Subsitution Reactions in Haloarenes

1. Nucleophilic Substitution Reaction

Aryl halides are less reactive towards nucleophilic substitution reaction. Their low reactivity is attributed due to the following reasons:

1. Due to resonance, C-X bond has partial double bond character.

2. Stabilisation of the molecule by delocalisation of electrons.

3. (Instability of phenyl carbocation.

However, aryl halides having electron withdrawing groups (like – NO2, -SO3H, etc.) at ortho and para positions undergo nucleophilic substitution reaction easily.

Presence of electron withdrawing group (-NO₂) increases the reactivity.

2. Electrophilic Substitution Reactions

Halogens are deactivating but o, p-directing. Thus, chlorination, nitration, sulphonation and

Friedel Craft’s reaction give a mixture of o- and p- chloro substituted derivatives.

In all this reactions Electrophile is generated which attacks on benzene ring due its negative charge holding nature. This particular ion complex is stabilized and then Electrophile is added as per the Ortho-Para / Meta directing nature of compound. Mostly para compounds are stable.

For Halogenation X⁺ is Electrophile [X = Cl, Br, F, I]

Nitration NO₂⁺ is Electrophile

Sulphonation SO₃ is Electrophile

Fridel Crafts Alkyl / Acyl group becomes electrophile

(i) Halogenation

(ii) Nitration

(iii) Sulphonation

(iv) Friedel-Crafts reaction