A Guidebook to Mechanism in Organic Chemistry

Elimination Reactions

ing as quite separate competing reactions and lends support to a car­
bonium ion as the common intermediate; for otherwise the nature of
the leaving group would be expected to play a significant role leading
to a change in the proportion of elimination to substitution products
as it was varied. Variation of the structure of the alkyl group, however,
has a considerable effect on the relative amounts of elimination and
substitution that take place. It is found that branching at the /J-carbon
atom tends to favour El elimination; thus MeCH 2 CMe 2 Cl yields
only 34 per cent of olefine whereas Me 2 CH-CMe 2 Cl yields 62 per
cent. The reason for this may, in part at least, be stwis: the more
branched the halide, the more crowding is released when it is con­
verted to the carbonium ion intermediate, but crowding is again
introduced when the latter reacts with an entering group (-»• substi­
tution); by contrast, loss of a proton ( -*• elimination) results, if any­
thing, in further relief of strain and so is preferred. Study of a range
of halides shows that this is not the whole of the story however.
Hyperconjugation may also play a part, as will be seen below (p. 197)
in considering the preferential formation of one isomeric olefine
rather than another from a carbonium ion in which there is more than
one j3-carbon atom which can lose a proton. The El mechanism is
also encountered inrfhe acid-induced dehydration of alcohols:

* H®^ <B -H,O —H®

Me 3 COH —> MesCOH —> Me„C® —> MeaC=CHj

H

THE E2 MECHANISM

In the alternative E2 mechanism, the rate of elimination of, for ex­

ample, hydrogen halide from an alkyl halide induced by eOH is

given by:

Rate « [R-Hal] [©OH]

This rate law has been interpreted as involving the abstraction of a

proton from the /3-carbon atom by base, accompanied by a simul­

taneous loss of halide ion from the a-carbon atom:

HO^ H-4^C-^Hal * H,0+NvC=C// + HaI©

II / N#

It might be objected that there is no necessity for proton abstrac­

tion and halide elimination to be simultaneous, that initial removal