We have already seen in section 14.2 how substituents (such as alkyl groups) affect the reactivity of the benzene ring to which they are attached. However, as is common in chemistry, the effects are felt both ways – the alkyl group reactivity is affected by the benzene ring. Because alkylbenzenes are very common compounds – both synthetic and natural – this is worth examining further. Alkylbenzenes can also be made from simpler benzene compounds by the Friedel-Crafts reaction, as described in section 14.2.
The benzyl group is not what you might expect – a group based on benzene itself is called a phenyl group (abbreviated as Ph-). Rather unexpectedly, a benzyl group (abbreviated as Bn-) is a benzene ring attached to a side chain carbon, i.e., PhCH2– . This carbon next to a ring tends to be much more reactive than a normal alkyl carbon, because any charges or unpaired electrons on that carbon can be stabilized by resonance.
For example, chloroethane reacts only very slowly with water, because an SN1 reaction is impossible for this primary alkyl halide (the primary alkyl carbocation would be too unstable). In contrast, benzyl chloride reacts fairly rapidly with water, because it can form a resonance-stabilized benzyl carbocation.
The rate determining step for SN1 is formation of a carbocation via a heterolysis step; in the case of benzyl chloride, the benzyl carbocation that forms has several resonance forms:
As Hammond’s Postulate tells us, this stable carbocation will form fairly quickly, making the SN1 reaction viable even though benzyl chloride is formally a primary alkyl halide. In other reactions that might involve a benzyl carbanion or a benzyl radical, these will also proceed more quickly because of resonance stabilization.
As we have seen, the benzylic carbon is more reactive than a normal alkyl carbon. This also applies to redox reactions. As we saw previously an aryl ketone can be reduced to an alkylbenzene using H2/Pd. An alkylbenzene can also be oxidized to an aryl ketone, or to a carboxylic acid, as seen in the later sections in this chapter.