Polar effects of substituents on the acid- and base- catalysed enolisation of benzyl phenyl ketone (1960)
AuthorsFischer, A.show all
Substituent effects on both acid- and base-catalysed enolisation of ketones are not yet well understood. A survey of kinetic and some preparative evidence recorded in the literature shows that replacement of an α-hydrogen by a substituent is accompanied by several effects. On the basis of preparative evidence it has previously been postulated that α-alkyl substituents stabilise, by hyperconjugation, the developing enolic double bond in the transition state of acid-catalysed enolisation. However, recorded kinetic results appear to be in conflict with this suggestion. To obtain further information on the importance of conjugative effects, the kinetics of acid- and base-catalysed enolisation of substituted benzyl phenyl ketones have been followed. In this system substituent effects are limited to the electronic effects (a) the inductive effect, and (b) the conjugative effect between developing enolic double bond and substituent. From the results it is concluded that-M substituents conjugated with the developing enolic double bond stabilise the transition state of both acid- and base-catalysed reactions. There is no observable stabilisation of the transition state by electron-donating substituents through a +M effect. The dissociation constants of the conjugate acids of the ketones have also been determined. Using these dissociation constants and the rates of acid-catalysed enolisation of the ketones, the rates of deprotonation (to enol) of the conjugate acids have been determined. Rates of deportation of other conuate acids have also been evaluated from recorded data. Both deprotonation of conjuage acid and of ketone are aided by electron-withdrawing (-I) substituents and hindered by electron-donating (+I) ones.