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AROMATIC HYDROCARBONS

Structure of Benzene:

2
In benzene, C- atom is sp hybridised with planar structure. It has the following resonating
structures.

Aromaticity: Benzene is aromatic due to following conditions.

(i)Planarity. (ii)Complete delocalisation of ∏- electrons in the ring.

(iii)Presence of (4n+2) ∏ electrons in the ring. Where n is an integer (n = 0, 1, 2, ……….)
Methods of Preparation of Benzene:

1. Cyclic polymerization of Ethyne:
3 CH CH Red hot iron tube/873K C6H6
2. Decarboxylation:

3. Reduction of Phenol:
C6H5-OH + Zn heat C6H6 + ZnO
Chemical Properties:
1. Electrophilic substitution reaction:
(a) Halogenation: C6H6 + Cl2 Anhyd. AlCl3 C6H5-Cl + HCl
(b) Nitration: C6H6 + Conc. HNO3 conc.H2SO4 C6H 5-NO2 + H2O
(c) Sulphonation: C6H6 + Conc.H2SO4 C6H5-SO3H + H2O
(d)Friedel-Craft Alkylation: C6H6 + CH3Cl Anhyd. AlCl3 C6H5-CH 3 + HCl
(e)Friedel-Craft Acylation: C6H6 + CH3COCl Anhyd. AlCl3 C6H5-COCH3 + HCl
(f)With excess of Cl2: C6H6 + 6Cl2 Anhyd. AlCl3/ dark, cold C6Cl6 + 6HCl
2. Addition reactions:
(a) Addition of H2: C6H6 + 3H2 Ni/heat C6H12
(b) Addition of halogen: C6H6 + 3Cl2 U.V/500K C6H6Cl6
3. Combustion: When heated with air benzene forms CO2 and H2O
C6H6 + 15/2 O2 6CO2 + 3H2O
Directive influence of functional group in monosubstituted benzene:
1. Ortho and para directing groups: This group directs the incoming group to ortho
and para positions. These are electron donating and ring activating groups.
Ex: -OH, -OR, -R, -NH2, -NHR, -NHCOCH3,
2. Meta directing groups:
This group directs the incoming group to meta positions. These are electron
withdrawing and ring deactivating groups. Ex: -NO2, -CN, -CHO, C=O, -COOH, -COOR