Question

Which of the following pair of compounds on heating gives butanenitrile?

• A. Propanol and alcoholic KCN
• B. Butanol and alcoholic KCN
• C. n -Butylchloride and alcoholic KCN
• D. n -Propylchloride and alcoholic KCN

Hint:

KCN adds one carbon! If you want a nitrile with $N$ carbons, start with an alkyl halide having $N-1$ carbons.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Alkyl nitriles can be prepared through a nucleophilic substitution reaction of alkyl halides with potassium cyanide ($\text{KCN}$) in an alcoholic medium. This reaction replaces the halide ion with the cyanide ($-\text{CN}$) ion, which effectively increases the carbon chain length of the principal molecule by one carbon atom.
Step 2: Key Formula or Approach:
Approach: Identify the total number of carbon atoms in the desired product (butanenitrile). Then, evaluate each reactant pair to find which combination of alkyl group and cyanide group yields that exact carbon count.
Step 3: Detailed Explanation:
The target molecule is butanenitrile, which has the chemical structure $\text{CH}_3-\text{CH}_2-\text{CH}_2-\text{CN}$. This molecule contains a total of four carbon atoms. In the reaction with $\text{KCN}$, the cyanide group provides one carbon atom to the new chain. Therefore, the starting alkyl halide must contain exactly three carbon atoms (a propyl group). Let's evaluate the given options: - (A) & (B) Alcohols (Propanol, Butanol) generally do not react directly with $\text{KCN}$ to form nitriles because the hydroxide ion ($\text{OH}^-$) is a poor leaving group. - (C) n-Butylchloride (4 carbons) + alcoholic $\text{KCN}$ (1 carbon) $\longrightarrow$ Pentanenitrile (5 carbons). - (D) n-Propylchloride (3 carbons) + alcoholic $\text{KCN}$ (1 carbon) $\longrightarrow$ Butanenitrile (4 carbons). Reaction equation for D: \[ \text{CH}_3-\text{CH}_2-\text{CH}_2-\text{Cl} + \text{KCN}(\text{alc.}) \xrightarrow{\Delta} \text{CH}_3-\text{CH}_2-\text{CH}_2-\text{CN} + \text{KCl} \] Step 4: Final Answer:
The pair that gives butanenitrile is n-Propylchloride and alcoholic KCN.