A Guidebook to Mechanism in Organic Chemistry

3

NUCLEOPHILIC SUBSTITUTION AT A

SATURATED CARBON ATOM

A TYPE of reaction that has probably received more study than any
other—largely due to the monumental work of Ingold and his school—
is nucleophilic substitution at a saturated carbon atom: the classical
displacement reaction exemplified by the conversion of analkyl halide
to an alcohol by the action of alkali:
HO © + R—Hal - HO—R + Hal©
Investigation of the kinetics of such reactions has shown that there
are essentially two extreme types, one in which
Rateoc [R-Hal][©OH].. (1)
and another in which:
Rateoc [R-Hal] i.e. is independent of [©OH] (2)
In many examples the kinetics are mixed, showing both types of rate
law simultaheously, or are otherwise complicated, but cases are known
which do exempliffthe simple relations shown above.

RELATION OF KINETICS TO MECHANISM

The hydrolysis of methyl bromide in aqueous alkali has been shown to

proceed according to equation (1) and this is interpreted as involving

the participation of both halide and hydroxyl ion in the rate-determin­

ing (i.e. slowest) step of the reaction. Ingold has suggested a transition

state in which the attacking hydroxyl ion becomes partially bonded to

the reacting carbon atom before the incipient bromide ion has become

wholly detached from it; thus part of the energy necessary to effect the

breaking of the C—Br bond is then supplied by that produced in

forming the HO—C bond. Calculation shows that an approach by the

hydroxyl ion along the line of centres of the carbon and bromine atoms

is that of lowest energy requirement. This can be represented:

H H H ~\ H

HOe + H»-C—Br -+HOC Br -* HO—C-H + Br©

/ I \

H H H

Transition state