A Guidebook to Mechanism in Organic Chemistry

Structure, Reactivity and Mechanism

electrons that may result either on the close approach of a reagent or,
more especially, in the transition state, lying between reactants and
products, that may result from its initial attack. These time-variable
factors corresponding to the permanent effects discussed above have
been named the inductomeric and electromeric effects, respectively.
Any such effects can be looked upon as polarisabilities rather than as
polarisations, for the distribution of electrons reverts to that of the
ground state of the molecule attacked if either of the reactants is
removed without reaction being allowed to take place or, if a transi­
tion state is actually formed, it decomposes to yield the starting
materials again. \
Such time-variable effects, being only temporary, will not, of course,
be reflected in the physical properties of the compounds concerned.
It has proved impossible to distinguish experimentally between
permanent and time-variable effects in a number of cases, but it cannot
be too greatly emphasised that despite the difficulties in distinguishing
what proportions of a given effect are due to permanent and to time-
variable factors, the actual close approach of a reagent may have^e
profound effect in enhancing reactivity in a reactant molecule and
so in promoting reaction.

(iv^Iyperconjugation

The inductive effect of alkyl groups is normally found to be in the

order f

Me Me

v • \

CHs-i- < Me-^-CHs-v- < CH-»- < Me->--C-»~

/ /

Me Me

as would be expected. When, however, the alkyl groups are attached

to an unsaturated system, e.g. a double bond or a benzene nucleus,

this order is found to be disturbed and in the case of some conjugated

systems actually reversed. It thus appears that alkyl groups are

capable, in these circumstances, of giving rise to electron release by

a mechanism different from the inductive effect and of which methyl

is the most successful exponent. This has been explained as proceed­

ing,by an extension of the conjugative or mesomeric effect, delocali­

sation taking place in the following way: