Addition to Carbon-Carbon Double Bonds
Thus conjugated dienes are reduced to dihydro-derivatives by sodium
and alcohol whereas non-conjugated dienes or simple alkenes are
unaffected.
It might be expected that in the addition of, for example, chlorine
to butadiene, reaction could proceed through a cyclic chloronium ion
CH==CH
/ \
CH2 CH2
CI
that would be largely unstrained. That this is not formed, however, is
shown by the fact that the above addition results in the formation of
the trans compound
ClCHa
\
CH=CH
\
CH 2 CI
and not the corresponding cis compound that would have been
obtafflRTby the attaSk of CI® on the cyclic chloronium ion. Add
ition thus probably proceeds through a delocalised carbonium ion,
cf. the addition of hydrogen halide below.
(i) Hydrogen halide
With butadiene itself a proton may initially form a n complex and
then a a complex with hydrogen on a terminal carbon atom (XXXIV).
Protonation takes place at C, rather than C 2 as the former yields a
secondary carbonium ion that is stabilised by delocalisation, whereas
the latter would yield a primary carbonium ion (XXXIII) that is not.
The resulting allylic cation (XXXIV) can take up Br® at either C 2 or C 4
leading to 1:2 and 1:4 overall addition, i.e. (XXXVa) and (XXXV6),
respectively (see p. 151).
The presence of conjugation dies not make 1:4-addition obliga
tory: it merely makes it possible, and whether this^r l:2-addition
actually takes place is governed by the relative rates of conversion of
the cation (XXXIV) to the alternative products and also by the
relative stability of these products. By and large, 1:2-addition tends