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Hydrolysis of Amides: a Kinetic Study of Substituent Effects on the Acidic and Basic Hydrolysis of Aliphatic Amides

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Hydrolysis of Amides: a Kinetic Study of Substituent Effects on the Acidic and Basic Hydrolysis of Aliphatic Amides University of Wollongong Research Online University of Wollongong Thesis Collection 1954-2016 University of Wollongong Thesis Collections 1969 Hydrolysis of amides: a kinetic study of substituent effects on the acidic and basic hydrolysis of aliphatic amides Grahame Leslie Jackson Wollongong University College Follow this and additional works at: https://ro.uow.edu.au/theses University of Wollongong Copyright Warning You may print or download ONE copy of this document for the purpose of your own research or study. The University does not authorise you to copy, communicate or otherwise make available electronically to any other person any copyright material contained on this site. You are reminded of the following: This work is copyright. Apart from any use permitted under the Copyright Act 1968, no part of this work may be reproduced by any process, nor may any other exclusive right be exercised, without the permission of the author. Copyright owners are entitled to take legal action against persons who infringe their copyright. A reproduction of material that is protected by copyright may be a copyright infringement. A court may impose penalties and award damages in relation to offences and infringements relating to copyright material. Higher penalties may apply, and higher damages may be awarded, for offences and infringements involving the conversion of material into digital or electronic form. Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong. Recommended Citation Jackson, Grahame Leslie, Hydrolysis of amides: a kinetic study of substituent effects on the acidic and basic hydrolysis of aliphatic amides, Doctor of Philosophy thesis, Department of Chemistry, University of Wollongong, 1969. https://ro.uow.edu.au/theses/1157 Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: [email protected] HYDROLYSIS OF AMIDES A KINETIC STUDY OF SUBSTITUENT EFFECTS ON THE ACIDIC AND BASIC HYDROLYSIS OF ALIPHATIC AMIDES by G. L. JACKSON B.Sc.(Hons). A.R.A.C.I. v-i. Chemistry Department Wollongong University College UNIVERSITY OF N.S.W. December 1969 898750 ni SUMMARY Kinetic rate constants over a range of temperature, enthalpies of activation and entropies of activation have been measured for the dilute acid hydrolysis of fourteen aliphatic amides together with similar data for the alkaline hydrolysis of thirteen aliphatic amideso These results, in conjunction with those of previous workers, indicate that the dilute acid hydrolysis of aliphatic amides is governed by a combination of steric and hyperconjugative substituent effects. The reaction series is well correlated by the Taft-type equation Log k = 1.12 ECs - 0.564(n-3) + log ko Q in which Eg denotes a pure steric substituent parameter and n is the number of alpha-hydrogens in the substituent. The results obtained for the halogeno-amides indicate that the acid hydrolysis of amides may show a slight sensitivity to polar effects. The results for the alkaline hydrolysis of a similar series of amides are well correlated by the equation log k = 1.6 7 <r* + 0.985 E°S - 0.573(n-3) + log k u 1 o e the alkaline hydrolysis is governed by a combination of polar, steric and hyperconjugative substituent effects The deviant behaviour of* the cyclic alkyl substituent in the above correlations is discussed in terms of the conformational inhibition of hyperconjugative effects in the hydrolysis TABLE OF CONTENTS Page OBJECTIVES i INTRODUCTION 3 A: Kinetic and Thermodynamic Relationships 4 B: Substituent Effects 14 C: Theory of the Taft Equations 21 D: Hydrolysis of Amides 29 EXPERIMENTAL 39 A: Preparation and Purification of Amides 40 B: Kinetic Methods 42 C: Techniques used in the Present Work 47 EXPERIMENTAL RESULTS Volume 2 DISCUSSION 66 A: Discussion of the Results of the 67 Acid Hydrolysis of Amides B: Discussion of the Results of the 96 Basic Hydrolysis of Amides C: Analysis of the Taft Substituent Parameters in Terms of the Hydrolysis 108 of Amides D: Extrathermodynamic Relationships 122 BIBLIOGRAPHY 128 ACKNOWLEDGEMENTS 134 APPENDIX 1 - Computer Programmes 135 APPENDIX 2 - Statistical Tests Employed 149 APPENDIX 3 - Publication involving the Cover Candidate Envelope 1 OBJECTIVES 2 The hydrolysis of amides is a reaction of considerable interest from both a practical and theoretical point of view. Practically, because amides may be considered as the basic units of protein molecules and a knowledge of their reactivity is clearly of considerable importance. The theoretical interest in the hydrolysis of amides lies in the mechanistic similarity of this reaction to the hydrolysis of esters, the defining reaction for the Taft'*' family of linear free energy relationships. A very extensive study of the hydrolysis of aliphatic amides has been made by Bruylants and his various co-workers. Later work by Bolton and Wilson, 3 9 4 has shown that most of Bruylants* data on the acidic hydrolysis of amides is in serious error due to an unfortunate choice of catalyst acid. A re-investigation of this important reaction is therefore justified. In the present project, the amides used have been carefully selected so as to provide the maximum amount of information on the effects of structural changes on the hydrolysis reaction in both acidic and basic solutions. 3 INTRODUCTION 4 A: KINETIC AND THERMODYNAMIC RELATIONSHIPS 5 The mechanism generally accepted Tor the hydrolysis of amides in alkaline and dilute acid solution indicates that the reaction is first order with respect to both the catalyst and the amide and thus is of overall second order. In view of this the kinetic data collected during this investigation were processed using the standard second order rate equations described below. Integrated Rate Equations Three rate equations are commonly used for the study of second order two component systemso The relative magnitude of the initial concentrations of the two components determines which equation is applicable for a specific set of kinetic data. a) Reactant Concentrations Unequal The stoichiometric equation for the hydrolysis of amides may be represented as A + B ■> Products and the reaction rate by dc -dc A B (i) dt dt 5 If a and b represent tbe initial molar concentrations of A and B respectively and x is tbe product concentration at time t tben C. = a-x and C_, = b-x A B Substitution for and in equation (l) yields dx = k(a-x)(b-x) ••••••....... ••••••••(2) dt which on integration, between the limits of t from 0 -- >t and x from 0 — ^ x, by the method of partial fractions, yields kt = 1 In f b (a-x) \ ........ .......... • • A a-b \ a(b-x) j b) Reactant Concentrations Equal If the initial concentrations of A and B are equal then equation (2) reduces to dx = k (a-x) ^ dt and on integration, between the limits described above, becomes kt x B a(a-x) c) Reactant Concentrations Marginally Different In many cases the initial concentrations of the reactants differ only by a small amount. Such kinetic data can be processed in two ways;- 6 i. By averaging the initial concentrations of the reactants and calculating rate constants using equation i.e. equal concentrations equation. ii. If the difference in the initial reactant concentrations is too large to allow accurate calculations using the "averaging technique", but too small to justify the use of equation A , then a correction procedure derived by Benson^ can be employed. For the general reaction A + B ---> Products dx = k.A.B............................ (3) dt at any time t B = B - x o = B - (A - A) o N o 7 = A + B - A o o = A + A where /\ = B - A i.e. the small difference in the ^ ^ o o initial concentrations of A and B. Substitution in equation (3 ) for B yields 7 dx k. a (a + A. ) dt k (A' * 4 -)(A’ - - t ) w h e r e A = A + A 2 “1 2 • * • dx = k A dt (4) Hence dx = k 4 ) dt •*? It ¿\± is small compared with A o then the term in brackets changes only minimally during the reaction, e.g. if /\A = Ao 1 , the term in brackets changes from 0.99 initially. 4 to O .96 after one half life. It may therefore be considered a constant when integrating and replaced by its mean value over the part of the reaction studied. The mean value (geometric mean) is F = 1 - A (5) 4a o 1 x A^1<£ where A^ 1 is the final value of A 1 measured. Substitution for F in equation (4) yields / \ 2 dx = k.F. (A j dt V ' which on integration, between the previously described limits, becomes k.F.t = 1 1 A 1 1 Ao - x A 8 or finally 1 1 k.F . t C where F has the value defined by equation (5) or, in terms of initial concentrations 2 F = 1 - ^ (2Aq + ^ ) (2Af. + ^ ) With the aid of the above rate equations it was possible to calculate rate constants for reactions in which the kinetics of the reaction were followed by chemical techniques. Correlation of Physical Properties with Concentration. For a physical property to be of use for kinetic studies it must vary linearly with concentration. For the reac tion A + B + C ------ > P where P denotes all the products let A be the value of such a physical property at time t. Then where the contribution from the medi^., and m i s A is the contribution from all the products. 9 If a linear variation with concentration is assumed then ^ $ where -0^ is a proportionality constant and A = r + * c ( c 'x) + ■ ■ Initially, i0e0 at t = o *o = + + % 8 * ^ ................
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