Although benzene molecule is unsaturated yet it does not give the test for unsaturation, it behaves as stable molecule and gives electrophilic substitution reaction. This property of benzene is known as aromaticity. Organic compounds which have similar chemical behaviour as benzene show aromatic property. Some important properties of the these compounds are shown below:
(i) Aromatic compounds although possess double bonds yet they do not give addition reaction. On the other hand they show electrophilic substitution reaction like nitration, sulphonation, alkylation, halogenations etc.
(ii) According to molecular structure these compounds should shoe high unsaturation but they do not give test for unsaturation.
(iii) Aromatic compounds are highly unstable as shown by their low enthalpy of hydrogenation.
(iv) Aromatic compounds generally contain five, six or seven membered ring and are found to be planer structure.
1. Huked (4n +2) π rule
W. Huckel in 1931 gave a rule according to it “Monocyclic planer molecules of sp2 hybridized carbon atoms containing (4n +2) π electrons (n = 0, 1, 2, 3, —–etc.) shows aromatic character”.
(a) 2 π electron system : (n = 0; 2 π) – It is obvious that there cannot be a ring of two carbon atom hence a three membered ring with a double bond and a positive charge on the third carbon namely cyclopropeny1 cation should be stable. Example, dipropyl cyclopropenium perchlorate.