Ethyne (Acetylene) — Properties, Preparation, Reactions, and Uses

Flashcards covering key concepts about ethyne (acetylene): its properties, preparation, reactions (addition, hydration, oxidation, substitution, polymerization), tests for unsaturation, and uses.

What is the chemical formula and structure of ethyne (acetylene)?

Formula: C2H2; structure: H-C≡C-H.

How is ethyne prepared from calcium carbide in the lab?

CaC2 + 2H2O → Ca(OH)2 + C2H2 (gas).

What impurity is removed during purification of ethyne and how is it removed?

Ammonia (NH3), H2S and Phosphine (PH3) impurity is removed by passing the gas through an acidified copper(II) sulfate solution.

Why is the preparation setup for ethyne performed on a heap of sand?

To prevent the flask from cracking due to the large amount of heat released.

List two physical properties of ethyne.

Ethyne is a colorless gas with a sweet odor; it is sparingly soluble in water.

What is the storage condition for ethyne?

Ethyne is dissolved in propanone (acetone) and stored in steel cylinders at about 12 atm.

Describe the combustion of ethyne in air and in pure oxygen.

In air: smoky, luminous flame due to high carbon content. In pure O2: complete combustion with a very hot, non-luminous flame (~3000°C).

What practical use does the oxy-acetylene flame have?

Used for welding and cutting metal.

How does ethyne react with hydrogen in the presence of a nickel catalyst at ~200°C?

Sequential addition of H2 to form ethene (C2H4) and then ethane (C2H6).

What happens when ethyne reacts with chlorine or bromine in the presence of a metallic halide catalyst at room temperature?

Formation of halogenated compounds (e.g., vinyl halides); without a catalyst, reaction with Cl2 can be explosive, producing C and HCl.

What is formed when ethyne is hydrated with dilute sulfuric acid and a Hg(II) catalyst at ~60°C?

Addition of water to form ethanal (acetaldehyde); ethenol (vinyl alcohol) is unstable and tautomerizes to ethanal.

What is the outcome of oxidizing ethyne with acidified potassium manganate(VII) (KMnO4)?

Ethyne is oxidized to ethanedioic acid (oxalic acid); acidified KMnO4 decolorizes, while alkaline KMnO4 turns the solution green at room temperature.

Describe the copper(I) chloride and silver nitrate substitution reaction with ethyne.

Precipitates of copper(I) dicarbide (Cu2C2) and silver dicarbide (Ag2C2) form; they are explosive when dry and can regenerate ethyne with dilute acid.

How does ethyne polymerize to benzene?

On passage through a hot tube with a complex organo-nickel catalyst: 3 C2H2 → C6H6.

How can you test for unsaturation in ethyne, and what is a distinguishing test for terminal alkynes?

Unsaturation is shown by decolorization with bromine/bromine water; KMnO4 also decolorizes but is not distinctive. Terminal alkynes give a white precipitate with silver trioxonitrate(V) (when dissolved in alcohol or ammonia).

List some uses of ethyne.

Fuel in miners’ lamps; oxy-acetylene flame for welding and cutting; source of ethanal and solvents (1,1,2-trichloroethene and 1,1,2,2-tetrachloroethene) used in industry and dry cleaning.

What is the general formula and degree of saturation for alkynes, according to Table 30.9?

General formula: CnH2n-2; highly unsaturated with a high carbon-to-hydrogen ratio; capable of addition reactions.

What is a key test result that distinguishes terminal alkynes from other unsaturates using silver trioxonitrate(V)?

Only terminal alkynes give a white precipitate with silver trioxonitrate(V) (in ammonia or alcohol).