Q9P - Organic Chemistry (Mcmurry)

Question

Show the products obtained from addition of methylmagnesium bromide to the following compounds:

1. Cyclopentanone
2. Benzophenone (diphenyl ketone)
3. 3-Hexanone

Step-by-Step Solution

Answer

a.

b. 

c.

Step 1: Grignard reaction to synthesize alcohols

Alcohols are synthesized from Grignard reagents (R-MgX). When ketones are treated with Grignard reagents, the product formed is always tertiary alcohol. The intermediate formed in this reaction is an alkoxide ion, and it further gets protonated by ${\mathrm{H}}_{\mathrm{3}}{\mathrm{O}}^{\mathrm{+}}$ .

Step 2: The addition of methylmagnesium bromide to cyclopentanone

(a)  In this reaction, methylmagnesium bromide is a Grignard reagent, and it reacts with a ketone to give tertiary alcohol as a product.

Addition of methylmagnesium bromide to cyclopentanone

Step 3: The addition of methylmagnesium bromide to benzophenone

(b)  In this reaction, when methylmagnesium bromide (Grignard reagent) is added to benzophenone, the product obtained is a tertiary alcohol. The –OH and methyl group is attached in place of the ketone group in the product.

Addition of methylmagnesium bromide to benzophenone

Step 4: The addition of methylmagnesium bromide to 3-Hexanone

c. When 3-Hexanone is treated with methylmagnesium bromide (Grignard reagent), the product obtained is a tertiary alcohol. The addition of –OH and methyl groups in place of the ketone group is seen in the product. 

Addition of methylmagnesium bromide to 3-Hexanone