Benzene and other organic compounds which resemble benzene in certain characteristic properties are called aromatic compounds. These characteristic properties constitute what is commonly known as aromatic character or aromaticity. Such important properties are summarised here.
(i) Unusual stability- Aromatic compounds are highly stable as shown by their low heats of hydrogenation and low heats of combustion. For example, heat of hydrogenation of benzene is only 49.8 kcal/mole as compared to that of the hypothetical cyclohexatriene (85.8 kcal/mole). Similarly, heat of combustion of benzene (789.1 kcal) is low as compared with that for cyclohexatriene (about 824.1 kcal).
(ii) Substitution rather that addition reactions- Aromatic compounds although possess double bonds, they do not undergo addition reactions. On the other hand, they undergo electrophilic substitution reactions like nitration, halogenation, sulphonation and Friedel Craft's reactions.
(iii) Resistant to oxidation- Aromatic compounds are resistant to oxidation by aq. KMnO4 HNO3 and other mild oxidising agents.
(iv) Cyclic flat molecules- Aromatic compounds generally contain five-, six or seven-membered rings and are found to have flat (or nearly flat) structures.
