Q22-61E - Organic Chemistry (Mcmurry)

Question

Attempted Grignard reaction of cyclohexanone with tert-butyl magnesium bromide yields only about 1% of the expected addition product along with 99% unreacted cyclohexanone. If D₃O⁺ is added to the reaction mixture after a suitable period, however, the “unreacted” cyclohexanone is found to have one deuterium atom incorporated into it. Explain.

 

Step-by-Step Solution

Answer

The compound tert-butyl magnesium bromide is a bulky group. It gives only about 1% of the expected addition product along with 99% unreacted cyclohexanone.

Step 1: The mechanism for the formation of 99% unreacted cyclohexanone

The alpha-carbon atom of the cyclohexanone ring reacts with tertiary butyl magnesium bromide and forms the respective enolate. On deutyrolysis, the alpha-deutero ketone is formed.

 

                                                                  Mechanism of the reaction

Step 2: The large size of the attacking species

The tert-butyl magnesium bromide is a bulkier group; it is very difficult to attack at the carbonyl position of the reactant since the bulkier group causes steric hindrance in the compound, making it an unstable product hence giving only a 1% addition product.

But when D₃O⁺ has been added, it gives a stable desired product because it is small in size and can attack carbonyl carbon tert-butyl magnesium bromide.

Hence give 99% additional product.