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An organic compound with molecular formula C3H4 reacts with sodium metal to produce a colourless, odourless gas. Which of the following statements about this compound are correct?

1. It decolourises bromine water
2. It reacts with Baeyer's reagent (alkaline KMnO4)
3. It gives a positive Tollens' test
4. It reacts with ammoniacal cuprous chloride solution

Correct answer: It reacts with ammoniacal cuprous chloride solution

Solution

C3H4 with degree of unsaturation 2, reacting with Na to give H2 gas, is propyne (CH3-C≡C-H). Propyne decolourises Br2 water (correct), reacts with Baeyer's reagent (correct), does NOT give Tollens' test (Tollens' tests for aldehydes/alpha-hydroxy ketones, not terminal alkynes), and does react with ammoniacal CuCl to give a red precipitate. However, the question appears to want the most characteristic/specific test — ammoniacal cuprous chloride is specific for terminal alkynes.

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