Alcohol Synthesis by Electrophilic Hydration Thermodynamic Control

ALCOHOL SYNTHESIS BY ELECTROPHILIC HYDRATION THERMODYNAMIC CONTROL 

So far, we have seen attack on the double bond by a proton, followed by nucleophilic attachment of its counterion to the intermediate carbocation. Can other nucleophiles participate Upon exposure of an alkene to an aqueous solution of sulfuric acid, which has a poorly nucleophilic counterion, water acts as the nucleophile to trap the carbocation formed by initial protonation. Overall, the elements of water add to the double bond, an electrophilic hydration. The addition follows the Markovnikov rule in that adds to the less substituted carbon and the OH group ends up at the more substituted one. Because water is a poor nucleophile, carbocation rearrangements can intervene in the hydration process. 

This addition process is the reverse of the acid-induced elimination of water from alcohols (dehydration, Section 11-7). Its mechanism is the same in reverse, as illustrated in the hydration of 2-methylpropene, a reaction of industrial importance leading to 2-methyl-2-propanol (tert-butyl alcohol). 

Alkene hydration and alcohol dehydration are equilibrium processes 

In the mechanism of alkene hydration, all the steps are reversible. The proton acts only as a catalyst and is not consumed in the overall reaction. Indeed, without the add, hydration would not occur alkenes are stable in neutral water. The presence of acid, however, establishes an equilibrium between alkene and alcohol. This equilibrium can be driven toward the alcohol by using low reaction temperatures and a large excess of water. Conversely, we have seen (Section 11-7) that treating the alcohol with concentrated acid favors dehydration, especially at elevated temperatures. 

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