Relation of Kinetics to MechanismThe negative charge is spread in the transition state in the course of
being transferred from hydroxyl to bromine, arid the hydrogen atoms
attached to the carbon atom attacked pass through a position in
which they all lie in one plane (at right angles to the plane of the paper
as drawn here). This type of mechanism has been named by Ingold
SJV2, standing for Substitution Nucleophilic bimolecular.
A certain element of confusion is to be met with both in text-books
and in the literature over the use of the term bimolecular, particularly
in its confusion with second order as applied to reactions. It is probably
simplest to reserve the latter nurely for a description of the type of
kinetic equation that a reaction follows: thus equation (1) represents a
second order reaction, being first order in each reactant and equation
(2) represents a first order reaction; while molecularity is reserved for
a description of the mechanism proposed, being used in the sense
of specifying the number of species that are actually undergoing
covalency changes in the rate-determining stage. Thus, hydrolysis of
methyl bromide under the conditions specified, not only exhibits
second order kinetics but, as represented mechanistically, is clearly a
bimolecular reaction.
By contrast, the hydrolysis of t-butyl chloride in alkali is found
kinetically to follow equation (2), i.e. as the rate is independent of
[eOH], this can play no part in the rate-determining step. This has
been interpreted as indicating that the hllide undergoes^low
ionisation (in fact, completion of the R->—Hal polarisation that
has already been shown to be present in such a molecule) as the rate-
determining step, followed by rapid, non rate-determining attack by
®OH or, if that is suitable, by solvent, the latter often predominating:Me
HO—C-«Me
Me Me Me eotyr \
slow © \ /' ^tet ' Me
Me*-C—CI—>CI C®.
/ I
Me H Me
I -H®
MCB-C—O—H Me»-C—OH
- / © /
Me Me
This type of mechanism has been named SNl, i.e. Substitution
Nucleophilic unimolecular. The energy necessary to effect the initial
Me