Structure, Reactivity and Mechanism , •H.ORCH 2 CH 2 Hal -> Hal® + R • CH 2 • CH 2 -
(L)Substitution-H
RCH=CH 2 ,
Eliminationas simple as might, at first sight, be supposed. Thus in a very simple
case, if the aqueous hydrolysis of the alkyl halide, R-Hal, is found to
follow the kinetics
Rate oc [R-Hal]
it is not safe to conclude that the rate-determining step does not in
volve the participation of water because [H 2 0] does not figure in the
rate equation, for if water is being used as the solvent its concentra
tion would remain virtually unchanged whether or not it actually
participated in the reaction. Its participation might be revealed by
carrying out the hydrolysis with only^ small concentration of water
in another solvent; then the hydrolysis may be found to. follow the
kinetics
Rate oc [R-Hal] rHjO]
but the actual mechanism by which the reaction proceeds may have
changed on altering the environment, so that we are not, of necessity,
any the wiser about what really went on in tie aqueous solution to
begin with!
The vast majority of organic reactions are carried out in solution
and quite small changes in the solvent used can have the profoundest
effects on reaction rates and mechanisms. Particularly is this so when
ionic intermediates; for example carbonium ions and carbanions, are
invoked for such ions normally carry an envelope of solvent mole
cules about with them, which greatly affects their stability (and hence
their ease of formation), and which is higher influenced, by the com
position and nature of the solvent employed.
Light may be thrown on reaction mechanisms from sources other
than kinetic experiments, however. Thus a study of alternative or
by-products in a reaction can often be of value. A case in point is the
elimination reaction (p. 189) that often accompanies a substitution
reaction (p. 201) in the action of alkali on alkyl halides and which can
be interpreted in some instances as involving a carbonium ion (L) as
a common intermediate:
RCH 2 CH 2 OH+H^8
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