10.4 : 付加反応によるアルコールの調製

Alcohols can be prepared through the reaction of alkenes with water using three synthetic routes: acid-catalyzed hydration, oxymercuration–demercuration, and hydroboration–oxidation.

In the acid-catalyzed hydration of alkenes, a proton is added to the less-substituted carbon in the double bond, forming a more stable carbocation. Subsequently, hydroxyl is added to the more substituted end of the double bond, which follows Markovnikov's rule.

Since a trigonal planar carbocation intermediate is involved in the reaction pathway, the addition of the water molecule on either face is equally probable, and a racemic mixture of alcohols is formed.

Alcohol synthesis via oxymercuration–demercuration of alkenes does not have a conventional carbocation intermediate. Instead, the nucleophilic attack of the double bond on the positively-charged mercury acetate creates a three-membered mercurinium ion intermediate, which can be considered a resonance hybrid with minor carbocation character.

The subsequent hydration of the mercurinium ion is consistent with Markovnikov's rule. Following proton transfer, sodium borohydride replaces the mercury species with a proton to give the final product — alcohol.

Alcohols can also be synthesized via the hydroboration–oxidation reaction. Here, borane adds to the double bond of an alkene to form an alkyl borane transition state.

Boron acquires a partial negative charge, and the carbon on the opposite end acquires a partial positive charge. Due to the steric effects, borane addition occurs at the less substituted carbon, which also stabilizes the partial positive charge on the more substituted carbon, forming an alkylborane.

Further, two more alkene molecules sequentially add to give a trialkyl borane, which upon further oxidation with hydrogen peroxide and sodium hydroxide results in an anti-Markovnikov product.