Elimination v. SubstitutionBr
(XXVII)nor to compounds in which the bridge comprises five or more atoms,
for a sufficiently planar conformation can then be attained without
too great an overall straining of the system.
Finally, it should be emphasised that in El eliminations where
more than one possible olefine can be obtained, the product which is
stabilised by hyperconjugation will almost always predominate.
The leaving group, having already departed, can exert no influence,
and the process is completed by loss of proton from that ^-position
which will yield the stabler olefine, i.e. (XXVIII) rather than (XXIX):
RCH=CMe,
(XXVIII)RCH 2 C(Me)=CH 2- (XXIX)
m» •
ELIMINATION v. SUBSTITUTION
Broadly speaking changes in reaction conditions that would be
expected to promote an SJV2 reaction at the expense of an SNl
(p. 60) will promote the often competing E2 reaction at the ex
pense of an El and, of course, vice-versa. The features that will
favour overall elimination at the expense of substitution are a little
more subtle, though some passing attention has already been paid
to them; thus in the El reaction reference has already been made to
steric features. The more crowded a halide, for example, the greater
is the release of strain when the carbonium ion intermediate is formed.
This strain is reintroduced on attack by a nucleophile but is not in
creased, and may even be further reduced, on removal of a proton
to yield the olefine. The sheer steric effect here becomes merged with
other features, liowever, for increasing alkyl substitution may also
lead to the possible formation of olefines that are increasingly stabi
lised by hyperconjugation, thus favouring their formation at the
expense of substitution. This, of course, is the reason for the greater
©/
Me
RCH 2 CMe 2 Br R-CHa-C
Me