Name the reaction where Lewis acid is used as catalyst for aromatic substitution.

1. Arenes:

(i) Arenes are aromatic hydrocarbons. They contain at least one benzene ring.

(ii) Aromatic hydrocarbons undergo mainly electrophilic substitution reactions inspite of high degree of unsaturation present in them.

(iii) Aromaticity in aromatic compounds is due to the presence cyclic, delocalized system of $(4\mathrm{n} + 2)\mathrm{\pi }$ electrons.

(iv) Electron releasing groups increase the reactivity of benzene ring towards electrophilic substitution reactions. Examples of such groups are —R,—OH,—NH2,—OR,—NHCOCH3, etc.

(v) Electron withdrawing groups decrease the reactivity of benzene ring towards electrophilic substitution reactions.

2. Benzene:

(i) The six carbon atoms in benzene are ${\mathrm{sp}}^2$ hybridised.

(ii) The resonance stabilisation energy for benzene is $36 \mathrm{K} \mathrm{cal}/\mathrm{mole}$.

(iii) Benzene can be isolated from the light oil fraction of coal tar.

(iv) Benzene undergoes electrophilic substitution reactions like nitration, sulphonation, halogenation and Friedel craft's reaction.

(v) Benzene and many polynuclear hydrocarbons containing fused benzene rings are known to possess carcinogenic (cancer causing) properties.