Ch 9 : Preps. of alkynes via Elimination

Chapter 9 : Alkynes

Preparation of Alkynes via Elimination reactions

Reaction Type: Elimination (E2)

Summary

The two p bonds of an alkyne can be formed using two consecutive elimination reactions.
The leaving groups are usually halides (esp. Br or Cl)
Reagent : usually NaNH₂ (a strong base, pK_a = 35)
Either geminal (1,1-) or vicinal (1,2-) dihalides can be used.
Since 1,2-dihalides can be prepared by addition of X₂ to an alkene, an alkene can be converted into an alkyne in two steps.
These reactions are typically E2 reactions and occur via an alkenyl halide.
The strong base is needed in order to cause elimination of the alkenyl halide.

What is the alkyne product from the reactions of the following with NaNH₂:
(a) 2,2-dibromopropane	ANSWER	(c) 1,2-dibromohexane	ANSWER
(b) 1,1-dibromooctane	ANSWER	(d) 2,3-dibromohexane	ANSWER