Which of the following reagents converts a nitrile into the corresponding aldehyde?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → i. DIBAL-H, ii. H3O+
Diisobutylaluminium hydride (DIBAL-H) reduces a nitrile only as far as the imine; hydrolysis of the imine then gives the aldehyde:
\( \displaystyle \ce{R-C#N} \xrightarrow{\text{i) DIBAL-H}} \ce{R-CH=NH} \xrightarrow{\text{ii) }\ce{H3O+}} \ce{R-CHO} \)
Why the other options are incorrect
Option (2) NaBH4 — Incorrect. Sodium borohydride is used to reduce aldehydes and ketones to alcohols; it is not a reagent for turning a nitrile into an aldehyde.
Option (3) i. LiAlH4, ii. H3O+ — Incorrect. Lithium aluminium hydride reduces a nitrile all the way to a primary amine, \( \ce{R-CH2NH2} \).
Option (4) NaOH, H2O, heat — Incorrect. This hydrolyses the nitrile, through the amide, to the carboxylic acid (as its salt); no reduction takes place.
NCERT (2026-27 edition):
8.2.2 Preparation of Aldehydes – “Alternatively, nitriles are selectively reduced by diisobutylaluminium hydride, (DIBAL-H) to imines followed by hydrolysis to aldehydes:”
\( \displaystyle \ce{RCN} \xrightarrow[\text{2. }\ce{H2O}]{\text{1. }\ce{AlH(i-Bu)2}} \ce{R-CHO} \)