Chapter 11 : Benzylic systems

Chapter 11 : Arenes and Aromaticity

Benzylic systems

As we saw in chapter 10, the positions adjacent to C=C, the allylic position, often show enhanced reactivity compared to simple alkanes due to the proximity of the adjacent p system.
Similarly, the positions adjacent to a benzene ring, known as the benzylic position also show enhanced reactivity compared to simple alkanes.

Students often confuse the term benzyl with phenyl, for example compare the location of the bromine atoms in bromobenzene and benzyl bromide:

		Highlight benzylic hydrogen atoms Highlight benzylic carbon atom Highlight phenyl hydrogen atoms Highlight phenyl carbon atoms Reset colours
Bromobenzene (phenyl bromide)	Benzyl bromide

Benzylic carbocations

The p system of a benzene ring can stabilize an adjacent carbocation by donating electron density through resonance. Remember that delocalising charge is a stabilising effect.

Note that in the resonance forms of the benzylic cation, the positive charge is located on the ortho and para positions of the benzene ring, but not the meta positions. This is reflected in the resonance hybrid.
Due to the stability of these benzylic cations, they are readily formed as intermediates during chemical reactions, for example SN1 reactions of benzylic halides. Note that 2-chloro-2-phenylpropane is 600 times more reactive than the 2-methyl analogue.

Benzylic radicals

Benzyl radicals can also be stabilized by resonance in the same manner as shown above for carbocations.