Which of the following does not give Cannizzaro reaction? 

Acetaldehyde 

The correct answer is option 4. Acetaldehyde.

Let us dive deeper into why acetaldehyde does not undergo the Cannizzaro reaction compared to the others.

Cannizzaro Reaction Overview

The Cannizzaro reaction is a disproportionation reaction where a non-enolizable aldehyde (without α-hydrogens) is both oxidized and reduced in a basic solution. One molecule of aldehyde is oxidized to a carboxylate salt, while another is reduced to the corresponding alcohol.

Reaction Conditions and Characteristics:

Non-enolizable Aldehydes: Aldehydes that lack α-hydrogens are more likely to undergo the Cannizzaro reaction. These aldehydes cannot form enolates (intermediates required for other reactions like aldol condensation) because they do not have α-hydrogens to lose.

Enolizable Aldehydes: Aldehydes with α-hydrogens can form enolates, making them prone to aldol condensation rather than the Cannizzaro reaction.

Analysis of Each Aldehyde:

1. 1-Methylcyclohexanecarbaldehyde:

This aldehyde is non-enolizable because it lacks α-hydrogens (no hydrogen atoms on the carbon adjacent to the aldehyde group).  It can undergo the Cannizzaro reaction because it fits the criteria of a non-enolizable aldehyde.

2. Benzaldehyde:

Benzaldehyde has no α-hydrogens; the aldehyde group is directly attached to a benzene ring. It readily undergoes the Cannizzaro reaction due to its lack of α-hydrogens.

3. Formaldehyde:

The simplest aldehyde with the formula \(\text{HCHO}\), it lacks α-hydrogens. It undergoes the Cannizzaro reaction because it is non-enolizable.

4. Acetaldehyde:

Acetaldehyde (\(\text{CH}_3\text{CHO}\)) has α-hydrogens (the hydrogens on the carbon adjacent to the aldehyde group). The presence of α-hydrogens means acetaldehyde can form enolates and participate in aldol condensation reactions. Aldol condensation is favored over the Cannizzaro reaction in this case.

Why Acetaldehyde Does Not Undergo Cannizzaro Reaction:

Aldol Condensation: Acetaldehyde has α-hydrogens that allow it to form enolates. This intermediate can react with another molecule of acetaldehyde to form an aldol product, rather than undergoing Cannizzaro reaction.

Cannizzaro Reaction Preference: Cannizzaro reaction is typically observed with aldehydes that lack α-hydrogens, where the possibility of forming enolates is not present.

Conclusion:

Acetaldehyde does not undergo the Cannizzaro reaction due to the presence of α-hydrogens, which makes it more likely to participate in aldol condensation instead. The other aldehydes mentioned—1-methylcyclohexanecarbaldehyde, benzaldehyde, and formaldehyde—lack α-hydrogens and thus can undergo the Cannizzaro reaction. Therefore, acetaldehyde is the aldehyde that does not give the Cannizzaro reaction.