A Guidebook to Mechanism in Organic Chemistry

Neighbouring Group Participation

results in inversion of configuration at this carbon and the formation
of the cyclic intermediate (XI), an alkylene oxide or epoxide. The
three-membered ring is then reopened by the action of eOH, attack
taking place at the less heavily substituted of the two carbon atoms
(i.e. the one that originally carried the chlorine in this case) as this will
be the more positive of the two, its electron availability being enhanced
by the inductive effect of only one rather than of two alkyl groups.
This attack will also be of the Sjv2 type, from the side to which the
chlorine was originally attached, resulting in a second inversion of
configuration at this carbon atom. The reaction is completed by con­
version of (he alkoxide ion (XfT) to the alcohol (XIII). This alcohol will
have the same configuration as the original chloride (IX) but it would
hardly be true to say that the overall reaction had proceeded with
retention of configuration for, in fact, the apparent retention of con­
figuration has been brought about by two successive inversions. This,
of course, distinguishes such a reaction type from Sw/which is attended
by a real retention of configuration.
Evidence for this overall picture of the reaction is provided by the
actual isolation of the epoxides postulated as intermediates and the
demonstration that they may be hydrolysed under the conditions of
the reaction and with results as observed above.
A similar apparent retention of configurationTccurs in the hydro­
lysis of a-halogenated acids in dilute alkaline felution •»

O

R C R C

\ / \ internal SJV2 \ /

c^Os

0

•*

HO0

-*

C

o

<=>OH. ,.

normal Sjy2 xx / \

c o®

/—V ' first inversion y VI second inversion 4 \

Br H H Cb HO H

(XIV)

but here the suggested cyclic intermediate (XIV), an a-lactone, has not

actually been isolated. If the reaction is carried out in extremely

concentrated base, [eOH] is then large enough to compete effectively

with the internal attack by -CO 20 and the normal one stage Sw 2

reaction then yields a product whose configuration has undergone

inversion. At intermediate concentrations of base both reactions take

place, i.e. retention of configuration with a varying degree of racemi­

sation is observed.