A Guidebook to Mechanism in Organic Chemistry

Structure, Reactivity and Mechanism

that much more difficult and, hence, less likely to proceed. The exact
nature of the transition state is not always known with certainty,
however, and the influence of structural factors on reactivity can then
only be considered, less satisfactorily, with reference to the original
reactant molecule.
Many common reactions are, however, less simple than this, pro­
ceeding not through a single transition state as in (XLV) but involv­
ing the formation of one, or more, actual intermediates as in the
two-stage process (XLVII):

X

/ 1 y

Reactants J

/ AF, ]

/ i

/ i

' ±

Inter­

mediate

/af'|\

A \

1

i

/ AF, ]

/ i

/ i

' ±

\ •

1

IAF

l

i

Jr.

\ Products

(XLVII)

This is essentially two separate reactions, reactants -»-intermediate

with a free energy of activation of JFf and intermediate -»• products,

with JF|; the free energy of the overall reaction being JF. The stage

with the higher free energy of activation—the first, with JFf in the

above case—will usually be the slower and, therefore, rate-determin­

ing step of the overall reaction, for clearly the overall reaction cannot

proceed more rapidly than its slowest stage and it will be this that

will be measured in a kinetic investigation of the overall reaction.

The degree of real difference between an activated complex and an

actual intermediate depends on the depth of the dip or energy mini­

mum characterising the latter. If it is sufficiently pronounced the

intermediate may actually be isolated as in the Hofmann reaction,

in which salts of the anion (XLVIII) may be recovered during

the conversion of an N-bromoamide to an isocyanate (c/.'p. 93):