Introduction to "Benzene's" Chemistry: Benzyl Groups

The aromatic system also influences the chemical behavior of carbon atoms that are (directly) adjacent to it. Such carbons, which are directly bound to an aromatic carbon, are called benzylic carbons. The reactivity of the benzylic position is similar to that of an allylic position. The 2p orbital of an ${\text{sp}}^2$-hybridized benzylic carbon in a benzyl radical or cation overlaps with the 2p orbital of the adjacent aromatic carbon. Thus, it is conjugated to the aromatic π system, resulting in a higher degree of stabilization as compared to an alkyl radical or cation. Therefore, in many reactions, such as nucleophilic substitutions, during which a benzylic radical or cation intermediate is formed, benzylic compounds are more reactive than alkyl compounds. Benzyl chloride, for instance, reacts about 150 times more rapidly with ethanol in an ${\text{S}}_{\text{N}}1$ reaction than isopropyl chloride does, since the intermediate benzylic carbenium ion's positive charge is delocalized in the aromatic system. Like allyl compounds, benzyl compounds may also react in ${\text{S}}_{\text{N}}2$ reactions. These reactions proceed more rapidly than the corresponding reactions with alkyl compounds, as well. This is because in the transition state, the benzylic carbon is also partially positively charged and ${\text{sp}}^2$-hybridized. It is therefore stabilized analogous to the benzylic carbenium ion in an ${\text{S}}_{\text{N}}1$ reaction.

The oxidation of alkylbenzenes with $\text{K}\text{Mn}{\text{O}}_4$ or ${\text{H}}_2{\text{Cr}}_2{\text{O}}_7$ is one example of the considerable reactivity found with benzyl radicals. In contrast to non-benzylic alkanes, the oxidation of alkylbenzenes yields benzoic acid. In other words, carbon-carbon single bonds are broken. However, the alkylbenzene must contain at least one benzylic hydrogen atom. Therefore, tert-butylbenzene is not oxidized. In addition, the reaction mixture must be heated for a considerable amount of time in order to obtain complete conversion to benzoic acid.