# Which of the following hydrocarbons react with sodium in liquid ammonia?

### Problem

The hydrocarbon which can react with sodium in liquid ammonia is

(1) $$\ce{CH3CH2CH2C#CCH2CH2CH3}$$
(2) $$\ce{CH3CH2C#CH}$$
(3) $$\ce{CH3CH=CHCH3}$$
(4) $$\ce{CH3CH2C#CCH2CH3}$$

### Solution

Solve it as a case of terminal alkynes, otherwise all alkynes react with sodium in liquid ammonia.

### Question

I know sodium in liquid ammonia is used to produce trans-alkenes from alkynes. But from the options, it seems all of them react with sodium in liquid ammonia.

The solution doesn't make any sense to me. If all of the alkynes given react with sodium in liquid ammonia, then why the question? And what difference does it make if the alkyne is terminal?

• Could the answer actually be all of them? Sometimes they do that on multiple choice questions. In any case the second choice should have an additional carbon between the triple bond and the methyl group on the right. They apparently mistyped. Apr 6, 2018 at 9:24
• The terminal alkyne will not give the birch reduction as they will just form the aklyne anion. Maybe this was something the textbook was referring to. Aug 10, 2018 at 4:55
• A Birch reduction uses alcohol as a proton source. No mention of alcohol here. Choice (2) was probably meant to be 1-butyne. Aug 9, 2020 at 16:35

The terminal alkyne has acidic $$\ce{H}$$ and therefore will give acid base reaction with $$\ce{NaNH3}$$ and go $$\ce{NH4+}$$ and $$\ce{RC#C- Na+}.$$