A Guidebook to Mechanism in Organic Chemistry

Orientation in E2 Eliminations

R • CH, • CH,—CH—CH,;

RCH,CH,CH=CH,

(XIX)

(XVIII)

Y RCH,CH=CHCH,

(XX)

(Y=Hal, NR',orSR'J

Three factors, essentially, influence the relative proportions of
olefine that^je actually obtained: (a) the relative ease with which a
proton can be lost from the available, alternative /^-positions, (b) the
relative stability of the olefines, once formed (more accurately, the
relative stability of the transition states leading to them), and (c)
steric effects (arising from substitution at the /J-positions, the size of
the leaving group Y, and the size of the base used to induce the
elimination). The relative significance of, and conflict between, these
factors has led in the past to the empirical recognition of two opposing
modes of elimination: Saytzeff elimination, leading preferentially to
the olefine carrying the larger number of alkyl groups, i.e. (XX) rather
than (XIX), and Hofmann elimination, leading preferentially to the
olefine carrying the smaller number of alkyl groups, i.e. (XIX) rather
than (XX). *
The Saytzeff mode, which is principally encountered in the llimin-
ation reactions of halides, is easy to justify in terms of (b) for the
olefine carrying the larger number of alkyl groups can be shown by
combustion experiments to be more stable than its less alkylated
isomers, a fact that may be explained by hyperconjugation. Thus
(XX) has five C—H linkages adjacent to the double bond compared
with only two for (XIX) and a greater number of forms such as (XXI)
can therefore contribute to its stabilisation by delocalisation (cf.
p. 21):

It should be remembered, howwer, that it is the hyperconjugative

effect of alkyl groups in the E2 transition state rather than in the end

product, that is of prime importance: alkyl hyperconjugation with

the forming double bond lowers the energy of that transition state

in which it occurs and hence favours its preferential formation.

R CH, CH—CH=CH, H

(XXI)