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

The problem requires an explanation of the given reactions along with their corresponding chemical equations.

1. Wolff–Kishner Reduction:

This reaction converts aldehydes or ketones into hydrocarbons. The carbonyl compound is first converted into its hydrazone using hydrazine (N₂H₄). On heating with a strong base such as KOH, the hydrazone decomposes to give the hydrocarbon with the evolution of nitrogen gas.

General equation:

\( R_2C=O + N_2H_4 \xrightarrow{\text{KOH, heat}} R_2CH_2 + N_2 + H_2O \)

Example: Conversion of acetone to propane:

\( (CH_3)_2C=O + N_2H_4 \xrightarrow{\text{KOH, heat}} (CH_3)_2CH_2 + N_2 + H_2O \)

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2. Etard Reaction:

The Etard reaction oxidizes toluene or substituted toluenes (having a benzylic methyl group) to the corresponding aldehydes. Chromyl chloride (CrO₂Cl₂) in CCl₄ is used, followed by hydrolysis. A complex is formed as an intermediate, which decomposes to the aldehyde.

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: Conversion of toluene to benzaldehyde:

\( C_6H_5CH_3 + 2CrO_2Cl_2 \xrightarrow{\text{CCl}_4} \text{Complex} \xrightarrow{\text{H}_2\text{O}} C_6H_5CHO \)

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3. Cannizzaro Reaction:

Aldehydes that do not contain α-hydrogen atoms undergo the Cannizzaro reaction in the presence of a strong base such as NaOH. In this disproportionation reaction, one molecule of the aldehyde is reduced to an alcohol while another is oxidized to a carboxylate salt.

General equation:

\( 2RCHO + \text{OH}^- \rightarrow RCH_2OH + RCOO^- \)

Example: Benzaldehyde:

\( 2C_6H_5CHO + \text{NaOH} \rightarrow C_6H_5CH_2OH + C_6H_5COONa \)

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Final Answer:

(a) Wolff–Kishner reduction converts aldehydes or ketones into 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 without α-hydrogen:
\( 2RCHO + \text{OH}^- \rightarrow RCH_2OH + RCOO^- \)