A Guidebook to Mechanism in Organic Chemistry

Elimination Reactions

In cases such as (V) where a sterospecific trans elimination cannot
take place cis elimination can be made to occur, though normally only
with considerable difficulty. The reaction then probably proceeds via
carbanion formation (cf. p. 193), a route that normally involves a
considerably higher free energy of activation than that via a normal
E2 'trans' transition state with consequent increase in the severity of
the conditions necessary to effect it.
In compounds in which no restriction of rotation about a bond
is imposed, the leaving groups will arrange themselves so as to
be as far apart as possible when they are elimination Thus meso
dibromostilbene (XIV) yields a cis unsaturated compound (XV),
whereas the corresponding DL-compound (XVI) yields the trans
form (XVII):

sQ H 20

HO'H ^ Ph Ph Ph

<\ A \ >

Ph*-C*-C—H -* C=C

Br' £flr Br' H

|XIV) (XV)

Br ©

sQ H 20

HO 'H • H Ph H

i^CJi-C—Ph ^ c=c

Br' ^JBr Br^ Vh

Br©

(XVI) (XVII)

A 'ball-and-stick' model will be found useful for confirming the true
stereochemical course of these eliminations.

(ii) Orientation in E2 eliminations Saytzeff v. Hofmann

The situation frequently arises in base-induced elimination reactions
e
of alkyl halides, R-Hal, and alkyl onium salts, such as RNR' 3 and
©
RSR'u, that more than one define can, in theory, be produced: