Chlorobenzene can be prepared by which of the following set of reactions ?
(A) Phenol, NaNO2 + HCl, CuCl
(B) Aniline, HCl/Δ
(C) Benzene + Cl2 in presence of Fe
(D) Aniline, NaNO2 + HCl (273-278 K), Cu2Cl2/HCl
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (C) and (D) only
Aryl halides such as chlorobenzene are made by two main routes:
Electrophilic substitution: benzene reacts with chlorine in the presence of a Lewis acid (Fe or \( \ce{FeCl3} \)), in the dark. Iron first reacts with chlorine to form \( \ce{FeCl3} \), which acts as the catalyst.
From diazonium salts (Sandmeyer reaction): a primary aromatic amine is diazotised with \( \ce{NaNO2} \) + HCl at 273–278 K, and the diazonium salt is treated with \( \ce{Cu2Cl2} \)/HCl.
Aryl halides cannot be made from phenols in the way alkyl halides are made from alcohols, because the C–O bond of phenol has partial double bond character and is hard to break.
(A) Phenol, \( \ce{NaNO2} \) + HCl, CuCl — Incorrect. Diazotisation needs an –NH2 group. Phenol has none, so no diazonium salt forms, and its C–OH bond is not replaced by Cl.
(B) Aniline, HCl/Δ — Incorrect. Aniline is a base; with HCl it only forms the salt anilinium chloride, \( \ce{C6H5NH3^{+}Cl^{-}} \). The C–N bond is not replaced.
\( \displaystyle \ce{C6H5NH2 + HCl -> C6H5NH3^+Cl^-} \)
(C) Benzene + \( \ce{Cl2} \) in presence of Fe — Correct. Iron first forms \( \ce{FeCl3} \), the Lewis acid that generates the electrophile \( \ce{Cl^+} \); electrophilic substitution then gives chlorobenzene.
\( \displaystyle \ce{2Fe + 3Cl2 -> 2FeCl3} \)
\( \displaystyle \ce{C6H6 + Cl2} \xrightarrow{\ce{Fe}} \ce{C6H5Cl + HCl} \)
(D) Aniline, \( \ce{NaNO2} \) + HCl (273-278 K), \( \ce{Cu2Cl2} \)/HCl — Correct. This is diazotisation followed by the Sandmeyer reaction.
\( \displaystyle \ce{C6H5NH2} \xrightarrow[273-278\ \mathrm{K}]{\ce{NaNO2 + HCl}} \ce{C6H5N2^+Cl^-} \xrightarrow{\ce{Cu2Cl2}/\ce{HCl}} \ce{C6H5Cl + N2} \)
NCERT (2026-27 edition):
From Hydrocarbons by Electrophilic Substitution – “Aryl chlorides and bromides can be easily prepared by electrophilic substitution of arenes with chlorine and bromine respectively in the presence of Lewis acid catalysts like iron or iron(III) chloride.”
From Amines by Sandmeyer’s Reaction – “When a primary aromatic amine, dissolved or suspended in cold aqueous mineral acid, is treated with sodium nitrite, a diazonium salt is formed. Mixing the solution of freshly prepared diazonium salt with cuprous chloride or cuprous bromide results in the replacement of the diazonium group by –Cl or –Br.”