Haloarenes — Part 2: Electrophilic and nucleophilic substitution, metals and DDT: visual lesson, ISC Class 12 Chemistry

Lesson

Reactions of haloarenes

Part 2 — substitution on the ring, replacement of the halogen, reactions with metals, and DDT.

In this lesson you will

Draw the mechanism of nitration of chlorobenzene and explain the ortho/para directing but deactivating effect of halogen
Explain why haloarenes resist nucleophilic substitution and how ortho/para nitro groups assist it
Write equations for reduction, Wurtz–Fittig, Fittig and Grignard reactions
Draw the structure of DDT and explain why its use is restricted

Concept

Halogen: deactivating but ortho/para directing

Halogen has a strong −I effect (withdraws electrons through the σ bond) that makes the ring less reactive than benzene, and a +R effect (lone pair into the ring) that increases electron density at ortho and para positions. −I outweighs +R for reactivity; +R decides orientation.

Table of the inductive and resonance effects of halogen
Effect	Direction	Result
−I	Electrons withdrawn from ring	Deactivation: slower than benzene
+R	Lone pair donated to o, p	o/p directing

Concept

Mechanism: nitration of chlorobenzene

Step 1 (electrophile): $\ce{HNO3 + 2H2SO4 -> NO2+ + H3O+ + 2HSO4-}$ .

Step 2 (slow): NO₂⁺ attacks the ring at o or p, forming a σ-complex (arenium ion). For o/p attack, one resonance form puts + on the carbon bearing Cl, where Cl’s lone pair stabilises it (a chloronium form); meta attack has no such form.

Step 3 (fast): HSO₄⁻ removes H⁺, restoring aromaticity. Products: 1-chloro-4-nitrobenzene (major) and 1-chloro-2-nitrobenzene.

\ce{C6H5Cl + HNO3 ->[conc. H2SO4] ClC6H4NO2 + H2O}

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