Question

Vicinal dihalides undergo double dehydrohalogenation to give terminal alkyne. How many moles of $NaN{H}_2$ are used in the overall reaction ?

• A. One
• B. Two
• C. Three
• D. Four

Answer 1

The correct option is C Three

In the first step, one mole $NaN{H}_2$ is the base in an elimination reaction to give the alkenyl bromide.

In the second reaction, likewise a second equivalent of $NaN{H}_2$ performs a second elimination reaction to form the alkyne.

For a terminal alkyne, any excess $NaN{H}_2$ will remove the acidic hydrogen from terminal $C-H$ and give the alkynyl anion. So if a terminal alkyne is formed, three equivalents of $NaN{H}_2$ will be consumed; the alkyne is protonated upon workup, usually by adding water.