The Molecular Mechanism of Acid-Base Catalysis

The Molecular Mechanism of Acid-Base Catalysis 1. The General Nature of Acid-Base Catalysis 

Early views on the nature of catalysis regarded it as an indefinite influence of some kind. Somewhat later a rather more definite picture was formed of catalysis by the hydrogen ion (regarded as a bare proton), which was supposed to attract the reactants together in virtue of its powerful electric field. This explanation did not seem especially appropriate to hydroxyl ion catalysis and obviously would not apply to catalysis by uncharged molecules. 

These early views envisaged reactions which could take place in the absence of a catalyst, but which were facilitated by its presence. Evidence gradually accumulated to show that many of the reactions subject to acid-base catalysis could not take place at all in the complete absence of catalysts, apparently spontaneous reactions being often due to catalysis by acidic or basic solvent molecules, or by some adventitious acidic or basic impurity. This seemed to indicate that the catalyst took a fundamental part in the reaction, possibly in a chemical sense. It was soon realized that the essential property of acids and bases was their power respectively to lose and to add on a proton, and enquiry also showed that substrates involved in acid catalysis could always be supposed to have some basic properties, while those in base-catalyzed reactions could always in principle act as acids, though the acid-base properties of the substrates were often so weak as to elude detection by ordinary means. This led to the suggestion that acid-base catalysis always involves an acid-base reaction between the catalyst and the substrate. Such a reaction is also often termed a protolytic reaction, since it involves the transfer of a proton between the two reacting species. 

This view of acid-base catalysis is now generally accepted, and specific mechanisms have been proposed for a large number of types of reaction. For the purpose of illustration a few of these are given in the next subsection, but no attempt has been made at completeness. In many instances the mechanism involves two successive proton transfers, and it may be a matter of some difficulty to decide the relative rates of the two successive steps. This question is considered in Sec. III.3. 

We must now consider the mechanism by which acids and bases can effect the transformation of a keto to an enol form and in doing so shall have occasion to modify slightly the enolization view of halogenation. All compounds containing a carbonyl group have some basic properties. 

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