A Guidebook to Mechanism in Organic Chemistry

Nucleophilic Substitution at a Saturated Carbon Atom

(Hi) S,vl mechanism

As the carbonium ion formed in the slow, rate-determining stage
of the reaction is planar, it is to be expected that subsequent attack by
a nucleophilic reagent such as eOH or the solvent (e.g. H 2 0) will take
place with equal readiness from either side of the planar ion leading,
in fact, to a 50/50 mixture of species having the same and the opposite
configuration as the starting material, i.e. that racemisation will take
place yielding an optically inactive (±^ product.
What actually happens, depends on how rapidly the attack by a
nucleophile follow s*n the initial ionisation step. If the second reaction
follows closely upon trie first, it may be that the receding anion, e.g.
BrQ, may still be only a few molecular diameters away and thus attack
by an approaching nucleophile is inhibited on the side of the car­
bonium ion to which the bromine was originally attached. Attack on
the 'backside' of the carbonium ion is unaffected, however, and will
thus preponderate, leading to more inversion than retention of con­
figuration in the product, i.e. racemisation with some inversion will
be observed in the product.
What is thus observed in practice, under S^l conditions, may range
from virtually complete racemisation to almost total inversion of
configuration depending on how rapidly attack by a nucleophile fol­
lows on the initial ionisation. The most common situation is mainly
racemisation attended by some inversion, the relative proportions of
the two seen with a particular substrate being profoundly influenced
by the conditions under which the reaction is carried out. If the solvent
can act as a nucleophile, e.g. H 2 0, attack is likely to be more rapid,
because of its very large relative concentration, than if the presence of
an added nucleophile, Y°, is necessary, thus leading to a relatively