Rates, equilibria, and structures in proton-transfer reactions

This chapter will be concerned mainly with the relation between the equilibrium constants of acid-base reactions and their forward and reverse rates. Relations between equilibrium constants and structure have already been considered in Chapter 6, so that the present discussion also implies relations between rates and structure. Moreover, there are many cases in which rates are easier to measure (though more difficult to interpret) than equilibria and can be compared directly with structures. We shall first consider the general basis and experimental evidence for this type of relation, followed by its molecular interpretation, with special reference to exceptional cases. We have seen in the two preceding chapters that the rates of proton-transfer reactions can be measured either directly, or indirectly through the study of acid-base catalysis, and in the following discussion information from both sources will be used indifferently. 

One generalization has already emerged (p. 130 ) about the relation between the rate and equilibrium constants for a reaction of the type 

Even lower rates are encountered for many classes of carbon acids, as already illustrated in Table 20 for reactions with solvent species. The same result follows from the very extensive information derived from the base-catalysed reactions of ketones and similar substances, as described in Chapter 9. 

We have seen in the last chapter that many catalysed reactions are determined kinetically by a single acid-base reaction between the catalyst and the substrate, and when this is so /c or k in Equation (105) is just the second-order velocity constant for this reaction. The interpretation is not quite so simple when the reaction involves two consecutive proton transfers, of which the first is effectively at equilibrium. Thus for acid catalysis we may have 

Now the second stage of the reaction is equivalent to a one-stage basic catalysis of the substrate for w hich w e can w rite the usual type 

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