Which of the following reagents can be used to prepare ethyl carbylamine from ethyl iodide?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → AgCN
The cyanide ion is an ambident nucleophile: it can attack through carbon or through nitrogen. Which atom attacks depends on the nature of the metal cyanide:
KCN gives an alkyl cyanide (nitrile):
\( \displaystyle \ce{C2H5I + KCN -> C2H5CN + KI} \)
AgCN gives an alkyl isocyanide (carbylamine):
\( \displaystyle \ce{C2H5I + AgCN -> C2H5NC + AgI} \)
So ethyl carbylamine (ethyl isocyanide) is prepared from ethyl iodide using AgCN.
Why the other options are incorrect
Option (1) KCN — Incorrect. Being ionic, it gives ethyl cyanide (propanenitrile), \( \ce{C2H5CN} \), not the isocyanide.
Option (3) CuCN — Incorrect. It is the reagent for replacing the diazonium group of an arenediazonium salt by –CN (Sandmeyer reaction), not for making alkyl isocyanides.
Option (4) HCN — Incorrect. It is a weak acid that supplies few cyanide ions; it adds to aldehydes and ketones to form cyanohydrins.
NCERT (2026-27 edition):
Reactions of Haloalkanes – “… KCN is predominantly ionic and provides cyanide ions in solution. Although both carbon and nitrogen atoms are in a position to donate electron pairs, the attack takes place mainly through carbon atom and not through nitrogen atom since C—C bond is more stable than C—N bond. However, AgCN is mainly covalent in nature and nitrogen is free to donate electron pair forming isocyanide as the main product.”