A Guidebook to Mechanism in Organic Chemistry

Orientation of Addition

substituents on the double-bond carbon atoms; the following relative
rates are observed:
Me
\
CH,=CH—Br « CHa=CH—C0 2 H < CH 2 =CH 2 < CH=CH 2
0 03 1 2
Me

C=CH, <

Me

Me Me Me Me

\ / \ /

^ 'C=CH x < C=C '

/ / \

Me Me Me

10 13

The rate of addition increases with successive introductions of methyl
despite access to the double bond becoming progressively more
hindered sterically. By contrast, the presence of electron-withdrawing
substituents markedly slows down the rate of addition. The presence
of a benzene nucleus also speeds up reaction very markedly because
of the stability, and consequent ease of formation, of the carbonium
ion intermediate (XIII):

v \_rn=i CH=CH,

o

H,X

©

=CH—CH,X

etc.

(XIII)

ORIENTATION OF ADDITION

With hydrogen halide, addition of H® is the rate-determining step

via an initially formed n complex, the addition being completed by

subsequent attack of Hal®. In support of this, it is found that ease of

addition increases on going HF -»-HCl ->HBr —HI, i.e. in order of

increasing acid strength. The series not only implies relative ease of

proton donation, of course, but also increase of nucleophilicity in the

anion, Hal®, that completes the aftack; but this second stage is not

involved in theyate-determining step of the overall addition. When

the olefine is unsymmetrical, e.g. propylene, hydrogen bromide can

add to form two possible products, Me-CHBr-Me and Me-CHj*

CH 2 Br. In practice, however, we should only expect to get the former,