Which of the following is a disproportionation reaction?

Question

Accepted Answer

The correct answer to the question "Which of the following is a disproportionation reaction?" is Option 2: Cannizzaro reaction. Let's explore why this is the correct choice and why the other options are incorrect.

1. **Cannizzaro Reaction (Option 2):**
   - The Cannizzaro reaction is a classic example of a disproportionation reaction. In this reaction, an aldehyde that lacks an alpha hydrogen undergoes self-oxidation and reduction when treated with a strong base. One molecule of the aldehyde is oxidized to a carboxylic acid, while another molecule is reduced to an alcohol. This simultaneous oxidation and reduction of the same species (aldehyde) is characteristic of disproportionation reactions. For example, in the Cannizzaro reaction of formaldehyde, one molecule of formaldehyde is oxidized to formic acid, and another is reduced to methanol.

2. **Cross Aldol Condensation (Option 1):**
   - Cross aldol condensation is not a disproportionation reaction. It involves the reaction between two different aldehydes or ketones in the presence of a base to form a β-hydroxy aldehyde or ketone, which can further dehydrate to form an α,β-unsaturated carbonyl compound. This reaction involves the formation of carbon-carbon bonds and does not involve the simultaneous oxidation and reduction of the same species.

3. **Gattermann-Koch Reaction (Option 3):**
   - The Gattermann-Koch reaction is a formylation reaction used to introduce a formyl group into an aromatic ring, typically benzene, using carbon monoxide and hydrochloric acid in the presence of a Lewis acid catalyst like aluminum chloride. This reaction is not a disproportionation reaction as it involves the introduction of a functional group rather than the simultaneous oxidation and reduction of the same species.

4. **Hoffmann Bromamide Degradation Reaction (Option 4):**
   - The Hoffmann bromamide degradation reaction is a method for converting an amide to an amine with one fewer carbon atom. This reaction involves the use of bromine and a strong base, typically sodium hydroxide. It is a rearrangement reaction rather than a disproportionation reaction, as it involves the migration of an alkyl or aryl group from the carbonyl carbon to the nitrogen atom, resulting in the loss of a carbon atom.

In conclusion, the Cannizzaro reaction is the only option among the given choices that involves the simultaneous oxidation and reduction of the same species, making it a disproportionation reaction. The other reactions listed do not fit the criteria for disproportionation, as they involve different types of chemical transformations.