NCERT · Class 12 · Chemistry · Haloalkanes and HaloarenesDiscuss the preparation of haloarenes via Sandmeyer reaction and Gattermann reaction. Explain why haloarenes are much less reactive towards nucleophilic substitution reactions compared to haloalkanes, providing at least three distinct chemical or structural reasons.

1. Preparation of Haloarenes\nHaloarenes can be prepared effectively through methods involving diazonium salts.

• Sandmeyer Reaction: When a primary aromatic amine is dissolved in cold aqueous mineral acid (like $HCl$) and treated with nitrous acid ($HNO_2$ or $NaNO_2 + HCl$) at $273-278K$, diazonium salt is formed. When this freshly prepared diazonium salt solution is reacted with cuprous chloride ($Cu_2Cl_2$) or cuprous bromide ($Cu_2Br_2$) dissolved in corresponding halogen acid, the diazo group is replaced by $-Cl$ or $-Br$ to form aryl halides (chlorobenzene or bromobenzene). $$\text{Ar-N}_2^+\text{Cl}^- \xrightarrow{Cu_2Cl_2 / HCl} \text{Ar-Cl} + \text{N}_2$$
• Gattermann Reaction: This is a modification of the Sandmeyer reaction. Instead of using cuprous halides, the freshly prepared benzene diazonium chloride solution is treated with copper powder in the presence of corresponding halogen acid ($HCl$ or $HBr$) to yield aryl halides. Although the yields are slightly lower than Sandmeyer, it serves as a prominent synthetic route. $$\text{Ar-N}_2^+\text{Cl}^- \xrightarrow{Cu / HCl} \text{Ar-Cl} + \text{N}_2 + \text{HCl}$$

2. Lower Reactivity of Haloarenes towards Nucleophilic Substitution\nHaloarenes are extremely unreactive towards nucleophilic substitution reactions under mild conditions compared to haloalkanes. The major reasons are:

• Resonance Effect: In haloarenes, the lone pairs of electrons on the halogen atom are in conjugation with the $\pi$-electrons of the benzene ring. This gives rise to several resonating structures, imparting partial double bond character to the carbon-chlorine bond. Due to this partial double bond character, the bond cleavage becomes difficult compared to the pure single bond in haloalkanes.
• Difference in Hybridization: In haloalkanes, the carbon atom attached to the halogen is $sp^3$ hybridized, whereas in haloarenes, the carbon atom is $sp^2$ hybridized. An $sp^2$ hybridized carbon is more electronegative, holds the electron pair more tightly, and has a shorter bond length ($169pm$) than an $sp^3$ hybridized carbon ($177pm$). Breaking a shorter and stronger bond requires significantly more energy.
• Repulsion between Nucleophile and Electron-Rich Arene Ring: Because the aromatic ring is electron-rich due to $\pi$-electron delocalization, an incoming negatively charged nucleophile experiences severe electrostatic repulsion, hindering its approach to the carbon atom.