Question

Explain the following with examples: (i) Aldol condensation (ii) Cannizzaro's reaction

Hint:

Aldol condensation results in the formation of α,β-unsaturated carbonyl compounds, while Cannizzaro's reaction involves the redox disproportionation of non-enolizable aldehydes.

Answer 1

Step 1: Aldol Condensation.
Aldol condensation is a reaction in which an aldehyde or ketone reacts with another aldehyde or ketone to form a β-hydroxy aldehyde or ketone, which then undergoes dehydration to form an α,β-unsaturated carbonyl compound. This reaction is catalyzed by a base or an acid.
Example of Aldol Condensation:
Consider the reaction between acetaldehyde molecules: \[ \text{CH}_3\text{CHO} + \text{CH}_3\text{CHO} \xrightarrow{\text{Base}} \text{CH}_3\text{CH(OH)CH}_2\text{CHO} \xrightarrow{\text{Dehydration}} \text{CH}_3\text{CH=CHCHO} \quad \text{(crotonaldehyde)} \] In this reaction, acetaldehyde (CH₃CHO) undergoes aldol condensation, leading to the formation of crotonaldehyde (CH₃CH=CHCHO).
Step 2: Cannizzaro's Reaction.
The Cannizzaro reaction is a type of disproportionation reaction in which non-enolizable aldehydes (those without an α-hydrogen) undergo a base-catalyzed redox reaction to produce an alcohol and a carboxylate ion. The aldehyde is simultaneously reduced and oxidized.
Example of Cannizzaro's Reaction:
Consider the reaction between two molecules of benzaldehyde: \[ 2\text{C}_6\text{H}_5\text{CHO} \xrightarrow{\text{Base}} \text{C}_6\text{H}_5\text{CH}_2\text{OH} + \text{C}_6\text{H}_5\text{COO}^- \] In this reaction, one molecule of benzaldehyde is reduced to benzyl alcohol (C₆H₅CH₂OH), and the other is oxidized to benzoate ion (C₆H₅COO⁻).
Step 3: Conclusion.
- Aldol condensation involves the formation of a β-hydroxy aldehyde or ketone followed by dehydration to yield an α,β-unsaturated compound.
- Cannizzaro’s reaction is a disproportionation reaction where non-enolizable aldehydes undergo reduction and oxidation in the presence of a base.