Aniline is to be converted into benzonitrile (cyanobenzene), C6H5CN. Which of the following sequences of reagents brings about this conversion?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \( \ce{NaNO2} \) and dilute HCl at 273–278 K, then CuCN/KCN
Step 1 – Diazotisation: the –NH2 group of aniline cannot be replaced directly. It is first changed into the diazonium group, which is a very good leaving group:
\( \displaystyle \ce{C6H5NH2 + NaNO2 + 2HCl} \xrightarrow{\text{273–278 K}} \ce{C6H5N2^+Cl^- + NaCl + 2H2O} \)
Step 2 – Sandmeyer reaction with CuCN/KCN: in the presence of the Cu(I) ion, the cyanide ion replaces the diazonium group and nitrogen escapes as a gas:
\( \displaystyle \ce{C6H5N2^+Cl^-} \xrightarrow{\ce{CuCN/KCN}} \ce{C6H5CN + N2 ^} \)
Final product: the –NH2 group of aniline has been replaced by –CN, giving benzonitrile, C6H5CN. This is the sequence of Option (2):
diazotisation comes first, so the ring carries a good leaving group;
the cyano group then comes in from CuCN/KCN, and nitrogen leaves as a gas.
Why the other options are incorrect
Option (1) Chloroform and ethanolic potassium hydroxide — Incorrect. This is the carbylamine reaction. Aniline gives phenyl isocyanide, C6H5NC, a foul smelling isomer of benzonitrile, not the nitrile:
\( \displaystyle \ce{C6H5NH2 + CHCl3 + 3KOH} \xrightarrow{\text{Heat}} \ce{C6H5NC + 3KCl + 3H2O} \)
Option (3) Copper powder and HCl, then aqueous KCN — Incorrect. The first two steps give chlorobenzene (Gattermann reaction). The cyano group cannot be introduced by nucleophilic substitution of chlorine in chlorobenzene, so KCN does not give benzonitrile.
Option (4) CuCN/KCN alone — Incorrect. Without diazotisation there is no diazonium group to be displaced; the –NH2 group itself is not replaced by the cyanide ion.
NCERT (2026-27 edition):
9.7 Method of Preparation of Diazonium Salts – “Benzenediazonium chloride is prepared by the reaction of aniline with nitrous acid at 273-278K. Nitrous acid is produced in the reaction mixture by the reaction of sodium nitrite with hydrochloric acid.”
9.9 Chemical Reactions – “1. Replacement by halide or cyanide ion: The Cl–, Br– and CN– nucleophiles can easily be introduced in the benzene ring in the presence of Cu(I) ion. This reaction is called Sandmeyer reaction.”
9.10 Importance of Diazonium Salts in Synthesis of Aromatic Compounds – “…The cyano group cannot be introduced by nucleophilic substitution of chlorine in chlorobenzene but cyanobenzene can be easily obtained from diazonium salt.”