Read the following passage and answer questions.
Nucleophiles attack at that part of the substrate molecule which is electron deficient. The reaction in which a nucleophile replaces already existing nucleophile in a molecule is called nucleophilic substitution reaction. Haloalkanes are substrates in these reactions. In this type of reaction, a nucleophile reacts with haloalkane (the substrate) having a partial positive charge on the carbon atom bonded to halogen. A substitution reaction takes place and halogen atom, called leaving group departs as halide ion. Since the substitution reaction is initiated by a nucleophile, it is called nucleophilic substitution reaction.
When KCN reacts with R-X, what is the major product?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Nitrile
$\text{KCN}$ provides cyanide ion $(\text{CN}^-)$ which is an ambident nucleophile.
Ambident means it can attack through:
- Carbon atom $\rightarrow$ gives nitrile
- Nitrogen atom $\rightarrow$ gives isonitrile
With $\text{KCN}$, attack occurs through carbon.
Reaction
$R-X + \text{CN}^- \rightarrow R-C\equiv N + X^-$
Product formed is nitrile.
Option-wise Detailed Explanation
Option 1: Alcohol
Alcohol forms when $\text{OH}^-$ replaces halogen.
Example:
$\text{R-X} + \text{OH}^- \rightarrow \text{R-OH}$
Since $\text{OH}^-$ is not present here, alcohol is not formed.
Option 2: Ether
Ether formation requires alkoxide ion $(\text{RO}^-)$.
Example:
$\text{R-X} + \text{RO}^- \rightarrow \text{R-OR}$
Since $\text{RO}^-$ is not present, ether cannot form.
Option 3: Isonitrile
Isonitrile forms when nucleophilic attack happens through nitrogen. This happens when $\text{AgCN}$ is used. $\text{AgCN}$ is covalent $\rightarrow$ nitrogen end attacks.
Reaction:
$\text{R-X} + \text{AgCN} \rightarrow \text{R-NC}$
But here $\text{KCN}$ is used, not $\text{AgCN}$.
Option 4: Nitrile
In $\text{KCN}$, $\text{CN}^-$ is ionic and free. Carbon end attacks electrophilic carbon.
Reaction:
$R-X + \text{KCN} \rightarrow R-C\equiv N$
Thus nitrile is formed.