Q.How will you obtain chlorobenzene from aniline?
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The Core Intuition
Imagine you have a benzene ring with an amino group (−NH2) attached. You want to replace that amino group with a chlorine, bromine, or cyano group. Direct substitution is nearly impossible — the amino group is stubbornly attached. But there's a clever detour.
The trick is to first convert the amino group into a diazonium salt (Ar−N2+). This diazonium group is a fantastic leaving group — it's practically begging to leave. The problem is that if you just heat it, it leaves as nitrogen gas and you get a mess. You need to control what replaces it.
Sandmeyer found that if you use a cuprous halide (CuCl, CuBr) or cuprous cyanide (CuCN), the copper helps transfer the halogen or cyano group onto the ring in a clean, high-yielding reaction. The copper acts like a molecular taxi — it picks up the halogen and drops it off exactly where the diazonium group left.
The Precise Statement
Ar−N2+X−CuX or CuCNAr−X+N2
where X=Cl,Br,CN
The Sandmeyer Reaction is the replacement of the diazonium group (−N2+) on an aromatic ring by chlorine, bromine, or a cyano group, using the corresponding cuprous halide or cuprous cyanide as the reagent.
The Mechanism (What Actually Happens)
The reaction proceeds through a radical mechanism, not a simple substitution. Here's the sequence:
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The cuprous ion (Cu+) transfers one electron to the diazonium ion. This breaks the N≡N bond, releasing nitrogen gas and generating an aryl radical (Ar∙).
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The cupric ion (Cu2+) that forms now has a halogen attached. The aryl radical abstracts this halogen from copper, giving you the final product Ar−X and regenerating Cu+ to continue the cycle.
The key insight: copper changes oxidation state (Cu+→Cu2+→Cu+), shuttling electrons to break the diazonium bond and then delivering the halogen. This is why the reaction works — the copper mediates a radical pathway that simple heating cannot achieve.
Why This Matters for Exams
Scope: The reaction works only for Cl, Br, and CN. You cannot get F or I this way — fluorine is too reactive, and iodine requires different conditions (the Gattermann reaction using copper powder instead of cuprous salts).
Yields: Sandmeyer gives excellent yields (70-90%), much better than the older Gattermann method. This is why it's the preferred industrial route for these substituted aromatics. …
The amino group of aniline cannot be replaced by chlorine directly, so it is first converted into a diazonium salt, which acts as a good leaving group that a copper reagent can then displace with chlorine. …
Diazotise aniline to benzenediazonium chloride, then displace the diazonium group with Cl using a copper-catalysed reaction.
Step 1 — Diazotisation: Aniline is treated with nitrous acid (generated in situ from NaNO2+HCl) at low temperature (273–278 K) to form benzenediazonium chloride:
C6H5NH2NaNO2/HCl273−278KC6H5N2+Cl−
Step 2 — Replacement of –N₂⁺ by –Cl: The diazonium salt is then decomposed in the presence of copper, which replaces the diazonium group with chlorine:
- Sandmeyer reaction: treat with Cu2Cl2/HCl (cuprous chloride), or …
- CBSE 2025Set ANNUAL1 markMCQQ.Which one of the following is not Sandmeyer's reaction?(a) C6H5N2Cl + Cu2Br2 --HCl-->(b) C6H5N2Cl + Cu (powder) --HCl, heat-->(c) C6H5N2Cl + Cu2Cl2 --HCl-->(d) C6H5N2Cl + CuCN --KCN-->
›Reveal solutionSolution
Sandmeyer's reaction specifically uses cuprous salts (Cu2Cl2, Cu2Br2, CuCN) to convert a diazonium salt into the corresponding aryl halide/nitrile; using copper powder itself instead of a cuprous salt is the Gattermann reaction, a related but distinct method.
Benzenediazonium chloride (C6H5N2Cl), formed by diazotisation of aniline, can be converted to various aryl derivatives:
- C6H5N2Cl + Cu2Cl2/HCl → C6H5Cl (chlorobenzene) — Sandmeyer's reaction
- C6H5N2Cl + Cu2Br2/HCl → C6H5Br (bromobenzene) — Sandmeyer's reaction
- C6H5N2Cl + CuCN/KCN → C6H5CN (benzonitrile) — Sandmeyer's reaction …
- CBSE 2025Set ANNUAL1 markQ.The reaction (benzenediazonium chloride, C6H5-N2+Cl-, treated with Cu/HCl) giving (chlorobenzene, C6H5-Cl, + N2 + CuCl) is named as ........... .
›Reveal solutionSolution
Replacing a benzenediazonium salt's -N2+Cl- group with -Cl using elemental COPPER POWDER and the corresponding halogen acid (HCl) is the Gattermann reaction — a variant of the better-known Sandmeyer reaction, which instead uses a cuprous salt (Cu2Cl2) rather than copper metal.
C6H5-N2+Cl- --Cu/HCl--> C6H5-Cl (chlorobenzene) + N2 + CuCl
…
- CBSE 2025Set ANNUAL1 markMCQQ.Benzenediazonium chloride (structure shown, a benzene ring with a –N2+Cl– substituent) + Cu2Cl2 --HCl--> ? The product of this reaction is (structures shown as options):(a) Benzene ring with an –OH substituent (phenol)(b) Benzene ring with an –NCl substituent(c) Benzene ring with a –Cu substituent(d) Benzene ring with a –Cl substituent (chlorobenzene)
›Reveal solutionSolution
Treating benzenediazonium chloride with Cu2Cl2 (cuprous chloride) and HCl replaces the −N2+Cl− group with −Cl, giving chlorobenzene — a classic Sandmeyer reaction.
The diagram shows benzenediazonium chloride reacting with Cu2Cl2/HCl. This is the Sandmeyer reaction, in which the diazonium group of an aryldiazonium salt is replaced by a halogen (Cl or Br) using the corresponding cuprous halide as catalyst:
C6H5N2+Cl−Cu2Cl2HClC6H5Cl+N2↑
…
- CBSE 2024Set ANNUAL1 markMCQQ.One of the reagents for Sandmeyer reaction is(a) Cu/HBr(b) CuBr/HBr(c) Cu at 573K(d) None of these
›Reveal solutionSolution
The Sandmeyer reaction converts an aryl diazonium salt to an aryl halide using a cuprous halide (Cu(I) salt) together with the corresponding hydrohalic acid.
ArN2+ Cl- --(CuBr / HBr)--> ArBr + N2 + ...
CuBr/HBr installs -Br (Sandmeyer); analogously CuCl/HCl installs -Cl, and CuCN/KCN installs -CN.
…
- CBSE 2024Set ANNUAL1 markQ.Complete the following reaction: C6H5N2ClCuCN?
›Reveal solutionSolution
Diazonium salts undergo Sandmeyer-type substitution with cuprous salts; CuCN replaces the diazonium group with a cyano group, releasing nitrogen gas.
Benzenediazonium chloride, C6H5N2+Cl−, reacts with cuprous cyanide (CuCN) in a Sandmeyer reaction. The copper(I) species facilitates a radical-mediated substitution in which the highly stable N2 molecule is expelled and the aromatic ring becomes bonded directly to the cyano group:
C6H5N2+Cl−CuCNC6H5−CN+N2↑+CuCl
…
- CBSE 2023Set ANNUAL1 markQ.How chlorobenzene is prepared from Benzene Diazonium Chloride? Write chemical reaction only.
›Reveal solutionSolution
Chlorobenzene is obtained from benzenediazonium chloride by the Sandmeyer/Gattermann reaction.
Benzenediazonium chloride, when treated with cuprous chloride and hydrochloric acid (Sandmeyer reaction), or with copper powder and HCl (Gattermann reaction), loses nitrogen gas and the diazonium group is replaced by chlorine: …
- CBSE 2022Set ANNUAL1 markMCQQ.The reaction which converts benzene diazonium chloride to chlorobenzene using CuCl in HCl is known as ______.(a) Swarts reaction(b) Sandmeyer reaction(c) Finkelstein reaction(d) Kolbe's reaction
›Reveal solutionSolution
Replacing the diazonium group of benzenediazonium chloride with -Cl using cuprous chloride (Cu2Cl2/CuCl) in HCl is called the Sandmeyer reaction.
Diazonium salts are versatile: the -N2+ group can be substituted by many other groups. When benzenediazonium chloride is treated with cuprous chloride in the presence of HCl, the diazonium group is replaced by -Cl, releasing N2 gas:
C6H5N2+Cl- --(CuCl/HCl)--> C6H5Cl + N2
…
- CBSE 2020Set ANNUAL1 markQ.Benzene diazonium chloride when treated with Cu2Cl2 and HCl, the product formed is chlorobenzene. This reaction is known as ______.
›Reveal solutionSolution
Converting a benzene diazonium salt to chlorobenzene using Cu2Cl2/HCl (or CuCl/HCl) is called the Sandmeyer reaction.
Diazonium salts are versatile intermediates: the -N2+ group can be replaced by many other groups. When benzenediazonium chloride is treated with cuprous chloride (Cu2Cl2) in the presence of HCl, the diazonium group is replaced by -Cl to give chlorobenzene, with evolution of N2 gas:
C6H5N2+Cl- --(Cu2Cl2/HCl)--> C6H5Cl + N2
…
- CBSE 2020Set OC1 markMCQQ.Identify the compound Y in the following reaction: aniline (benzene ring--NH2) treated with NaNO2/HCl at (273-278) K gives the diazonium salt (benzene ring--N2+ Cl-), which is then treated with CuCl2 to give Y + N2 (a Sandmeyer reaction).(a) chlorobenzene (benzene ring with a single -Cl substituent)(b) benzene (unsubstituted ring, no substituent)(c) 1,3-dichlorobenzene (benzene ring with two -Cl substituents, meta to each other)(d) 1,4-dichlorobenzene (benzene ring with two -Cl substituents, para to each other)
›Reveal solutionSolution
Benzenediazonium chloride, when treated with cuprous/cupric chloride, loses N2 and the diazonium group is replaced by a chlorine atom, giving chlorobenzene — this substitution is the Sandmeyer reaction.
Aniline is first converted to benzenediazonium chloride by diazotization at low temperature (273–278 K, to keep the unstable diazonium salt from decomposing):
C6H5NH2+NaNO2+2HCl273–278 KC6H5N2+Cl−+NaCl+2H2O
When this diazonium salt is then warmed with cuprous chloride (generated in situ from CuCl2 under the reaction conditions, or used directly with Cu2Cl2/HCl), the −N2+ group is displaced by chlorine and nitrogen gas is evolved — a copper-catalysed radical substitution known as the Sandmeyer reaction:
C6H5N2+Cl−CuCl2/CuClC6H5Cl+N2↑
…
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