Question

Explain Cannizzaro reaction with suitable example.

Hint:

Aldehydes WITH $\alpha$-H (like Acetaldehyde) undergo Aldol Condensation. Aldehydes WITHOUT $\alpha$-H (like Benzaldehyde or Formaldehyde) undergo Cannizzaro reaction.

Answer 1

Step 1: Understanding the Concept:
The Cannizzaro reaction is a disproportionation reaction (self-redox) of aldehydes that do not have an alpha-hydrogen atom.
Step 2: Detailed Explanation:
In the presence of a strong base (like 50% NaOH), one molecule of aldehyde is reduced to an alcohol while another molecule is oxidized to a carboxylic acid salt.
Example: Formaldehyde
Two molecules of formaldehyde ($HCHO$) react with sodium hydroxide:
\[ 2HCHO + NaOH(\text{conc.}) \rightarrow CH_3OH + HCOONa \]
Products: Methanol (reduced product) and Sodium formate (oxidized product).

Example: Benzaldehyde
\[ 2C_6H_5CHO + KOH(\text{conc.}) \rightarrow C_6H_5CH_2OH + C_6H_5COOK \]
Products: Benzyl alcohol and Potassium benzoate.
Step 3: Final Answer:
Aldehydes without $\alpha$-hydrogen atoms react with conc. alkali to form an alcohol and a salt of a carboxylic acid.