Hydration of Alkenes

The hydration reaction of alkenes is the net addition of water across the double bond, resulting in an alcohol.

There are a few ways to perform the hydration of alkenes.

Acid-catalyzed hydration

Water reacts too slow to react with the alkene on its own, but we can use an acid catalyst to speed it up.

The mechanism involves electrophilic addition of a proton from the acid to form a carbocation intermediate. This happens using Markovnikov's rule of deprotonation occurring at the least substituted carbon to form the most stable carbocation intermediate.

In the next step, an oxonium ion is formed through the addition of water. And a simple deprotonation gives an alcohol as the product.

The proton in the oxonium intermediate can be deprotonated by any base present, including the conjugate base of the acid used as a catalyst. Deprotonation can even be by another alkene molecule, which would generate another carbocation intermediate and propagate the chain mechanism.

Oxymercuration

Transition metals can also aid the addition of water to alkenes by breaking the double bond. A common compound is mercuric acetate Hg(OAc)2. The coordination of the metal to the alkene, leaving it susceptible to the nucleophilic attack from water.

The transition metal behaves the same way as the acid catalyst. The alkene undergoes electrophilic addition, forming a bridged cyclic structure known as a mercurinium ion.

The water attacks to form a hydroxyalkyl mercury complex and lastly, demercuration occurs via reduction with sodium borohydride.

Hydroboration

A process involving addition of a B–H bond of borane, BH3, to an alkene to yield an organoborane intermediate, RBH2.

The BH2 group is then replaced by an OH group using hydrogen peroxide.