A Guidebook to Mechanism in Organic Chemistry

Energetics of Reaction

proportion of the activated molecules will satisfy this condition. In
addition to the straightforward energy term, an entropy factor is also
involved expressing essentially the relative randomness, and hence
probability, of the initial and final states of the reaction, i.e. of
reactants and products. The amount of work necessary to get the
reactants up the top of the hump, including both the energetic and
probability factors, is called the free energy of activation, AFX.
The top of the energy hump, x, corresponds to the least stable
configuration through which the reactants pass on their way to pro­
ducts and this is- generally referred to as the transition state or
activatedTomplex. It shoule emphasised that this is merely a state
that is passed through in a dynamic process and the transition state
is not an intermediate that can actually be isolated. A typical transi­
tion state is (XLVI) met with in the alkaline hydrolysis of methyl
iodide

HOe + H^C-

/

H

H H

a- "V / a-

HO C 1

H

HO—C^H+I^9

\

H H

*rfXLVI)

in which the C—OH bond is being formed before the C—I bond is
completely broken and the three hydrogen atoms are passing Tflrough
a configuration in which they all he in one plane (at right angles to
the plane of the paper}. This, reaction is discussed in detail below
(p. 66).
It will be realised from what has already been said that in discussing
the influence of structural factors, both electronic and steric, on the
reactivity of compounds, it is more pertinent to consider what effect
these factors will have on the transition state in a reaction rather than
on the ground state of the reactant molecule. For any factor that
serves to stabilise the transition state, i.e. to lower its energy content,
lowers A F* (the height of the hump that has to be surmounted) and so
offers an easier and less demanding path for the reaction to traverse.
It is at this point that the time-variable, i.e. polarisability factors (p.
19), consequent on the close approach of reagent and substrate, may
exert their most potent, and possibly determining, effect on the course
of a reaction. Steric effects are also of the utmost significance at this
point, for a transition state in which the groups are highly crowded
will be notably loth to form and the reaction, therefore, rendered