A Guidebook to Mechanism in Organic Chemistry

Elimination Reactions

then heated. Removal of the nitrogen from the compound by one

such treatment indicates that it was present in a side-chain while

elimination after two or three treatments, indicates its presence in a

saturated ring or at a ring junction, respectively. The resultant olefine

is then investigated so as to shed further light on the structure of the

original natural product.

The presence of a phenyl group on the a- or /3-carbon atom very

markedly promotes E2 eliminations because of its stabilisation of the

resultant olefine by delocalisation. The effect is more marked in the

/?- than in the a-position, however, because of the additional effect

of phenyl in increasing the acidity of the j8-hydrogens from this posi­

tion and so facilitating their removal. The effect is sufficiently pro­

nounced so as to control the orientation of elimination, resulting in

the Saytzeff mode even with onium salts:

Me

le ©OH

PhCHjCH,—N—CHaCH 3 * Ph • CH=CH,+Me,N • CH, • CH,

I

Me

A vinyl group willjiave much the same effect.

A^steric limitation on elimination reactions is codified in Bredt's

rule that reactions which would introduce a double bond on to a

bridgehead carbon atom in bicyclic systems do not take place. Thus

(XXV) does not yield the bicycloheptene (XXVI) which has, indeed,

never been prepared:

(XXV) (XXVI)

This is presumably due to !he bond angles required by the
rigid ring system preventing any degree of attainment of the planar
configuration required for significant w bonding to the adjacent
carbon atom. It should be emphasised in this connection that the
Bredt rule does not thus apply to compounds such as (XXVII)