Equilibria in hydration

The unexpectedly low K value of crotonaldehyde can be easily explained on the basis of electronic effects, excepting for the observation that its rate of hydration is considerably greater than those of crotonic acid or of propene. A mechanism involving proton attack upon the ethylenic bond would require the sequence of reactivity 

Specific acid catalysis (by H ) was found [F. G. Ciapella and M. Kilpatrick, J. Am. Chem. Soc.. 70 (1948) 639] in the hydration of isobutene general acid-catalysis was not, and for this reason it was thought that the transition state comprised only a proton and the olefin. Schubert et a P, however, showed that formic acid itself could protonate p-methoxy-a-methylstyrene, and hence that general acid catalysis occurred. 

The most reasonable explanation of the situation is to postulate a pre-equilibrium in which the carbonyl group of the aldehyde is protonated, followed by a rate-determining attack by water upon the conjugate acid, viz-  

In the hydration of mesityl oxide, the slow step in the sequence appears to be the uptake of a proton by the 2,4-dihydroxy-2-methyI-3-pentene which results from a fast pre-equilibrium hydration of the mesityl oxide. In this situation, however, the rate of the overall reaction does not vary linearly with either [H+ ] or ho at higher acidities this is in keeping with the requisites of the reaction mechanism which is neither strictly A-1 or A-2.  

In an A-1 mechanism, only the protonated substrate, SH+, is involved in the rate-determining stage, and the effective concentration of this ion, and hence the observed reaction rate, ought to depend upon ho- In an A-2 mechanism, SH undergoes attack by a nucleophile in the rate-determining stage. For the special, but common, case when this nucleophile is water, it has been shown that the rates of some acid-catalysed reactions are dependent upon [H+] (see ref. 12 for the derivation of this relationship for an A-2 process). The proposal that such a relationship is diagnostic of the A-2 mechanism has been criticised . 

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