Chemistry - Carbonyl compounds Question with Solution | TestHub

The given reaction is an aldol condensation between 2,2-dimethylcyclopentanone and benzaldehyde in the presence of a base (NaOH) and heat.

First, we identify the possible enolates. 2,2-dimethylcyclopentanone has only one enolizable $\alpha$-carbon (C5), as the other $\alpha$-carbon (C2) is blocked by two methyl groups. Thus, the enolate will form exclusively at the C5 position.

Next, we consider the possible condensation products:

1. Crossed Aldol Condensation: The enolate attacks the carbonyl carbon of benzaldehyde. Subsequent dehydration (loss of water) forms a double bond between the C5 carbon of the cyclopentanone ring and the benzylic carbon. This yields an $\alpha ,\beta$-unsaturated ketone, 2-benzylidene-5,5-dimethylcyclopentanone. Both $E$ and $Z$ geometric isomers can be formed, which correspond to the structures in Option (1) and Option (3).

2. Self-Aldol Condensation: The enolate of 2,2-dimethylcyclopentanone can also attack the carbonyl carbon of another molecule of 2,2-dimethylcyclopentanone. The intermediate aldol addition product will have a hydroxyl group on the former carbonyl carbon of the electrophile.

For dehydration to occur under basic conditions (E1cB mechanism), an acidic $\alpha$-proton must be removed. The only available $\alpha$-proton is on the C5 carbon of the nucleophilic ring. Removal of this proton and the hydroxyl group leads to an exocyclic double bond, forming an $\alpha ,\beta$-unsaturated system. This corresponds to the structure in Option (4).

Option (2) shows a product with an endocyclic double bond in the second ring. This is a $\beta ,\gamma$-unsaturated ketone. Forming this product would require the removal of a proton that is not $\alpha$ to any carbonyl group, which is not acidic enough to be removed by NaOH. Furthermore, aldol condensation is driven by the thermodynamic stability of the resulting conjugated $\alpha ,\beta$-unsaturated system.

Therefore, the unconjugated product in Option (2) cannot be obtained.

Answer: