The mechanism of this reaction is usually depicted to consist of the following steps-

•  Complexation of the organomagnesium species with the substrate (Scheme 2).

Scheme 2

• The next step involves nucleophilic attack of organic moiety of Grignard reagent on the electron deficient carbon of carbonyl group via a molecular complex (Scheme 3).

Scheme 3

• The intermediate formed in the above step is hydrolyzed to give a tertiary alcohol. Alternatively, if the carbonyl group is attached with a leaving group (i.e., if R ' = OR) then the tetrahedral adduct can break down to produce a C=O group that can proceed a fast second addition step to afford tertiary alcohol (Scheme 4).

Scheme 4

A number of methods have been developed to stop the reaction at the aldehyde or ketone stage. Such protocols involve the formation of a masked carbonyl compound that releases the desired compound on hydrolysis (Scheme 5).