Q.51. - Organic Chemistry

Question

Question: In theory, the intramolecular aldol reaction of 6-oxoheptanal could yield the three compounds shown. It turns out, though, that 1-acetylcyclopentene is by far the major product. Why are the other two compounds formed in only minor amounts? Draw a stepwise mechanism to show how all three products are formed.

Step-by-Step Solution

Answer

Answer

Mechanism of the three products of the given reaction

Step 1: Reactants of intramolecular aldol reaction

Out of the two carbonyl groups present, one carbonyl carbon with an  $\mathit{\alpha }$-hydrogen is converted into an enolate. The enolate attacks the carbonyl carbon of the other carbonyl group to form a ring.

Step 2: Stability of enolate

The enolate with a more substituted carbanionic center is more stable.

Step 3: Mechanism of the given reaction

The attack of the enolate as a nucleophile on the carbonyl carbon of another carbonyl carbon depends on the steric hindrance around the carbonyl carbon.

Upon reaction with a base, 6-oxoheptanal forms three enolates. The most stable enolate A forms 1-acetylcyclopentene as the major product. 

The stability of enolate A is because the carbanionic center is most substituted among the three enolates. In addition to this, the other carbonyl group in enolate A is less hindered, hence the attack of the nucleophile is easily possible. Overall, the product containing a five-membered ring is also very stable.  

The stepwise mechanisms for the three products are shown here below:

Mechanism of the three products of the given reaction