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Chapter 11 : Arenes and Aromaticity |
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Benzylic systems
- As we saw in chapter 10, the
positions adjacent to C=C in alkenes is referred to as the allylic position. They often show
enhanced reactivity compared to simple alkanes due to the proximity of the
adjacent π 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:
Benzylic carbocations
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| The p system of a benzene ring can stabilise 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 that the 2-methyl analogue.
Benzylic radicals
Benzyl radicals can also be stabilised by resonance
in the same manner as shown above for carbocations.
