Question

Write the mechanism of the following reaction:

\[\ce{{n}BuBr + KCN ->[EtOH-H2O] {n}BuCN}\]

Answer 1

This reaction is a first-order nucleophilic substitution (S_N1). The mechanism can be stated as:

Step 1: Generation of nucleophile:

\[\ce{KCN ->[EtOH-H2O]K+ + \overset{—}{C} ≡ N}\]

Step 2: Nucleophilic attack and formation of transition state:

Step 3: Generation of product:

Therefore, we have 

\[\ce{CH3 - CH2 - CH2 - CH2 - Br + KCN ->[EtOH/H2O]CH3CH2CH2CH2CN + KBr}\]

Answer 2

KCN is the resonating hybrid of the following structures:

\[\ce{K^+ [^- :C ≡ N: ↔ :C = \overset{\bullet\bullet}{N} :^-]}\]

Therefore, CN⁻ acts as an ambident nucleophile. It can attack the carbon atom of the C-Br bond in n-BuBr through either the carbon (C) or nitrogen (N) atom. Since the C-N bond is weaker than the C-C bond, the attack occurs at the carbon atom, leading to the formation of n-butyl cyanide.

\[\ce{K^+CN^- + \underset{n-butyl bromide}{CH3CH2CH2\overset{δ+}{C}H2 - \overset{δ-}{B}r} -> \underset{n-butyl cyanide}{CH3CH2CH2CH2CN} + KBr}\]