Answer the question on the basis of passage given below:
Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, the presence of two relatively large substituents in ketones hinders the approach of nucleophile to carbonyl carbon more than aldehydes having only one such substituent. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl more effectively than in former.
For the preparation of phenylethanoic acid from benzyl alcohol, reagents required are
A. \(HBr\)
B. \(CO_2\)
C. \(KCN\)
D. \(\Delta / H_3O^+\)
E. \(AgNO_3\)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A, C and D only
Phenylethanoic acid, \( \ce{C6H5CH2COOH} \), has one carbon more than benzyl alcohol, \( \ce{C6H5CH2OH} \). The extra carbon is brought in as the cyanide ion, and the nitrile is then hydrolysed to the acid:
\( \ce{C6H5CH2OH} \) + HBr → \( \ce{C6H5CH2Br} \) + \( \ce{H2O} \)
\( \ce{C6H5CH2Br} \) + KCN → \( \ce{C6H5CH2CN} \) + KBr
\( \ce{C6H5CH2CN} \) → (\( \ce{H3O^{+}} \), Δ) → \( \ce{C6H5CH2COOH} \) (via the amide)
A. HBr — Required. It converts the alcohol into benzyl bromide.
B. CO2 — Not required. \( \ce{CO2} \) adds a carbon only to a Grignard reagent, and no magnesium is given in the list.
C. KCN — Required. Cyanide replaces Br and adds the extra carbon (benzyl cyanide).
D. Δ/H3O+ — Required. Acid hydrolysis turns the –CN group into –COOH.
E. AgNO3 — Not required. Silver nitrate plays no part in this sequence.
NCERT (2026-27 edition):
From Nitriles and Amides – “Nitriles are hydrolysed to amides and then to acids in the presence of H+ or OH− as catalyst. Mild reaction conditions are used to stop the reaction at the amide stage.”
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| – | – | – | 12-06 S3 10-06 S1 | – |