91Ó°ÊÓ

Grignard reagent react with the carbonyl (C=O) carbon in aldehydes or ketones to give alcohols. The substituents on the carbonyl determine the nature of the product alcohol. Formaldehyde gives primary alcohols, aldehydes give secondary alcohols and ketones give tertiary alcohols.

The presence of acidic proton assures the formation of desired alcohol from an intermediate metal alkoxide.

Mechanism:The nucleophilic C of the organometallic reagent is added to the electrophilic carbon of the carbonyl group.

Electrons from the carbon of carbonyl, move to the electronegative O generating an intermediate metal alkoxide complex. Then protonation of the alkoxide oxygen creates an alcohol product.

Nucleophilic Addition of Grignard- reagent

Hydrogen cyanide is added to aldehydes and ketones forming cyanohydrins.

In the first step, the cyanide ion acts as a nucleophile and forms a C-C bond with the electrophilic carbon of the carbonyl group, the electrons of the pi bond between the carbonyl carbon and oxygen are shifted onto oxygen forming an alkoxide ion intermediate.

In the second step, the alkoxide ion is protonated by HCN which regenerates the cyanide ion.

Mechanism

Nucleophilic Addition of HCN

Aldehydes are reduced with sodium borohydride ${\mathbf{NaBH}}_4$to give primary alcohols, while ketones are reduced to give secondary alcohols.

Hydride ions from sodium borohydride act as a nucleophile.

Mechanism

Nucleophilic Addition of Sodium borohydride

As already discussed, Grignard reagent react with the carbonyl (C=O) carbon in aldehydes or ketones to give alcohols.

Nucleophilic Addition of Grignard- reagent

All these reaction are nucleophilic addition reactions where the nucleophilic part of the reagent adds to the electrophilic carbonyl carbon to form an intermediate and then occurs protonation.