Question

(A) Explain the following reactions and write chemical equations involved:
(a) Wolff-Kishner reduction
(b) Etard reaction
(c) Cannizzaro reaction

Hint:

In the Wolff-Kishner reduction, remember that hydrazine and strong base under heat are used to reduce carbonyl compounds, whereas in the Cannizzaro reaction, aldehydes without alpha hydrogens undergo disproportionation.

Answer 1

To solve the problem, we need to explain the given reactions and provide their chemical equations.

1. Wolff-Kishner Reduction:
This reaction reduces aldehydes or ketones to hydrocarbons by treating their hydrazone derivatives with a strong base (e.g., KOH) and hydrazine (\( N_2H_4 \)) at high temperatures. It proceeds via the formation of a hydrazone intermediate, which decomposes to form a hydrocarbon and nitrogen gas.

General equation:
\( R_2C=O + N_2H_4 \xrightarrow{\text{KOH, heat}} R_2CH_2 + N_2 + H_2O \).
Example: Acetone to propane:
\( (CH_3)_2C=O + N_2H_4 \xrightarrow{\text{KOH, heat}} (CH_3)_2CH_2 + N_2 + H_2O \).

2. Etard Reaction:
This reaction oxidizes toluene or its derivatives (with an alkyl side chain) to aldehydes using chromyl chloride (\( CrO_2Cl_2 \)) in \( CCl_4 \), followed by hydrolysis. The reaction forms a complex intermediate that decomposes to the aldehyde upon hydrolysis.

General equation:
\( C_6H_5CH_3 + 2CrO_2Cl_2 \rightarrow C_6H_5CH_3 \cdot (CrO_2Cl_2)_2 \xrightarrow{\text{H}_2\text{O}} C_6H_5CHO + 2CrOCl + 2HCl \).
Example: Toluene to benzaldehyde:
\( C_6H_5CH_3 + 2CrO_2Cl_2 \xrightarrow{\text{CCl}_4} \text{Complex} \xrightarrow{\text{H}_2\text{O}} C_6H_5CHO \).

3. Cannizzaro Reaction:
Aldehydes lacking α-hydrogens (e.g., formaldehyde, benzaldehyde) undergo this reaction in the presence of a strong base (e.g., NaOH). One molecule is reduced to an alcohol, and another is oxidized to a carboxylic acid salt.

General equation:
\( 2RCHO + \text{OH}^- \rightarrow RCH_2OH + RCOO^- \).
Example: Benzaldehyde:
\( 2C_6H_5CHO + \text{NaOH} \rightarrow C_6H_5CH_2OH + C_6H_5COONa \).

Final Answer:
(a) Wolff-Kishner reduction converts aldehydes/ketones to hydrocarbons: \( R_2C=O + N_2H_4 \xrightarrow{\text{KOH, heat}} R_2CH_2 + N_2 + H_2O \).
(b) Etard reaction oxidizes toluene to benzaldehyde: \( C_6H_5CH_3 + 2CrO_2Cl_2 \xrightarrow{\text{H}_2\text{O}} C_6H_5CHO \).
(c) Cannizzaro reaction involves disproportionation of aldehydes: \( 2RCHO + \text{OH}^- \rightarrow RCH_2OH + RCOO^- \).