Question

Give the two reactions of acetylene which are used to distinguish it from alkenes.

Short answer

The two characteristic reactions of acetylene that distinguish it from alkenes are: 1. Formation of a white precipitate with ammoniacal silver nitrate solution (Tollens' reagent). The reaction is: 2AgNO₃ + 2NH₄OH + C₂H₂ -> Ag₂C₂ ↓ + 2NH₄NO₃ + 2H₂O 2. Formation of a reddish-brown precipitate with cuprous chloride solution (Cu₂Cl₂) in the presence of ammonia. The reaction is: 2CuCl + C₂H₂ + 4NH₄OH -> Cu₂C₂ ↓ + 4NH₄Cl + 2H₂O + 2NH₃

Step-by-step solution

Reaction 1: Formation of Precipitate with Ammoniacal Silver Nitrate Solution

To test for the presence of acetylene, pass the gas through an ammoniacal silver nitrate solution (Tollens' reagent). If the gas is acetylene, a white precipitate of silver acetylide (Ag₂C₂) is formed, which is not observed with alkenes as they don't react with ammoniacal silver nitrate solution. The reaction is as follows: 2AgNO₃ + 2NH₄OH + C₂H₂ -> Ag₂C₂ ↓ + 2NH₄NO₃ + 2H₂O

Reaction 2: Formation of Reddish Brown Precipitate with Cuprous Chloride Solution

To further distinguish acetylene from alkenes, pass the gas through a cuprous chloride solution (Cu₂Cl₂) in the presence of ammonia. A reddish-brown precipitate of copper acetylide (Cu₂C₂) is formed when acetylene reacts with the cuprous chloride solution, whereas alkenes do not produce any precipitate in this reaction. The reaction is as follows: 2CuCl + C₂H₂ + 4NH₄OH -> Cu₂C₂ ↓ + 4NH₄Cl + 2H₂O + 2NH₃ These two reactions are characteristic of acetylene and help in distinguishing it from alkenes.