CUET Preparation Today
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.
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When KCN reacts with R-X, what is the major product? |
Alcohol Ether Isonitrile Nitrile |
Nitrile |
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:
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. |
