A Guidebook to Mechanism in Organic Chemistry

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