Modèle mathématique de transfert concerté de proton dans l'hydrolyse d'acétals alkylarylés dérivés du benzaldéhyde

Abstract

AbstractThe catalytic constants of six acids (pka ranging from 2. 6 to 8) as well as those of hydroxonium ion and water have been measured in the hydrolysis of six ethyl‐substituted phenylbenzaldehyde acetals.A good Brønsted correlation is obtained for all compounds, the α coefficient decreasing regularly from 0.7 to 0.5 with the basicity of the leaving group. 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 transfer is presented which extends and puts in a mathematical form those previously proposed by Jencks, 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 general‐acid‐catalysed reaction depending upon the strength of the catalyst, (ii) the α Brønsted coefficient is not a measure of the “degree of proton transfer in the transition state” but is rather a measure of the easiness of cleavage of the C‐OAr bond.