 The reaction is first order with respect to each reactant with a sec- and -order rate constantk2 = 1.60(±. 09) x 102 M 1sec 1 at = 0. 075 m and20. 4°. 2- -d[CHSlidt = k[CHSi[S ] 65 2 65 306 When sulfite was removed from the reaction by addition of formalde- hyde, the rate law remained the same, but the stoichiometry and the second-order rate constant changed. 2- 2- 2- 2CH + S30 C6H5SSC6H5+ S203 + S03 65S 2 1 1 k= 3.4( ±0. 3) 2 10 M sec atII = 0. 075 m and 20.4° The reaction of triphenylphosphine with trithionate 2- H2O -I-(C P +S0 C H ) PS +S032-+S042_+2H+ 65 3 36 was found to be first order with respect to each reactant with a sec.- ond-order rate constantk= 6. 1(±. 3) x 103M 1 2 M sec at = 0. 075 m andt = 20.4 °. -d[(C6H5)3P]idt = k2[S306211(C6H5)3P1 The activation parameters fork are.,Ht= 6. 9(*0.4) kcal /mole 2 and ,6,S*= -45(±2) eu. The reaction of cyanide with trithionate in basic solution 2- 2- CN +S3062-+ 20H SCN + SO3 + SO4 + H2O also is first order in trithionate and cyanide. -d[CNi/dt = k2[CNi[S3062-] 4 1sec1 k= 5. 0(±. 5) x 10M 2 atp. = 0. 15 m and20. 4°. The reaction of thioethoxide with trithionate 2CH + S062- CHSSCH5 +S2032_+S032- 25S 25 2 showed first-order behavior in both reactants with a second-order lsec 1 rate constantk= 0. 85(*. 04) M atp. = 0. 075 m and 20.4°. 2 -d[C2H5S-1/d.t = k2[C2si[s3062-] The activation parameters obtained fork were 2 H*= 6. 3(±. 3) kcal/mole and AS*= -36(±2) eu. The mechanism proposed for each reaction involves a rate- determining nucleophilic attack on the bridging sulfur in the trithio- nate anion, /SO3 n- 2 Nun + S Nu-__g_--S0I NuSSO11.- + S032- 3 3 \SO 3 SO3 followed by various fast steps depending on the particular reaction. A quantitative reactivity series toward divalent sulfur can be formu- lated since all reactions involve S displacements at one site.The N2 thiophilicity series includes two nucleophiles whose exchange reaction with trithionate had been studied earlier by Fava and co-workers. 2-> 2- C2H5S > C6H5S > (CH > CN >0 S 65)3P The relative order of nucleophiles shows that both basicity and polarizability are factors in determining their reactivity toward di- valent sulfur. Kinetics and Mechanisms of Displacement Reactions Involving Trithionate Ion by Ronald Dale Ritter A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy June 1970 APPROVED: Redacted for Privacy Associate Professor of Che try in charge of major Redacted for Privacy Chairman of the Department of Chemistry Redacted for Privacy Dean of Graduate School Date thesis is presented Typed by Clover Redfern for V Ronald Dale Ritter ACKNOWLEDGMENTS I would like to thank Dr. Krueger for his interest and direction in this work.Also, the suggestions made by Dr. J. L. Kice, Dr. B. Saville, and Dr. A. Fava were certainly appreciated.I would like to thank Dr. Yoke's research group for access to its equipment and instruments.Also, many thanks to my wife for typing the rough draft. TABLE OF CONTENTS Page INTRODUCTION 1 Nucleophilicity 1 Nucleophilic Displacement on Di-Coordinated Sulfur 5 Previous Kinetic Studies Involving Nucleophilic Displacement on Trithionate 9 EXPERIMENTAL 12 Reagents 12 Solvent 16 Kinetic Method and Temperature Control 17 Temperature Calibration 17 Thiophenoxide-Trithionate Reaction 19 Buffer System 19 Air Oxidation 20 Kinetic Experiment 20 Stoichiometry 22 Intermediate 26 Triphenylphosphine T rithionate Reaction 27 Buffer System 27 Kinetic Experiment 27 Stoichiometry 29 Cyanide - Trithionate Reaction 31 Kinetic Experiment 31 Stoichiometry 33 Thioethoxide-Trithionate Reaction 33 02 Removal 33 Buffer System 35 Stability of Trithionate in Basic Solution 36 Kinetic Experiment 37 Stoichiometry 38 RESULTS 41 Thiophenoxide-Trithionate Reaction 41 Preliminary Kinetic Studies 41 Rate Law in the Presence of Added Sulfite 43 Effect of Added Formaldehyde 48 Temperature Effect 49 Triphenylphosphine-Trithionate Reaction 52 Pseudo-Order Kinetic Studies 52 Temperature Effect 56 Page Cyanide Trithionate Reaction 58 Thioethoxide - T rithionate Re action 62 Kinetic Studies 62 Tempe rature Effect DISCUSSION 72 General Mechanism 72 Specific Mechanisms 73 Thiophenoxide 73 T riphenylphosphine 79 Cyanide 81 Thioethoxide 84 Nucleophilic Displacement by Sulfite and Thiosulfate on Trithionate 86 Relative Reactivities of Nucleophiles 88 Factors Determining Nucleophilicity Toward Trithionate 90 BIBLIOGRAPHY 97 LIST OF TABLES Table Page 1. Kinetic data for the second-order reaction of trithio- nate and thiophenoxide at p. = 0. 075 m and 20.4° with sulfite present. 46 2.Temperature dependence of the thiophenoxide-trithionate reaction. 50 3.Kinetic data for the second-order reaction of triphenyl- phosphine and trithionate at [I =. 075 mand 20.4°. 55 4.Temperature dependence of the triphenylphosphine- trithionate reaction. 57 5. Kinetic data of the second-order reaction of cyanide and trithionate. 64 6. Kinetic data for the second-order reaction of thioethoxide and trithionate at p. = 0. 075 m and 20. 4° with [OH-] = 6. 67 x 10-2M. 68 7.Temperature dependence of the thioethoxide- trithionate reaction. 70 8.Nucleophilic reactivity toward trithionate. 88 9.Second-order rate constants for nucleophilic reactions involving methyl iodide, Pt(II), and trithionate. 92 10.A comparison of basicity and nucleophilic reactivity. 92 11.A comparison of Edwards En values and nucleophilic reactivity. 94 LIST OF FIGURES Figure Page 1. Degassing chamber used on vacuum line. 34 2. First-order dependence of thiophenoxide at p. = 0. 075 m and 20.4 °. 42 3. Pseudo-first-order plots for the thiophenoxide- trithionate reaction at p. = 0. 075 m and 20. 4°. 45 4.Temperature dependence plot of the second-order rate constant for the thiophenoxide-trithionate reaction. 51 5. Pseudo-first-order plots for the triohenylphosphine- trithionate reaction at H. = 0. 075 m and 20. 4°. 54 6.Temperature dependence plot of the second order rate constant for the triphenylphosphine-trithionate reaction. 59 7. Second-order plot for the cyanide-trithionate reaction in 50 wt methanol-water at p. = 0. 15 m and 20. 4° 61 8. Second-order plot for the cyanide-trithionate reaction in water at p. = 0. 16 m and 20.4 °. 63 9.Second-order plot for the thioethoxide-trithionate reac- tion at p. = 0. 15 m and 20. 4° (Curve A). 66 10.Second-order plot for the thioethoxide-trithionate reac- tion at p. = 0. 15 m and 20. 4° (Curve B). 67 11.Temperature dependence plot of the second-order rate constant for the thioethoxide-trithionate reaction. 71 KINETICS AND MECHANISMS OF DISPLACEMENT REACTIONS INVOLVING TRITHIONATE ION INTRODUCTION Nucleophilicity A "nucleophile" is defined as a reagent which has a tendency to form a new covalent bond by sharing its electron pair (9, 34, 58). This is very similar to the definition of a Lewis base, and the two should not be confused.The term basicity is used in a thermodynamic sense, while the term nucleophilicity pertains to kinetic behavior.Swain and Scott proposed that nucleophilic reactivity be used for rate phenomena, and basicity, for equilibria in order to avoid this confusion (58). A general reaction which represents nucleophilic displacement is (1) Nu + S-X [Nu--- S- --X] Nu-S + X where Nu is the incoming nucleophile,S-Xis the substrate, and X is the leaving group.The two main factors determining this nucleophilic behavior are basicity and polarizability. A direct correlation between nucleophilic reactivity and basicity has been found in cases where, for a given group of nucleophiles con- taining the same nucleophilic atom, the structural features in the vicinity of the site of attack are similar.For example, Smith reports 2 the general base catalyzed hydrolysis of the chloroacetate ion in which the rate-determining-step involves the displacement of chloride by various nucleophilic reagents (57). (2) CICH0G- + Nu NuCH2COO + Cl 2 For 32 nucleophiles the rates of reaction increased from k= 3. 0 x108to1. 04 x103Mlsec1while the basicity in- 2 creased fromlog Kb = -1. 74 to15. 14. However, there are many cases where rates of nucleophilic reactions are much faster than basicities would predict.For instance, even in Smith's work the high reactivity of5032 -and52032 -could not be explained by basicity alone.These seem to involve primarily polarizable nucleophiles. For example, Pearson and co-workers have looked kinetically at the attack on Pt(py)2C12 by a large number of nucleophiles (53). k 2 (3) Pt(py)2C12 + Nu Pt(py)2NuCl + Cl - fast - (4) Pt(py)2C1Nu + Nu Pt(PY)Nu+ Cl 2 2 They found that the more polarizable nucleophiles were extremely re- active, and in no way could reactivities be correlated with basicity. The term "polarizable" is not completely understood, but it has been stated by Edwards and Pearson to result from "the existence of 3 low-lying excited states which, when mixed with the ground state, produce polarity" (16).In other words, the polarizable nucleophiles are those which have empty orbitals available which are low in energy.
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