Advices For Studying Organic Chemistry

7.9 ALKENES BY DEBROMINATION OF VICINAL DIBROMIDES

1. Vicinal (or vic) and geminal (or gem) dihalides:

CC

XX

CC

X

X

A vic-dihalide A gem-dihalide

1) vic-Dibromides undergo debromination:

CC

Br Br

+ 2 NaI acetone CC + I 2 + 2 NaBr

CC

Br Br

+ Zn CH^3 COor^2 H CC + Zn

CH 3 CH 2 OH

Br 2

A Mechanism for the Reaction

Mechanism:
Step 1

CC

Br

Br

I − + CC+ I Br + Br −

An iodide ion become bonded to a bromine atom in a step that is, in

effect, an SN2 attack on the bromine; removal of the bromine brings

about an E2 elimination andtheformationofa bouble bond.

Step 2

+ +

Here, an SN2-type attack by iodide ion on IBr leads

to the formation of I 2 and a bromide ion.

I − I Br II Br −