Frequently Asked Question

There are actually only three possible aldol condensation products when acetone and propanal are mixed in the presence of base without dehydration. Here's a mechanistic explanation:
Step 1: Enolate Formation
Both acetone and propanal can form enolates in the presence of base. The enolate of acetone is more stable due to the electron-donating effect of the two methyl groups. The enolate of propanal is less stable due to the electron-withdrawing effect of the aldehyde group. Therefore, acetone will preferentially act as the nucleophile in the aldol reaction.
Step 2: Aldol Condensation
The enolate of acetone attacks the carbonyl group of propanal, forming an aldol product. This product is a β-hydroxyaldehyde.
Possible Products
Since acetone is symmetrical, there's only one possible enolate. However, propanal has two different alpha hydrogens. Thus, the enolate of acetone can attack propanal on either side, forming two different aldol products:
1. Product 1: The enolate of acetone attacks the carbonyl group of propanal at the alpha carbon next to the aldehyde group.
2. Product 2: The enolate of acetone attacks the carbonyl group of propanal at the alpha carbon next to the methyl group.
Note: Since propanal can't enolize on the side of the aldehyde group, there is no possibility of a third product where propanal acts as a nucleophile and acetone as an electrophile.
Mechanism:
The following is a general mechanism for the aldol condensation:
[Mechanism Image]
Overall, the three possible aldol condensation products, without dehydration, are:
1. 3-Hydroxy-2-methylpentanal: Formed when the enolate of acetone attacks the carbonyl group of propanal at the alpha carbon next to the aldehyde group.
2. 4-Hydroxy-2-methylpentanal: Formed when the enolate of acetone attacks the carbonyl group of propanal at the alpha carbon next to the methyl group.
3. 4-Hydroxy-3-methylhexanal: Formed when the enolate of acetone attacks the carbonyl group of propanal at the alpha carbon next to the methyl group (this product forms when propanal acts as a nucleophile, but it's less likely to form due to the less stable enolate).