A Guidebook to Mechanism in Organic Chemistry

Addition to Carbon-Carbon Double Bonds

exemplifying the greater tendency to form a secondary rather than a

primary carbonium ion (cf. p. 109):

H. .... H

H©

Me—CH—CH, Me—CH=CH, Me—CH—CH,

I © /\ 9 I

Br© I I Br©

H H

I ' I

Me—CH—CH, Me—CH—CH,

1

I •

Br • Br

This is found to be the case and is the theoretical justification for

the empirical generalisation of Markownikov: 'In the addition of

unsymmetrical adducts to unsymmetrical olefines halogen, or the

more negative group, becomes attached to the more highly substituted

of the unsaturated carbon atoms.'

The study of the addition of hydrogen halides to defines presents a

number of experimental difficulties. In solution in water or hydroxylic

solvents, acid-catalysed hydration, etc. (see below) constitutes a

competing reaction, while in less polar solvents radical formation is

encouraged and tflrtnechanism changes, resulting with HBr in anti-

MafUlwmikov addition to yield Me-CH,-CH 8 Br via the preferen­

tially formed intermediate, MeCHCHjBr. The radical mechanism

of addition and the reasons for its occurrence are discussed subse­

quently (p. 244).

Addition of HBr to vinyl bromide, CH^CHBr, is also of

some interest. Under polar conditions, CH 3 -CHBr„ rather than

CHgBr-CHgBr, is obtained reflecting the greater stability of the

carbonium ion intermediate (XlVa) rather than (XIV6):

H H H

CH,—CH ^Br «-• CH 2 —CH=Br CH,—CH—Br

(XlVa) (XIV6)

Nevertheless the rate of addition Is, as described above, about thirty
times slower than with ethylene, indicating the inductive effect of the
bromine atom in reducing overall electron availability at the double
bond:
CH,=CH-»-Br

y

142