Aromatic hydrocarbons are highly unsaturated molecules but behave like saturated hydrocarbons. Those which contain a benzene ring are called benzenoids but those which contain some highly unsaturated ring other than benzene are called non-benzenoids. Their aromatic character can be ascertained through Hückel rule. Aromatic hydrocarbons are, however, more reactive than alkanes but are less reactive than alkenes and alkynes. The lower reactivity of arenes is because of the extra stability associated with these molecules due to delocalization of -electrons. Their stability is measured in terms of resonance energy which can be estimated from either heat of combustion or heat of hydrogenation data. They normally undergo electrophilic substitution reactions. In presence of a Lewis acid catalyst, nuclear halogenation occurs but in absence of Lewis acid catalyst and in presence of light, halogens add to the benzene ring. If an aromatic hydrocarbon contains an alkyl side chain, then in presence of heat/light side chain halogenation occurs in preference to addition of halogens to the benzene ring. The reactivity of aromatic hydrocarbons towards electrophilic substitution reactions depends upon the electron density in the benzene ring. Electron-donating groups favour while electron-withdrawing groups retard these reactions. Orientation of electrophilic substitution reactions is governed by the nature of the substituent already present in the ring. Although aromatic hydrocarbons are resistant to oxidising agents `(KMnO_4, K_2Cr_2O_7`, etc.) they do undergo ozonolysis.
Which of the following is not a non-benzenoid aromatic compound ?
A. Tropylium cation
B. Azulene
C. Tropolone
D. Diphenyl
