NCERT · Class 12 · Chemistry · Haloalkanes and HaloarenesExplain the following name reactions with balanced chemical equations: Sandmeyer Reaction Wurtz-Fittig Reaction Friedel-Crafts Alkylation of Chlorobenzene

1. Sandmeyer Reaction

• Definition: The Sandmeyer reaction is used for the preparation of aryl chlorides, bromides, and cyanides by substituting the diazonium group from primary aromatic amines.
• Process: When a primary aromatic amine, dissolved in cold aqueous mineral acid, is treated with nitrous acid ($NaNO_2 + HX$) at 273-278 K, a diazonium salt is formed. Mixing this freshly prepared diazonium salt solution with corresponding cuprous halide ($Cu_2Cl_2$ or $Cu_2Br_2$) or cuprous cyanide ($CuCN$) results in the replacement of the diazonium group by $-Cl$, $-Br$, or $-CN$ group.
• Chemical Equation: $$C_6H_5NH_2 + HNO_2 + HCl \xrightarrow{273-278 K} C_6H_5N_2^+Cl^- + 2H_2O$$ $$C_6H_5N_2^+Cl^- \xrightarrow{Cu_2Cl_2 / HCl} C_6H_5Cl + N_2$$

2. Wurtz-Fittig Reaction

• Definition: A mixture of an alkyl halide and an aryl halide gives an alkylarene when treated with sodium in the presence of dry ether. This reaction is known as the Wurtz-Fittig reaction.
• Process: In this coupling reaction, two different halogen-containing molecules (one aliphatic and one aromatic) react with metallic sodium in anhydrous ethereal medium to form a substituted aromatic hydrocarbon (alkylbenzene).
• Chemical Equation: $$C_6H_5Cl + 2Na + Cl-CH_3 \xrightarrow{Dry Ether} C_6H_5-CH_3 + 2NaCl$$ (Chlorobenzene + Chloromethane $\rightarrow$ Toluene)

3. Friedel-Crafts Alkylation of Chlorobenzene

• Definition: Friedel-Crafts alkylation involves the introduction of an alkyl group into the benzene ring of chlorobenzene in the presence of a Lewis acid catalyst such as anhydrous aluminium chloride ($AlCl_3$).
• Process: Chlorobenzene reacts with an alkyl halide (e.g., methyl chloride) in the presence of anhydrous $AlCl_3$. Since chlorine is an ortho- and para-directing group due to resonance, a mixture of 1-chloro-4-methylbenzene (major product) and 1-chloro-2-methylbenzene (minor product) is obtained.
• Chemical Equation: $$C_6H_5Cl + CH_3Cl \xrightarrow{Anhydrous AlCl_3} o-Chlorotoluene + p-Chlorotoluene$$