A Guidebook to Mechanism in Organic Chemistry

Debromination

O

MeC—CH—CH,

I

Br \ O

(XXXII) MeC—CH=CH,

?

MeC—CH,—CH,

— Br

(XXXIII)

electron-withdrawing substituent is much greater in the )3- than in the
a-position. The influence of such activating groups is often sufficiently
great to lead to the elimination of more unusual leaving groups such
as OR and NH 2.
A rather interesting intramolecular reaction of this type is the loss
of C0 2 and bromide ion from the anion of a /3-bromo-acid, for
example cinnamic acid dibromide (XXXIV):

r

ol o o=c=o

Cf H Br „H

Br»5c^C—Ph -* ^CC*

Br H Ph

t

(XXXIV)

DEBROMINATION

Br©

Attention has been confined so far almost wholly to reactions in

which one of the leaving groups has been hydrogen, and although

these are the most common and important eliminations, the dehalo-

genation of 1,2-dihalides, particularly bromides, also has some

mechanistic and preparative interest. The most common classical

reagents for the purpose are metals such as zinc:

ZnBr

Zn: Br R * R R

A \ A

:—R' -> C=C

/ \

K R'' V Br£