Advices For Studying Organic Chemistry

A Mechanism for the Reaction

Dehydrohalogenation of vic-Dibromides to form Alkynes

Reaction:

RC

H

Br Br

CR^2 Br−

H + 2 NH 2 − RC CR +^2 NH 3 +

Mechanism:
Step 1

HN − H Br Br Br

HNH H

+ RCCR Br −

H H CC

R

H + +

Amide ion vic-Dibromide Bromoalkene Ammonia Bromide ion The strongly basic amide ion brings about an E2 reaction.

Step 2

CC Br

N H H

− HNH H

Br −

R

H + RCCR+

Bromoalkene Amide ion Alkyne A second E2 reaction produces the alkyne.

+

Ammonia Bromide ion

Examples:
CH 3 CH 2 CH CH 2 CH 3 CH 2 CHCH 2 Br
Br
Br

Br

Br 2 CCl 4

NaNH 2 mineral oil CH 3 CH 2 CH CH

H 3 CH 2 CC CH 2

+ H 3 CH 2 CC CH

H 3 CH 2 CC C −Na+ NH^4 Cl H 3 CH 2 CC CH + NH 3 + NaCl

110-160 oC NaNH 2 mineral oil 110-160 oC

NaNH 2