Question

Explain Cannizzaro’s reaction with the help of benzaldehyde. Write the reaction for the conversion of cyclohexene to adipic acid.

Hint:

A key way to remember the Cannizzaro reaction is "No alpha, Can't aldol". Aldehydes without alpha-hydrogens undergo Cannizzaro; those with alpha-hydrogens undergo the Aldol condensation in the presence of a dilute base.

Answer 1

Cannizzaro's Reaction
The Cannizzaro reaction is a redox disproportionation reaction given by aldehydes that do not have an \(\alpha\)-hydrogen atom. When heated with a concentrated alkali solution (like 50% NaOH), one molecule of the aldehyde is reduced to an alcohol, and another molecule is oxidized to the salt of a carboxylic acid.
Example with Benzaldehyde: Benzaldehyde (C\(_6\)H\(_5\)CHO) has no \(\alpha\)-hydrogen. When two molecules of benzaldehyde are treated with concentrated NaOH, one is reduced to benzyl alcohol, and the other is oxidized to sodium benzoate. \[ \underset{\text{Benzaldehyde}}{2\text{C}_6\text{H}_5\text{CHO}} + \text{conc. NaOH} \xrightarrow{\Delta} \underset{\text{Benzyl alcohol}}{\text{C}_6\text{H}_5\text{CH}_2\text{OH}} + \underset{\text{Sodium benzoate}}{\text{C}_6\text{H}_5\text{COONa}} \]
Conversion of Cyclohexene to Adipic Acid This conversion is achieved by the strong oxidation of cyclohexene. The double bond is cleaved, and the carbons of the double bond are oxidized to carboxylic acid groups. A common oxidizing agent for this is alkaline potassium permanganate (KMnO\(_4\)) followed by acidification, or hot concentrated nitric acid (HNO\(_3\)).
\[ \underset{\text{Cyclohexene}}{\text{C}_6\text{H}_{10}} \xrightarrow[\text{heat}]{\text{alk. KMnO}_4} \underset{\text{Adipic acid}}{\text{HOOC}-(\text{CH}_2)_4-\text{COOH}} \]