Elimination Reactions
a(i) Stereospecificity in E2 eliminations
It has been found that E2 elimination reactions exhibit a high degree
of stereospecificity, proceeding considerably more readily if the
groups to be eliminated are trans to each other.
Thus it is fourfO that of the stereoisomerides of benzene hexa-
chldride, C„H 6 CI 6 ^>ne isomer loses HC1 10,000 times more slowly
than any of the others and this is found to be the one (V) that has no
adjacent chlorine and hydrogen atoms trans with respect to each
other:
CI H
H/} b ci
" H ClH Cl v (V)This stereospecificity brings to mind the characteristic 'attack from
the back* of the SN 2 reaction (p. 65) and probably results from the
electrons released by removal ofthe proton from the /3-carbon atom
attacking the a-carbon atom, 'from the back', with displacement of
the leaving group (VI -*• VII, see p. 195).
It has been suggested that it is necessary that the attacking atom of
the base, the hydrogen atom to be eliminated, C„, Ca and the otherwe have thus now seen elimination reactions in which the H—C bond
is broken before (trichlorethylene above), simultaneously with (E2),
and after (El), the C—Y bond.
An E2 elimination will naturally be promoted, relative to an S^2
substitution, by any features that serve to stabilise the resultant
olefine or, more particularly, the transition state leading to it; a good
example is substitution by phenyl on the jS-carbon atom: