A Guidebook to Mechanism in Organic Chemistry

Effect of Structure

substituted, but the SJV2 mechanism involves a transition state in
which there are five groups around the carbon atom attacked, while
the Sjyl intermediate involves only three, and even these at their
maximum distance apart in a planar state. The divergence between the
two will naturally increase, with consequent favouring of SN\ at the
expense of S^, as alkyl substitution at the carbon atom being attacked
is increased.
A similar change of mechanism is observed, but considerably sooner,
in traversing the series:

•CH,—CI PhCH^-Cl Ph 2 CH—CI Ph,C—CI

Swl hydrolysis is here observed at the second member and with
Ph 3 C—CI the ionisation is so pronounced that the compound
shows electrical conductivity when dissolved in liquid SOa. The
reason for the greater promotion of ionisation, with consequent more
rapid changeover to the SN\ mechanism, is the considerable stabilisa­
tion of the carbonium ion that is here possible by delocalisation of the
positive charge

© o

i.e. a classical example of an ion stabilised by charge delocalisation
via the agency of the delocalised w orbitals of the benzene nucleus
{cf. the negatively charged phenoxide ion, p. 17). In terms of overall
reactivity, benzyl chloride is rather similar to t-butyl chloride; the
effect will become progressively more pronounced, and S^l attack
further facilitated, with PhaCHCl and Ph 3 CCI as the possibilities of
delocalising the positive charge are increased in the carbonium ions
obtainable from these halides.
Similar carbonium ion stabilisation can occur with allyl halides:

CH 2 =CH—CH 2 C1 -* [CH 2 =^CH^-CH 2 <_> CH 2 —CH=CH 2 ] + Cls

Sjyl attack is thus promoted and allyl, like benzyl, halides are
normally extremely reactive as compared with e.g. CHj'CHa-CHaCI
and Ph-CH 2 -CH 2 -CH 2 C1 respectively where such carbonium ion
stabilisation cannot take place.