Advices For Studying Organic Chemistry

8.6 A F BROMINE AND CHLORINE TO ALKENES

1

fo

DDITION O

. Alkenes react rapidly with chlorine and bromine in non-nucleophilic solvents to
rm vicinal dihalides.

CH 3 CH CHCH 3 + Cl (^2) − 9 oC CH 3 CH CHCH 3
Cl Cl
(100%)
CH 3 CH 2 CH CH 2 + Cl (^2) − 9 oC CH 3 CH 2 CH CH 2 (97%)
Cl Cl

• Br 2 −^5
  oC
  CCl 4
  Br
  HH
  Br


• enantiomer (95%)
  trans-1,2-Dibromocyclohexane
  (as a racemic form)

2. A test for the presence of carbon-carbon multiple bonds:

CC+ Br 2 rooin them temp dark,eratu CCrel

4

CC

Br Br

vic-Dibromide

Rapid decolorization of

Br 2 /CCl 4 is a test for

An alkene alkenes and alkynes.

(colorless) (colorless)

Bromine

(red brown)

1) Alkanes do not react appreciably with bromine or chlorine at room temperature

and in the absence of light.

RH + Br 2 rin the daoomtemrk, CClperatur 4 e no appreciable reaction

Alkane Bromine

(colorless) (red brown)

8.6A MECHANISM OF HALOGEN ADDITION:

1. In the first step, the π electrons of the alkene double bond attack the halogen. (In
   the absence of oxygen, some reactions between alkenes and chlorine proceed