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9. Aromatic Hydrocarbons eLearn.Punjab

They undergo electrophilic addition reactions easily. Being relatively
unstable, alkenes undergo polymerization reactions and they are also readily
oxidized.

Benzene is unique in its behaviour. It is highly unsaturated compound
and at the same time it is very stable molecule. The stability of benzene,
as described earlier, is due to the extensive delocalization of π -electrons.
It resembles alkenes when it gives addition reactions. The substitution of
benzene, however, does not involve free radicals. These are electrophilic
substitution reactions and involve electrophiles.

Its addition reactions require more drastic conditions than those for alkenes.
Benzene does not undergo polymerization and it is also resistant to oxidation.

KEYPOINTS

1. Aromatic hydrocarbons include benzene and all those compounds that are
structurally related to benzene.

2. Aromatic hydrocarbons containing one benzene ring in their molecules are
called monocyclic aromatic hydrocarbons.

3. Aromatic hydrocarbons containing two or more benzene rings in their molecules
are called polycyclic aromatic hydrocarbons.

4. Benzene was discovered by Michael Faraday in 1825.
5. The electrons in benzene are loosely held and the ring acts as a source of

electrons. Hence benzene is readily attacked by electrophiles in the presence
of a catalyst.
6. Since electrophilic substitutions reaction lead to resonance stabilized benzene
derivatives so substitutions are the main reactions of benzene.
7. Resonance energy of benzene is 150.5 kJ/mole.
8. Structure of benzene is the resonance hybrid of two Kekule’s structures and
three Dewar’s structures.
9. Groups like NH2, NHR, - OR, - SH, - OCOR, - X, - OH, etc. which increase the
electron density in the nucleus and facilitate further electrophilic substitutions
are known as ortho- and para-directing groups.

10. Groups like CN, - CHO, NH3, NR3, CCl3 which hinder further substitution in the
benzene nucleus are known as meta- directing groups.

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