Question

Which of the following reactions is shown by aldehydes that do not have alpha H-atom?

• A. Aldol condensation
• B. Cross aldol condensation
• C. Cannizzaro reaction
• D. Kolbe’s reaction

Answer 1

The Correct Option is C

Aldehydes that lack an alpha hydrogen atom undergo the Cannizzaro reaction. This chemical reaction involves the reduction and oxidation of aldehydes without the presence of an alpha hydrogen. During this reaction, two molecules of the aldehyde react with each other:

Step-by-Step Explanation: 

1. Consider an aldehyde, RCHO, without an alpha hydrogen.

2. In the Cannizzaro reaction, two aldehyde molecules undergo a disproportionation reaction, which is a redox process.

3. One molecule of the aldehyde is reduced to form an alcohol (RCH₂OH), while the other is oxidized to form a carboxylate (RCOO^-).

Chemical Reaction:

2RCHO → RCH₂OH + RCOO^-

Thus, aldehydes with no alpha hydrogen atom undergo the Cannizzaro reaction, making the answer: Cannizzaro reaction.

Answer 2

Aldehydes that lack an alpha hydrogen (hydrogens attached to the carbon adjacent to the carbonyl group) undergo the Cannizzaro reaction in the presence of a base. This reaction is a redox reaction where two molecules of the aldehyde react in the presence of a strong base to form an alcohol and a carboxylate ion.

In the Cannizzaro reaction, one molecule of the aldehyde undergoes reduction to form an alcohol, while the other molecule undergoes oxidation to form a carboxylate ion (the conjugate base of a carboxylic acid). This occurs due to the absence of a hydrogen atom on the alpha carbon, which prevents the typical aldol condensation from occurring. As a result, the reaction proceeds through a disproportionation mechanism.

An example of this reaction is the conversion of formaldehyde (HCHO) to methanol (CH₃OH) and formate ion (HCOO⁻), typically in the presence of a strong base such as sodium hydroxide (NaOH). The Cannizzaro reaction is particularly useful for synthesizing alcohols and carboxylates from aldehydes that cannot undergo aldol condensation due to the absence of alpha hydrogens.