A Mathematical Model for Concerted Proton Transfer in the Hydrolysis of Ethylphenylbenzaldehyde Acetals

Abstract

The hydrolysis oF seven ethyl substituted phenylbenzaldehyde acetals, () CR = C2H5, X = H), has been examined in details ; the catalytic constants of six buffer acids (pKa ranging -From 2.6 to 8) as well as those of hydronium ion and water have been measured. A good Brnsted correlation is obtained for all compounds, the a coefficient decreasing regularly from 0.7 to 0.5 when the basicity of the leaving group increases. On the contrary the Hammett plots obtained for each acid are bad, excepted those corresponding to hydronium ion and water. A model of concerted proton is presented which extends and puts in a mathematical form the models previously proposed by Jencics, Kreevoy and Guthrie. With this model we can demonstrate that : i) the hydrolysis of a given compound can be either a specific-acid or a generalacid catalysed reaction depending upon the strength of the catalyst, ii) The a Brnsted coefficient is not a measure of the 'degree of proton in the transition but is rather a measure of the of of the C OAr bond, iii) It is possible to predict the kind of catalysis which a reaction will be submitted to when we snow the two rate constants, kc and kH+; of the spontaneous' and of the hydronium ion catalysed hydrolyses. INTRODUCTION Acid and base-catalysed reactions, that is reactions involving a proton between the substrate and the external medium -, are of the utmost importance in organic chemistry and biochemistry. Since the very beginning of physical organic chemistry searchers in this field have been busy trying to find out the intimate mechanism of this proton particularly in those reactions involving the formation and the decomposition of tetrahedral intermediates. Broadly speaking we can distinguish two classes of acid and/or base-catalysed reactions those in which a proton in transferred to or from a carbon atom of the substrate, such as the hydration of alkenes, the enolization of ketones, etc... -, and those in which the proton takes place between two electronegative atoms of the substrate and of the catalyst. In this paper I'll deal only with this second class of reactions, more specifically with acid-catalysed reactions in which the proton is transferred from the catalyst to the substrate. Brnsted (1) has, many years ago, subdivided these reactions into two sub-classes, the general-acid catalysed and specific-acid catalysed reactions, depending upon whether the concentration of the weak acid catalyst appears or not in the rate law it is generally acknowledged that the kind of catalysis actually occurring in a reaction process depends only on the substrate : a given compound will always react with specific-acid or general-acid catalysis whatever the solvent and the acid catalyst. The specific-acid catalysis has usually been interpreted as indicating that the proton occurs in a rapid and reversible step and is followed by a ratelimiting decomposition of the conjugate acid of the substrate to give, in the case of acetals for exemple -, an oxocarbonium ion (Mechanism Al). On the other hand, in its classical interpretation the general-acid catalysis implies a slow pmton during the rate-determining step of the reaction to give the oxocarbonium ion and the conjugate base of the catalyst (Mechanism a The well-known Brnsted catalysis law, kA = G(KA) , where kA is the specific rate constant for catalysis by the acid whose acid dissociation constant is KA correlates the effectiveness of acids as catalysts with their acid strength. Many evidences have been accumulated, since it was first proposed, in support of its essential validity and it is now generally admitted that all reactions which show general 1837 1838 G. LANATY and C. MENUT acid catalysis must conform to the Brnsted relation. Besides its use as a means to make predictions of rate constants the Brnsted relation has received a widespread acceptance as an indicator of reaction mechanism, the most interesting aspect of this later application being that the Brnsted exponent m provides information about transition state structure the numerical value of the exponent m, always between 0 and 1, is usually said as being equal to the extend of proton transfer at the reaction transition state (for a review of current ideas on this topic see Kresge, Ref. 2). The purpose of this article is to show i) that the clear-cut distinction between the two types of catalysis is not necessarily imposed by the nature of the substrate and that systems can exist, at least the system we have in hands -, which belong to one kind or the other of mechanisms depending upon the circumstances, ii) that the Brnsted exponent a has nothing to do with the degree of proton at the transition it is rather a measure of the easiness of cleavage of the C — 0 bond which is broken during the slow step of the reaction.