Question

Give structures of A, B and C: $CH_3Cl \xrightarrow{KCN}$ A $\xrightarrow{LiAlH_4}$ B $\xrightarrow{CHCl_3 + \text{alc. } KOH, \Delta}$ C

Hint:

Carbylamine reaction ($CHCl_3$ + alc. $KOH$ + primary amine) ? Isocyanide (foul-smelling). This is a confirmatory test for $1^\circ$ amines.

Answer 1

Step 1: Step 1 — $CH_3Cl + KCN$ (A).
$CN^-$ from KCN is a nucleophile; it displaces $Cl^-$ from $CH_3Cl$ via $S_N2$, inserting one extra carbon. Product: Acetonitrile ($CH_3CN$). Compound A: $CH_3CN$.
Step 2: Step 2 — $CH_3CN + LiAlH_4$ (B).
$LiAlH_4$ reduces the nitrile group ($-C\equiv N$) to a primary amine ($-CH_2NH_2$). Product: Methylamine ($CH_3NH_2$). Compound B: $CH_3NH_2$.
Step 3: Step 3 — $CH_3NH_2 + CHCl_3 + $ alc. $KOH$, $\Delta$ (C).
This is the carbylamine reaction (isocyanide test), characteristic of primary amines. The product is methyl isocyanide ($CH_3NC$), which has a very foul odour. Compound C: $CH_3NC$.
Step 4: Summary.
A: $CH_3CN$ (acetonitrile); B: $CH_3NH_2$ (methylamine); C: $CH_3NC$ (methyl isocyanide).