Elimination Reactions41 kcal/mole and )>C=C<^->-C=C- 23 kcal/mole, though it
should be emphasised that these figures may be modified considerably,
depending on the structure of the original compound or the actual
atoms or groups that are eliminated during the change.
Some a-elimination reactions are known, however, in which both
groups are lost from the same carbon atom (p. 206) and also many
reactions in which the two carbon atoms losing groups are further
apart resulting in a cyclisation, as in the Dieckmann reaction (p. 178),
for example:
CH 2 —CH2 CH,—CH 2
I I QOEt I I
GHj CH 2 COsEt > CH 2 CH • C0 2 Et + EtOH
\ \ /
C OEt C
II II
o o
Elimination reactions are also known in which groups are lost from
atoms other than carbon: in the conversion of the acetates of aldox-
imes to nitriles, for example,
-Me CO.H
ArCH=NOCOMe > ArC=N
or i% the reversal^ the addition reactions of carbonyl groups
(p. 163)
H H
I -HCN I
R—C—OH > R—C=0
I
CN
though these reactions have been studied in less detail.
Elimination reactions have been shown to take place by either a
uni- or a bimolecular mechanism, designated as El and E2 respec
tively, by analogy with the SN\ and S^2 mechanisms of nucleophilic
substitution which they often accompany in, for example, the attack
of base on an alkyl halide:
R • CH=CH 2 +H 2 0 + Bre
©oH^y«» Elimination
RCH 2 *CH 2 Br #
©OHN^
RCH 2 CH 2 OH + Br©
Substitution