MP Board · Class 12 · Chemistry · Haloalkanes and HaloarenesDiscuss the preparation methods of haloarenes. Explain why haloarenes are much less reactive towards nucleophilic substitution reactions compared to haloalkanes, and describe their electrophilic aromatic substitution reactions with two examples.

1. Preparation Methods of Haloarenes

• From Hydrocarbons via Electrophilic Substitution: Aryl chlorides and bromides can be easily prepared by electrophilic substitution ofarenes with chlorine and bromine in the presence of Lewis acid catalysts like iron(III) chloride or iron(III) bromide.
• Sandmeyer's Reaction: When a primary aromatic amine, dissolved in mineral acid, is treated with cold nitrous acid ($HNO_2$), diazonium salts are formed. Mixing this solution with copper(I) chloride or copper(I) bromide results in the replacement of the diazonium group by $-Cl$ or $-Br$.

2. Reasons for Lower Reactivity of Haloarenes towards Nucleophilic Substitution\nHaloarenes are extremely less reactive towards nucleophilic substitution compared to haloalkanes due to the following factors:

• Resonance Effect: The lone pairs of electrons on the halogen atom are in conjugation with the $\pi$-electrons of the benzene ring. This gives the C-Cl bond partial double bond character, making it stronger and harder to cleave than a single bond.
• Difference in Hybridization: In haloalkanes, the carbon atom attached to halogen is $sp^3$ hybridized, whereas in haloarenes, it is $sp^2$ hybridized. An $sp^2$ hybridized carbon is more electronegative and holds the electron pair more tightly, resulting in a shorter and stronger bond.
• Instability of Phenyl Cation: In case of an $S_N1$ mechanism, the phenyl cation formed by the self-ionization of haloarene would not be stabilized by resonance, making the formation energetically unfavorable.
• Repulsion: Electron-rich nucleophiles experience electrostatic repulsion when approaching the electron-rich $\pi$-cloud of the benzene ring.

3. Electrophilic Aromatic Substitution Reactions\nHaloarenes undergo usual electrophilic reactions of the benzene ring such as halogenation, nitration, sulphonation, and Friedel-Crafts reactions. The halogen atom shows $-I$ effect and $+R$ effect. The $+R$ effect directs the incoming electrophile to ortho- and para-positions.

• Halogenation: Treatment of chlorobenzene with chlorine in the presence of anhydrous $AlCl_3$ gives 1,4-dichlorobenzene (major) and 1,2-dichlorobenzene (minor).
• Friedel-Crafts Alkylation: Reaction of chlorobenzene with alkyl chloride in the presence of anhydrous aluminum chloride yields 1-chloro-4-methylbenzene and 1-chloro-2-methylbenzene.