Alkylarenes
It is fairly easy to create an electron excess or deficiency at the benzylic position because the aromatic π cloud delocalizes it.
The carbon bonded to the aromatic ring, whether it is a carbocation, a radical or a carbanion, adopts sp2 hybridization and its "remaining" p orbital conjugates with the ring π orbitals.
Reactions going through a benzylic carbocation, radical or carbanion are easier than expected.
Benzylic halogenation
The aromatic compounds like benzene or pyridine do not react with bromine without a catalyst, as alkenes (addition) or alkanes (radical halogenation) do.
Click here to recall alkene halogenation.
Click here to review alkane halogenation.
Toluene and any of the three possible methylpyridines do react with bromine without a catalyst, but at the methyl group (radical halogenation) and with special ease.
Benzylic halogenation is especially favorable. It costs less energy than at a "normal" alkyl carbon. The C–H bonds at the benzylic position are slightly weaker than expected.
Nucleophilic substitution
The solvolysis of a tosylate is faster at the benzylic position because it probably goes through a benzylic carbocation, less unstable than a "normal" alkyl carbocation.
Deprotonation (acidity)
Toluene can lose a proton from its methyl group, which is apparently a normal sp3 carbon. That can be done with a strong base like butyllithium whose carbons are sp3 as well.
However, toluene's pKa is 9 units lower than butane's (toluene is 109 times more acidic than butane) because the benzylic carbanion is stabilized by resonance with the aromatic ring. That's why it is formed with less difficulty than expected.
Benzylic oxidation
Oxidation processes usually go through free radicals. That's why they take place easily at benzylic positions.
Benzylic hydrogenolysis
The selective hydrogenolysis of groups bonded to the benzylic position can be easily achieved, for instance that of an ether.
