Q.Benzenediazonium chloride is treated separately with $(a)$ $\text{CuCl}/\text{HCl}$, $(b)$ $\text{CuBr}/\text{HBr}$ and $(c)$ aqueous $\text{KI}$. Name the product in each case and state which two of these three are classified as Sandmeyer reactions.

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 …

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

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Treating benzenediazonium chloride with $Cu_2Cl_2$ (cuprous chloride) and HCl replaces the $-N_2^+Cl^-$ group with $-Cl$, giving chlorobenzene — a classic Sandmeyer reaction.

The diagram shows benzenediazonium chloride reacting with $Cu_2Cl_2$/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:

$C_6H_5N_2^+Cl^- \xrightarrow[HCl]{Cu_2Cl_2} C_6H_5Cl + N_2\uparrow$

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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.

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Diazonium salts undergo Sandmeyer-type substitution with cuprous salts; $CuCN$ replaces the diazonium group with a cyano group, releasing nitrogen gas.

Benzenediazonium chloride, $C_6H_5N_2^+Cl^-$, reacts with cuprous cyanide ($CuCN$) in a Sandmeyer reaction. The copper(I) species facilitates a radical-mediated substitution in which the highly stable $N_2$ molecule is expelled and the aromatic ring becomes bonded directly to the cyano group:

$C_6H_5N_2^+Cl^- \xrightarrow{CuCN} C_6H_5{-}CN + N_2\uparrow + CuCl$

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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: …

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

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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

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Benzenediazonium chloride, when treated with cuprous/cupric chloride, loses $N_2$ 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):

$C_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{273\text{–}278\ K} C_6H_5N_2^+Cl^- + NaCl + 2H_2O$

When this diazonium salt is then warmed with cuprous chloride (generated in situ from $CuCl_2$ under the reaction conditions, or used directly with $Cu_2Cl_2/HCl$), the $-N_2^+$ group is displaced by chlorine and nitrogen gas is evolved — a copper-catalysed radical substitution known as the Sandmeyer reaction:

$C_6H_5N_2^+Cl^- \xrightarrow{CuCl_2/CuCl} C_6H_5Cl + N_2\uparrow$

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