Cis-EliminationHydrogen halide is lost in a two-stage process to yield carbon dichlor-
ide, a carbene (cf. p. 93), as an intermediate in the hydrolysis. The
latter then reacts with water to yield CO as the primary product and
this then undergoes further slow attack by^0 OH to yield formate
anion. The initial attack on H rather than C (with expulsion of Cle)
by eOH is due to the electron-withdrawing effect of the chlorine
atoms increasing the acidity of the hydrogen atom, a property which
is reflected in its ready base-catalysed exchange with deuterium in
DaO. Confirmation of the existence of CC1 2 , i.e. of an a-elimination,
is provided bjtf he introduction of substrates into the system that would
be expected to react readily with such a species; thus olefines have been
converted into cyclo-propane derivatives :
Me Me Me Me(^0) OH l\ 9/
CHCU —*• ecu —•
^1
Another example of an a-elimination is seen in'the action of
potassium amide on 2,2-diphenylvinyl bromide (XXXV):
0
NHS NH 3Ph H (PIP
C=CV ->• ->Hb==Ce Ph—C±C—Ph -+ Ph—C=C—Ph
Ph Br Ph (^Br Br©
(XXXV) (XXXVI)Whether the whole process proceeds as a concerted operation or
whether the carbanion (XXXVI) is actually formed, as such, is not
known however.CIS-ELIMINATION
A number of esters, particularly acetates, are known to undergo
elimination reactions on heatingt^n the absence of solvent, to yield
olefines: #
O O
\ I II V / II
pCH—C—O—C—R -* ^>C=C<^+HO—C—R