A Guidebook to Mechanism in Organic Chemistry

Structure, Reactivity and Mechanism

That this difference is not due to differing electron availability at the
nitrogen atom in the two cases is confirmed by the fact that the two
amines differ very little in their strengths as bases (cf. p. 56): the uptake
of a proton constituting very much less of a steric obstacle than the
uptake of the relatively bulky BMe 3.
More familiar examples of steric inhibition, however, are probably
the difficulties met with in esterifying tertiary acids (XXXV) and
2,6-disubstituted benzoic acids (XXXVIa)

R 3 CCO,H

(XXXV)

and then in the hydrolysis of the-esters, or other derivatives such as
amides, once made. That this effect is indeed steric is suggested by its
being much greater in magnitude than can be accounted for by any
influence the alkyl substituents might be expected to have on electron
availability and also by its non-occurrence in the aromatic species if
th^^ubstituents are in the m- or /^-positions. Further, if the carboxyl
group is moved away from the nucleus by the introduction of a CH 2
group, the new acid (XXXVI6) may now bqjesterified as readily as the
unsubstituted species: the functional group is now beyond the steric
range of the methyl substituents.
It should be emphasised that.such steric inhibition is only an
extreme case and any factors which disturb or inhibit a particular
orientation of the reactants with respect to each other, short of pre­
venting their close approach, can also profoundly affect the rate of
reactions: a state of affairs that is often encountered in reactions in
biological systems.

COaH

CO,H\ Cff,

(XXXVIa). (XXXVI/))

CLASSIFICATION OF REAGENTS

Reference has already been made to electron-donating and electron-

withdrawing groups, their effect being to render a site in a molecule

electron-rich or electron-deficient, respectively. This will clearly in­

fluence the type of reagent with which the compound will most