91Ó°ÊÓ

The reaction of an aldehyde or a ketone with a Grignard reagent is a nucleophilic addition reaction.

The nucleophilic Grignard reagent adds to the carbonyl carbon then it forms an alkoxide ion that is complexed with the magnesium ion. In the end addition of dilute acid breaks up the complex and we get the desired alcoholic product.

On reacting with the Grignard reagent Aldehydes give secondary alcohol and reaction of the Grignard reagent with ketone gives tertiary alcohol.

As we know, the carboxylic acids do not undergo substitution reactions at the α-carbon position because a base removes a proton from the OH group instead of from the α-carbon, because the OH group is more acidic.

If, however,a carboxylic acid is treated with PBr₃and Br₂, only then α-carbon will be brominated. This halogenation reaction is called the Hell–Volhard–Zelinski reaction.

(a) In the first step of the following synthesis it involves treatment of cyclohexanone with Grignard reagent to produce tertiary alcoholic product, it then undergoes reduction with H₂SO₄ to form methyl cyclohexene, and this undergoes epoxidation with MCPBA (meta-chloroperoxybenzoic acid) to yield epoxide, which on alkylation and treatment with water to give desired product as shown below:

(b)The first step involved in the reaction is cyclization of the dione into following ketone. This ketone on treatment with the Grignard reagent to forms an alkoxide ion. In the end addition of water forms the tertiary alcohol to give our desired product as shown below: