Q.37. - Organic Chemistry

Question

Question: Explain why ketone K undergoes aldol reactions but ketone J does not.

Step-by-Step Solution

Answer

Answer

 

J cannot undergo aldol condensation as the intermediate enolate formation is forbidden because it has a double bond on the bridgehead carbon.

Step 1: Aldol condensation

Aldol condensation involves the reaction between 2 molecules of aldehyde in the presence of a strong base to form an  $\mathit{\alpha }\mathbf{,}\mathit{\beta }$-unsaturated ketone.

 

The reaction proceeds via the formation of an enol or an enolate, which further reacts with the other carbonyl compound.

Step 2: Mechanism of Aldol condensation

The general reaction scheme is given below.

Aldol condensation

 

The mechanism of aldol condensation is given below.

Mechanism of aldol condensation

Step 3: Comparison of two ketones

The two ketones given are shown below.

Structures of K and J

 

 

As mentioned before, aldol condensation occurs via an intermediate step where the ketones are treated with a base to form an enolate. The base attacks at the  $\alpha$-carbon position and abstracts a proton.

 

In both cases, the hydrogen that will get abstracted is shown below.

Hydrogen attached to the $\alpha$-carbon of ketones

 

It is observed that in the case of J, the acidic hydrogen is a part of the bridged carbon, and the double bond will form on the bridged C-C bond. According to Bredt’s rule, a double bond on the bridgehead or a bridged ring system is forbidden. 

 

Therefore, the enolate intermediate of J will not form.

 

On the other hand, one of the   $\alpha$-carbons of K, which is not the bridgehead carbon, gives a stable enolate intermediate where the double bond does not fall on the bridgehead or bridged ring. Hence, K can undergo aldol condensation, whereas J cannot.