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Q8.15(viii):
How will you bring about the following conversion in not more than two steps? (viii) Benzaldehyde to (\alpha)-Hydroxyphenylacetic acid

Solution :

Step 1: Conversion of Benzaldehyde to Mandelic Acid Nitrile (Cyanohydrin)

To convert Benzaldehyde ($C_6H_5CHO$) into $\alpha$-Hydroxyphenylacetic acid (also known as Mandelic acid), the first step involves a nucleophilic addition reaction with hydrogen cyanide ($HCN$).

Reaction: $C_6H_5CHO + HCN \rightarrow C_6H_5CH(OH)CN$

Mechanism: The nucleophilic cyanide ion ($CN^-$), generated from $NaCN/HCl$, attacks the electrophilic carbonyl carbon of the benzaldehyde. The carbonyl oxygen is subsequently protonated to form a cyanohydrin [Per Nucleophilic Addition Mechanism].

C6H5-CHO + HCN NaCN/HCl C6H5-CH(OH)-CN

Step 2: Acidic Hydrolysis of Cyanohydrin

The resulting mandelonitrile ($C_6H_5CH(OH)CN$) is subjected to complete acid-catalyzed hydrolysis. This converts the nitrile ($-CN$) functional group into a carboxylic acid ($-COOH$) group.

Reaction: $C_6H_5CH(OH)CN + 2H_2O + H^+ \rightarrow C_6H_5CH(OH)COOH + NH_4^+$

Justification: Nitriles undergo hydrolysis in the presence of strong aqueous acids (e.g., $H_2SO_4$ or $HCl$) to form amides as intermediates, which are further hydrolyzed to carboxylic acids [Per Nitrile Hydrolysis Protocol].

C6H5-CH(OH)-CN + 2H2O H3O+, Δ C6H5-CH(OH)-COOH
Step Reagents Product
1 $NaCN, HCl$ Benzaldehyde cyanohydrin
2 $H_3O^+, \Delta$ $\alpha$-Hydroxyphenylacetic acid

Final Solution: The conversion is achieved in two steps: first, nucleophilic addition of HCN to Benzaldehyde to form mandelonitrile, followed by the acid-catalyzed hydrolysis of the nitrile group to yield $\alpha$-Hydroxyphenylacetic acid ($C_6H_5CH(OH)COOH$).


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