14.14 : 酸ハロゲン化物からアルコールへ: グリニャール反応

In addition to lithium aluminum hydride, organomagnesium halides, known as Grignard reagents, convert acid halides to alcohols.

Unlike the first approach, where the final product is a primary alcohol, reduction with the Grignard reagent yields a tertiary alcohol.

The reaction requires two equivalents of the Grignard reagent and proceeds via a ketone intermediate.

Since the alkyl–magnesium bond is highly polar, the alkyl carbon acquires a carbanionic character and functions as a nucleophile.

The first step of the mechanism begins with a nucleophilic attack by the Grignard reagent at the carbonyl carbon, forming a tetrahedral intermediate.

In the second step, the carbon–oxygen double bond is reconstructed, and the halide ion departs as a leaving group to yield a ketone.

Next, the ketone is attacked by another equivalent of the Grignard reagent, generating an alkoxide intermediate.

Lastly, protonation of the alkoxide drives the reaction to completion, forming a tertiary alcohol as the final product.