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: Concept
Nitriles can be prepared by the nucleophilic substitution ($S_N2$) of alkyl halides with alcoholic potassium cyanide (KCN).

Step 2: Meaning
Butanenitrile ($\text{CH}_3\text{CH}_2\text{CH}_2\text{CN}$) contains 4 carbon atoms. Note that the cyanide group (—CN) adds one carbon to the existing alkyl chain.

Step 3: Analysis
To get a 4-carbon nitrile, we must start with a 3-carbon alkyl halide. $n\text{-Propyl chloride} (\text{C}_3\text{H}_7\text{Cl}) + \text{KCN} \rightarrow n\text{-Propyl cyanide} (\text{C}_3\text{H}_7\text{CN}) + \text{KCl}$. $n\text{-Propyl cyanide}$ is IUPAC named as butanenitrile.

Step 4: Conclusion
Heating $n$-Propylchloride with alcoholic KCN yields butanenitrile. Final Answer: (D)