Question

Draw the structure of the organic product(s) of the grignard reaction between phosgene (clcocl) and excess phenylmagnesium bromide, followed by aqueous workup.

Answer 1

This is a three step reaction.

Step 1:
In first step when Phenyl Magnesium Bromide (Grignard Reagent) is treated with phosgene it produces Benzoyl Chloride. In this step Grignard reagent act as a nucleophile while phosgene acts as electrophile.

Step 2:
The Benzoyl chloride formed in first step still has an electrophillic carbon, therefore, another grignard reagent (which is present in excess) is added to the carbonyl group and results in the elimination of chloride ion (good leaving group) and formation of Benzophenone.

Step 3:
The benzophenone (ketone) formed in second step contains a carbonyl group with electrophilic carbon. Hence, the grignard reagent is added through nucleophilic addition reaction and forms a tetrahedral intermediate, which upon hydrolysis yields triphenylmethanol (tertiary alcohol). This tertiary alcohol formed can not undergo reaction with grignard reacgent because it does not contain any electrophilic center.

Answer 2

Phosgene on reacting with phenylmagnesium bromide generates benzoyl chloride.

Since, phenylmagnesium bromide is added in excess. It would further react with benzoyl chloride to form benzophenone.

Benzophenone on further reacting with phenylmagnesium bromide, and aqueous treatment, gives triphenylmethanol.

Entire reaction pathways is shown below: