A Guidebook to Mechanism in Organic Chemistry

Stereochemistry of Addition

STEREOCHEMISTRY OF ADDITION

An intermediate such as (VII) would involve 'attack from the back'
by BrQ as access from that side would be sterically so much readier
than from the front where the bulky bromine atom would get in the
way.
The overall addition should thus be trans, one bromine atom
becoming attached from the front of the molecule and the other from
the back. Thus if we consider the addition of bromine to maleic acid
(VIII) to yield the dibrorho-analogue of tartaric acid (IX):

H H

\ /

C=C>

/ \

HO,C CO,H

(VIII)

BR,

Br® Br®

• c c
  / \ / \
  H0 8 C Br CO.H

(X).

H H

\ /

(IXa) Br»»C—C--Br

/ \

HO,C .COjF

III

HOsC H

\ /

(IXa') Br—C—C—Br

/ \

H CO.H

III

HO,C H

(IXa*) Br—j J—Br

H CO,H

H H

Br -C—C—Br (IX/3)

HOaC CO.H

III

H CO.H

\ /

CO.H

Br—C —C-/ Br

/ \

HOjCf H

ID

H CO.H

Br —Br

(IX/30

(IX/Y)

HO.C H

The first-formed bromonium ion (X) could be attacked 'from the

back' on either carbon atom, witb.equal facility as these are identi­

cally situated, to yield equal quantities of the dibromides (IXa) and

(1X6). Before tifese can be written in the more usual plane-projection

formulae, they must be rotated about the carbon-carbon single bond

so that the two bromine atoms are on the same side of the molecule,