Conjugated doable bonds

A complete definition of aromatic compounds must wait until Chapter 16. For the present we will define them as benzene and its substituted derivatives. They are also called arenes. These aromatic compounds have a six-membered ring with three conjugated doable bonds. It is this cycle of conjugated double bonds that makes arenes special. Examples include the following ... 

The 3H- and 4//-pyrazoles and 2H- and 4//-imidazoles (83UP40200) contain two doable bonds in the heterocyclic ring, but in each case the conjugation does not include all the ring atoms hence the compounds are not aromatic. 

Reaction with the Doable Bond of Dienes and Conjugated Systems... 

One of the most striking differences between conjugated dienes and typical alkenes is in their electrophilic addition reactions. To review briefly, the addition of an electrophile to a carbon-carbon doable bond is a general reaction of alkenes (Section 6.8). Markovnikov regiocheraistry is found because the more stable carbocation is involved as an intermediate. Thus, addition of HCl to 2-methylpropene yields 2-chloro-2-methylpropane rather than 1-chloro-2-methylpropane, and addition of 2 mol equiv of HCl to the noncon-jugated diene 1,4-pentadiene yields 2,4-dichloropentane. 

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