A Guidebook to Mechanism in Organic Chemistry

Structure, Reactivity and Mechanism

The actual rearrangement step is often followed by a displacement,
addition or elimination reaction before a final, stable product is
obtained.

ENERGETICS OF REACTION

The general path followed by the reactants in an organic reaction as
they are converted into products is normally too complex to be fol­
lowed in complete detail, but useful comments can be made on the
sequence of changes involved, and particularly on their energetics.
Broadly speaking, reactions proceed most readily when the products
constitute a more stable state than the original reactants, the difference
being the free energy of reaction, JF; nevertheless it is seldom, if
ever, that the change involves, energetically, a mere direct run down­
hill (XLIV): the more usual pictui$ is that an 'energy hump' has to
be surmounted on the way (XLV):

X

(XLIV) (XLV)

The horizontal co-ordinate in the above diagrams, often called the
reaction co-ordinate, need have no exact quantitative significance and
merely represents the sequence of the reaction. It will be seen that in
order for reaction to proceed in (XLV), energy will have to be supplied
to the reactants in order to carry them over the hump. This energy is
required, essentially, to stretch and ultimately to break any bonds as
may be necessary in the reactants. This proceeds more readily in
molecules that have absorbed energy and so become activated; the
well-known increase in reaction rates as the temperature is raised, is
indeed, due to the larger proportion of molecules in an activated state
as the temperature rises. A probability factor is also involved as
molecules, although activated, will often only react with each other if
they are in a particular orientation or configuration and only a certain