- Debromination by zinc takes place on the surface of the metal and the mechanism
is uncertain.
- Other electropositive metals (e.g., Na, Ca, and Mg) also cause debromination of
vic-dibromide.
- vic-Debromination are usually prepared by the addition of bromine to an alkene.
- Bromination followed by debromination is useful in the purification of alkenes
and in “protecting” the double bond.
7.10 SYNTHESIS OF ALKYNES BY ELIMINATION REACTIONS
- Alkynes can be synthesized from alkenes.
RCH CHR + Br 2
Br Br
RCCR
H H
vic-Dibromide
1) The vic-dibromide is dehydrohalogenated through its reaction with a strong base.
2) The dehydrohalogenation occurs in two steps. Depending on conditions, these
two dehydrohalogenations may be carried out as separate reactions, or they may
be carried out consecutively in a single mixture.
i) The strong base, NaNH 2 , is capable of effecting both dehydrohalogenations in
a single reaction mixture.
ii) At least two molar equivalents of NaNH 2 per mole of the dihalide must be used,
and if the product is a terminal alkyne, three molar equivalents must be used
because the terminal alkyne is deprotonated by NaNH 2 as it is formed in the
mixture.
iii) Dehydrohalogenations with NaNH 2 are usually carried out in liquid ammonia
or in an inert medium such as mineral oil.
