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Kinetic and Thermodynamic Acidities of Selected Weakly Acidic Hydrocarbons

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Kinetic and Thermodynamic Acidities of Selected Weakly Acidic Hydrocarbons Scholars' Mine Doctoral Dissertations Student Theses and Dissertations 1974 Kinetic and thermodynamic acidities of selected weakly acidic hydrocarbons Darrell R. Strait Follow this and additional works at: https://scholarsmine.mst.edu/doctoral_dissertations Part of the Chemistry Commons Department: Chemistry Recommended Citation Strait, Darrell R., "Kinetic and thermodynamic acidities of selected weakly acidic hydrocarbons" (1974). Doctoral Dissertations. 51. https://scholarsmine.mst.edu/doctoral_dissertations/51 This thesis is brought to you by Scholars' Mine, a service of the Missouri S&T Library and Learning Resources. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact [email protected]. KINETIC AND THERMODYNAMIC ACIDITIES OF SELECTED WEAKLY ACIDIC HYDROCARBONS by DARRELL REX STRAIT, 1942- A DISSERTATION Presented to the Faculty of the Graduate School of the UNIVERSITY OF MISSOURI - ROLLA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY in CHEMISTRY 1974 /:1. 0, \Mi=D~ v/ I( A This dissertation is dedicated to my wife and our parents. iii ABSTRACT The kinetic and thermodynamic acidities in methanol for bromoform and pheny1acetylene have been determined at various temperatures. The thermodynamic acidities of phenylacetylene were obtained through a quench method which involved tritium tagging of the phenylacetylene anions present at equilibrium. Hith the use of this method,. a pKa of 18.34 for phenyl acetylene at 0°C was obtained. Through the use of these acidities, it was shown that the kinetics for these proton exchange processes are not diffusion controlled. Isotope effects (kr/k0 ) were obtained for chloroform, bromoform and phenylacetylene at three temperatures (0°C, -l0°C and -20°C). These isotope effects at 0°C are 1.25, 1.07 and 0.92 respectively. The inverse isotope effects for phenylacetylene were explained as being due to the formation of stronger bonds in the transition state than are being broken in the ground state of the reactants. These isotope effects were used to calculate a Br¢nsted a of 0.55 for both phenyl­ acetylene and the haloforms. Through the use of the Br¢nsted relationship and these a•s, a pKa of 19.73 was determined for bromoform at 0°C. As a natural consequence of kinetic studies at various temperatures, activation energies of 18.91 Kcal/mol for phenylacetylene and 19.23 Kcal/mol for bromoform were obtained. These activation energies reinforced the conclusion that the kinetics in this study are not diffusion controlled. iv ACKNOWLEDGEMENTS In the pursuit of this goal~ I have relied on the friendship, knowledge~ experience, understanding and kindness of a great number of people. To list all of their names and contributions would be a dissertation in itself. I apologize to those whose names have not been included. Although unlisted, they are not forgotten and have my thanks for all of their help and encouragement. I wish to express my gratitude to Dr. James 0. Stoffer for the many hours spent in helping make this dissertation a reality. To Dr. Stoffer also goes a tremendous thanks for also being a friend and a person to whom communication was an easy task. To my wife, Susan, I am deeply indebted and wish to thank her for her companionship, love, help, and encouragement. Without these and her, it would have been impossible to accomplish or even attempt to accomplish this goal. For providing the financial support while at the University of Missouri-Rolla, I wish to thank Dr. William H. Webb and the Department of Chemistry for the Teaching Assistantships. I also thank the University of Missouri-Rolla and Dean Robert F. McFarland for summer assistantships in 1972 and 1974. Finally, special recognition and thanks is extended to Mr. and Mrs. Rex E. Strait and r1r. and Mrs. Earl S. Kennett for the encourage­ ment and help they have most lovingly given. They have many times reinforced the thought that they are among the "Greatest Parents in the World". v TABLE OF CONTENTS Page ABSTRACT ...................... ,. •.•. ., .............................. iii ACKNOWLEDGEMENTS. • . • • • . • . • • • . • . • . • . • . • . • . • . • . • . • . • . • . i v LIST OF FIGURES .•.•••••.•.•.•.•.•••.•.•.•.•.•.•••.• '· .•.•...•... viii LIST OF TABLES.................................................. ix I. INTRODUCTION.............................................. 1 I I. HISTORICAL................................................ 8 A. ACID-BASE CHE~USTRY OF WEAKLY-ACIDIC HYDROCARBONS..... 8 B. HALOFORM KINETIC AND THERMODYNAMIC ACIDITY ~.. 1EASUREMENTS. • • . • . • • • • • . • . • . • . • . • . • . • . • . • . • . • . • . 13 C. PHENYLACETYLENE KINETIC AND THERMODYNAMIC ACIDITY t~EASUREMENTS.................................. 15 D. SELECTED ACIDITY RESULTS IN METHANOL.................. 17 E. EQUILIBRIUM QUENCH METHODS............................ 22 F. SCINTILLATION COUNTING................................ 23 III. RESULTS AND DISCUSSION.................................... 24 A. EXPERIMENTAL.......................................... 24 1. Compounds and Solutions........................... 24 a. Chloroform .•.•...........•.•.•...•...........• 24 b. Chloroform-d.................................. 25 c. Bromoform •... ..•.•.....•.•.•......... e........ 25 d. Bromoform-d................................... 25 e. Phenyl acetylene............................... 25 f. Phenylacetylene-d.......... •.•.•. ..•.•. .. ... 26 g. Anhydrous Methano 1............................ 26 h. Toluene....................................... 26 vi Table of Contents (continued) Page i. Scintillation Solution.......................... 26 j. Sodium Methoxide Solution....................... 27 k. Benzene Sulfonic Acid Solution.................. 27 1. Tritiated Water................................. 27 2. Kinetic and Thermodynamic Procedures................ 27 a. Kinetic Baths................................... 28 b. Temperature Determination....................... 28 c. Radioactivity Determination..................... 29 d. Ex traction Process. • . • . • . • . • . .. ... 30 e. Miscellaneous................................... 32 f. Kinetic Runs.................................... 33 g. Equilibrium Quench Runs......................... 37 3. Calculations........................................ 40 a. Treatment of Raw Data from the Scintillation Counter ................................. ~········ 40 b. Least Squares Rate Constants.................... 41 c. Isotope Effects........................ .. .. .. 43 B. THEORY.................................................... 43 1. Kinetics............................................ 43 2. Primary Isotope Effects............................. 53 3. Equilibrium Quench Runs............................. 55 4. Equilibrium Isotope Effects......................... 57 C. RESULTS AND CONCLUSIONS................................. 59 1. Kinetic I so tope Effects............ • . .. .. .. .. 59 2. Kineti~cs ...•.•...•... e·•···························· 74 vii Table of Contents (continued) Page 3. pKa Va1ues from Quench Equilibrium Runs............. 81 4. pKa Determination from Br~nsted Relationship........ 83 5. Activation Parameters............................... 89 6. Equi 1 i bri urn Isotope Effects......................... 90 7. Base Catalyzed Decomposition of Hydrocarbons........ 93 I Vo SUMMARY • • • • • • • • • • . • • • . • • • • • . • • • • • • • .. • .. • .. • • o • • • , .. • • • • • • .. .. • • 9 5 BIBLIOGRAPHY ...................... 0......................... .. ... 96 VITA ...• Qo·············•·o···························· .. o·•······· 103 APPENDICES.... • . • . • . • . • . • . • .. • . • .. • . • . • . • . • . • . • . • . • . • . .. .. • . .. .. 104 CALCULATION......... • • • • • . • • • • • • • . • • • • • • • • • • . • . • . • 104 A. C00 B. EQUILIBRIUM QUENCH RUN CALCULATIONS •.•.•••...••....•.... 107 C. CALCULATION OF UNCERTAINITY (PROBABLE ERROR)............ 111 D. ABBREVIATIONS........................................... 112 viii LIST OF FIGURES Figure Page 1. Cross section of the kinetic assembly....................... 35 2. Experimental setup for a quench run......................... 39 3. Potential energy diagram for thesis reaction, showing the effect of changing from R-H to R-D.............. 68 ix LIST OF TABLES Table Page I. Relative acidities of some selected acids in various solvents......................................... 4 II. Kinetic rate data for selected haloforms .•...•.•......... 13 III. Combined kinetic data for some haloforms .•.•.•........... 15 IV. Summary of acidity results for phenylacetylene .......... 16 V. DPM's for normal set of washings vs. rewash ...•...•...... 32 VI. Data and results from bromoform kinetic run at -20.0°C (DRS 52) .•••...•.•.....•.•.•.•••.•.•.....•.... 42 VII. Results of phenylacetylene isotope effects runs ......•... 60 VIII. Results of bromoform isotope effects runs •...•.•......... 61 IX. Results of chloroform isotope effects runs •.•...•........ 62 X. Normalized slopes and isotope effects of hydrocarbons • .•.......................................... 63 XI. I R stretching frequencies .•.•.•.•.•.•.•.•.•.•.•.•.•...... 70 XII. Calculated AVt values obtained from equation (27) using kH/k 0(exp.) and 6vR from Table XI •.•.•.•.•.....•... 72 XIII. Isotope effect results from other authors ..•.•...•.•..... 73 XIV. Phenylacetylene kinetic results ...•.•.•.•.•.•.•.....•.... 75 XV. Bromoform kinetic results
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