80 Aromatic reactions
<< Aromatics^1 | Aromatic reactions | Ketones and aldehydes>>^2The lack of reactivity of arenes is notable when compared to the reactivities of typical
compounds containing multiple conjugated double bonds. For example, 1,3,5-hexatriene is
much more reactive than hexane, hexene, or any hexadiene. Benzene is much less reactive
than any of these. Any of the alkenes will be readily converted to alcohols in the presence
of a dilute aqueous solution of H 2 SO 4 , but benzene is inert. Similarly, alkenes react readily
with halogens and hydrogen halidesby addition to give alkyl halides, whereas halogens
react with benzene by substitutionand only in the presence of a catalyst. KMnO 4 or
chromic acid solutions (typically CrO 3 or K 2 Cr 2 O 7 ) cleave the double bonds of alkenes,
giving ketones or carboxylic acids, but do not react at all with benzene. Because of the
stability of aromatic compounds, however, reactions involving these have extremely high
activation energies, for passage to the transition state necessarily requires disruption of the
aromatic system, resulting in a temporary loss of aromatic stabilization energy. Instead of
reacting by addition and elimination, as nonaromatic compounds often do, benzene and its
derivatives usually react by electrophilic aromatic substitution.
1 Chapter 68 on page 241
2 Chapter 86 on page 287