A Guidebook to Mechanism in Organic Chemistry

Addition to Carbon-Carbon Double Bonds

(IXa') and (lXbr); on projection these then yield the formulae (IXa°)

and (1X6"). It will be seen that these are mirror images, and as equal

quantities are produced, the end result would be DL-maleic acid

dibromide (1,2-dibromosuccinic acid). This product is, in practice,

obtained and, exactly analogously, fumaric acid (XI) is found to yield

meso-1,2-dibromosuccinic acid (XII); attack by Bre on either

carbon of the cyclic bromonium intermediate here yielding the same

product:

H CO.H Br

-CO„H

C H- -CO.H

HOsC// Br

(XI) (XII)

Halogens are, in fact, found uniformly to add on by an overall

trans mechanism. While this is good evidence in favour of the

bromonium ion intermediate, it does not constitute actual confirma­

tion; for provided attack by Br^3 were rapid enough, a carbonium

ion intermediate,^!^ OO, could also undergo preferential attack

frorrtabe back befdK any significant rotation about the carbon-

carbon single bond had taken place, thus also leading to stereo-

specific trans addition. Acetylenes also add on one molecule of

halogen to yield a trans product; thus acetylene dicarboxylic acid,

H0 2 C-C=C-C0 2 H, yields the trans compound, dibromofumaric

acid:

Br CO.H

\ /

C

II

C

/ \

HO.C Br

EFFECT OF SUBSTITUENTS ON RATE OF ADDITION
If attack by incipient Br® to form a cation, whethef (V) or (VII), is
the rate-determining step of the reaction, it would be expected that
addition would be facilitated by the presence of electron-donating