Question

What happens when but-2-yne is treated with Na in liquid ammonia?
A.Cis-2-butene is formed
B.Trans-2-butene is formed
C.n-butane is the major product
D.It rearranges to but-1-yne
E.There is no reaction

Answer 1

Hint: Sodium in liquid ammonia reduces alkynes to alkenes. The reaction is called Birch reduction. In other words, it hydrogenates the alkynes to form alkenes.

Complete step by step answer:
The reduction of alkynes to alkenes is done with the help of sodium in liquid ammonia. This catalyst readily gives the trans-2-Butene (E-2-Butene). This reaction is known as Birch reduction. In order to obtain cis-2-Butene, Lindlar catalyst can be employed. Lindlar catalyst is a combination of Palladium, lead acetate and quinoline with hydrogen gas being pumped at high pressure.
But-2-yne when treated with sodium in liquid ammonia gives trans-2-butene.

The reaction of alkyne with sodium in liquid ammonia produces sodamide in the process converting alkynes to alkenes.
Mechanism of Birch Reduction:
-Sodium in liquid ammonia produces solvated electrons.
-The attack of solvated electrons forms a radical anion.
-Now, protonation of the radical anion takes place.
-The reduction of radicals to an anion by electrons occurs.
-Now through protonation of the anion by alcohol, desired alkene is formed.

So, the correct option is B.

Note:
Instead of sodium metal, even lithium (Li) metal can be used. Keep in mind sodium amide or with liq. Ammonia is not a Birch reduction. Since sodamide is a strong base and not a reducing agent.