A benzylic cation has five resonance contributors. Notice that the positive charge is
shared by four carbons.Not all allylic and benzylic cations have the same stability. Just as a tertiary alkyl
carbocation is more stable than a secondary alkyl carbocation, a tertiary allylic cation
is more stable than a secondary allylic cation, which in turn is more stable than the
(primary) allyl cation. Similarly, a tertiary benzylic cation is more stable than a
secondary benzylic cation, which is more stable than the (primary) benzyl cation.Because the allyl and benzyl cations have delocalized electrons, they are more
stable than other primary carbocations. (Indeed, they have about the same stability as
secondary alkyl carbocations.) We can add the benzyl and allyl cations to the group of
carbocations whose relative stabilities were shown in Sections 4.2 and 6.4.Notice that it is the primarybenzyl and the primaryallyl cations that have about the
same stability as secondaryalkyl carbocations. Secondary benzylic and allylic cations,
as well as tertiary benzylic and allylic cations, are even more stable than primary
benzyl and allyl cations.PROBLEM 8Which carbocation in each of the following pairs is more stable?a. or c. orb.d.e. or CHCH 2OCH 3CH 3 OCH+ +
CH 2 CCH 2or+
CHCH 3+
CHCH 3+ or +CH 3 OC+
CH 3 OCH 2 C H 2+
CH 3 NHC H 2+
CH 3 O H 2+
H 2most
stablemost
stableRCH 2 CH R >>++C CH 2 CHCH RRR+
> CH 2 CHCH 2+tertiary allylic cationrelative stabilitiestertiary benzylic cation secondary benzylic cationsecondary allylic cation allyl cationC+
CH >benzyl cation+
CH 2RSection 7.7 Stability of Allylic and Benzylic Cations 2793-D Molecules:
Allyl cation; Benzyl cationa benzylic cationCHR CHR CHR++++
+
CHR CHRRR RRRR CH 2 CH 2 CHCH 2++
CH 2 CH+
C+HC+HHH C+HHrelative stabilities of carbocationsC+a tertiary
carbocationbenzyl
cationallyl
cationa secondary
carbocationa primary
carbocationvinyl
cationmethyl
cationmost > ≈ ≈>>>
stableleast
stableBRUI07-263_297r4 21-03-2003 11:31 AM Page 279