Question

Aldol condensation cannot occur between (a) two aldehydes (b) an aldehyde and a ketone (c) two ketones (d) an aldehyde and an ester

Short answer

Answer: An aldehyde and an ester.

Step-by-step solution

Option (a): Two aldehydes

Two aldehydes could potentially undergo Aldol condensation because both have a carbonyl group and at least one α-hydrogen. Thus, this option is not the correct answer.

Option (b): An aldehyde and a ketone

Both aldehydes and ketones have carbonyl groups, and they usually have α-hydrogens. Therefore, an aldehyde and a ketone can also undergo Aldol condensation. Thus, this option is not the correct answer.

Option (c): Two ketones

Similar to aldehydes, ketones also possess carbonyl groups and α-hydrogens, so they can undergo Aldol condensation as well. Thus, this option is not the correct answer.

Option (d): An aldehyde and an ester

An aldehyde contains a carbonyl group and α-hydrogens which would make it capable of undergoing Aldol condensation. However, esters generally do not have α-hydrogens because the carbonyl group is attached to an -OR group. Consequently, esters cannot generate the required enol or enolate ion during an Aldol reaction. Therefore, the correct answer is option (d).

Key concepts

Carbonyl Group

At the heart of many organic reactions, including aldol condensation, is the carbonyl group. This functional group consists of a carbon atom double-bonded to an oxygen atom, represented as \( \text{C}=\text{O} \). The carbonyl group is highly polar, meaning it has a significant difference in electronegativity between the carbon and the oxygen.
This polarity makes the carbon atom electrophilic, attracting nucleophiles to attack it. In aldol condensation, this reactivity is crucial as the carbonyl group facilitates the formation of a new carbon-carbon bond.
The presence of the carbonyl group in aldehydes and ketones is a key reason they can participate in aldol reactions. The versatility of the carbonyl group contributes to its importance in synthetic chemistry, forming the backbone of many molecules involved in industrial and biological processes.

Alpha Hydrogen

For aldol condensation to occur, it's not just the carbonyl group that's important, but also the presence of alpha hydrogens. Alpha hydrogens are the hydrogen atoms attached to the carbon adjacent to the carbonyl carbon, known as the alpha carbon. These hydrogens are relatively acidic due to the electron-withdrawing nature of the carbonyl group.
In the process of aldol condensation, these alpha hydrogens can be easily removed to form a carbanion or an enolate ion. This enolate ion then acts as a nucleophile, attacking another carbonyl carbon, resulting in the formation of a new carbon-carbon bond.
Having at least one alpha hydrogen is essential for the mechanism of aldol reactions, as it allows for the formation of the enol or enolate intermediate. Without alpha hydrogens, as seen in esters in the given exercise, the necessary intermediate cannot form, thus preventing the condensation reaction.

Aldehydes and Ketones

Aldehydes and ketones are two classes of carbonyl-containing compounds that frequently participate in aldol condensation. Both contain the carbonyl group, but their structures differ:

• Aldehydes have at least one hydrogen atom attached to the carbonyl carbon, making them highly reactive.
• Ketones have two alkyl or aryl groups attached to the carbonyl carbon. They are generally less reactive than aldehydes.

Both aldehydes and ketones usually possess alpha hydrogens, enabling them to form enolates or enols. This ability facilitates their participation in aldol condensation through aldol addition, where a new carbon-carbon bond is made.
Interestingly, despite their reactivity, ketones are less prone to self-condensation compared to aldehydes. Nonetheless, the presence of both a carbonyl group and alpha hydrogens means that both types of compounds can engage in these important reactions, provided they are mixed with other appropriate reactants, explaining why combinations of both are often used in synthetic applications.