Nucleophilic Substitution at a Saturated Carbon Atomlatter has had time to collapse to the planar state, thus leading to a
product having the same configuration as the starting material. This
is admittedly an ad hoc explanation but support for it has been obtained
by the fact that compounds such as (VIII), alkyl chlorosulphites, can
actually be isolated and then shown to undergo extremely ready
conversion to alkyl halide+S0 2. If, however, the reaction is carried
out in the presence of base, e.g. pyridine, the hydrogen chloride
liberated in forming the chlorosulphite in stage (i) is converted to Cle;
this then readily attacks (VIII) 'from the back' with the expulsion of
GOSOCl. The reaction is now of ajiormal SN 2 type, proceeding
with inversion, and the somewhat specialised SNi (Substitution
Nucleophilic internal) mechanism, with retention of configuration, is
no longer observed.(?) Neighbouring group participation
There are also a number of other cases of replacement reactions in
which retention of configuration takes place but these can all be
shown to have one feature in common: an atom, close to the carbon
undergoing the displacement reaction, which carries a negative charge
or has an unshared pair of electrons, i.e.. which can act as an' internal'
nucleophilic reagent. Thus in the alkaline hydrolysis of the /J-chloro-
hydrin(IX) *R CRjOH R CR.Of R CR,
\„/ ©OH \y jin<«nalS*2 \_/|
C —>• C-« ' —> HOe-*C
ici
J \ v first inversion ^ vl
CI H CI H H Qp
(IX) (X)
normal
SN 2(XI)
©OH
second
inversion
R CR 8 OH R ' CR 8 Oe
\ / H.O \ /c <— c
- \ • * \
HO H HO H
(XIII) (XII)
the first stage is conversion of the OH to the corresponding alkoxide
ion (X). This ion,acting as a nucleophile, then attacks the carbon atom
carrying chlorine 'from the back' in an internal SN 2 reaction that