Hydration of Carbonyl Structures

Bronsted a values for a few general-acid-catalyzed reactions are given in Table 10.1. Our discussion in Section 9.3.5 showed that the a value is interpreted as the extent of protonation of the reactant by the acid in the transition state. The fact that the Bronsted plots are linear for carbc nyl hydrations means that the extent of protonation at the transition state remains the same regardless of the strength of the acid. 

A linear Bronsted plot, however, seems counter to the Hammond postulate. As the reaction becomes more exothermic with stronger acids, the transition state should shift toward the structure of the reactant. So, why is the extent of proton transfer in the transition state of a carbonyl hydration invariant Actually, this question is relevant not just to hydration reactions, but also to all reactions where linear Bronsted plots are found. 

A More O Ferrall-Jencks plot of hydration of a carbonyl compound. As the acid becomes stronger, the transition state has less nucleophilic attack (A to B), and the a value remains the same. 

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