Making ethyne

Ethyne is produced on an industrial scale from calcium carbonate (limestone) and coal:

CaO + 3C → CaC₂  + CO

Calcium carbide and water are then reacted to produce ethyne and calcium hydroxide:

CaC₂  + 2H₂O → Ca(OH)₂  + C₂H₂

Ethyne can also be manufactured by the partial combustion of methane with oxygen, or by the cracking of hydrocarbons.

In this demonstration, I prepare ethyne and do a series of simple tests on the gas to reveal some of its properties.

Kit

• retort stand
• bowl
• pipette
• side-arm tube with bung
• rubber tubing and delivery tube
• six test tubes with bungs
• 0.002 M acidified potassium manganate(VII) solution, approximately 1 cm³
• 0.002 M bromine solution, approximately 1 cm3
• solid calcium carbide, CaC₂ (IRRITANT, HIGHLY FLAMMABLE)

Procedure

Clamp the side-arm tube vertically and add two or three pieces of calcium carbide. Replace the bung. Connect the delivery tube to the side-arm with the rubber tubing, making sure the end of the delivery tube is under water in an adjacent bowl. Put six test tubes in the bowl and allow them to fill with water. Remove the bung of the side-arm tube and add two or three drops of water. Replace the bung and collect six test tubes of ethyne gas by downward displacement of water. As the gas leaves the delivery tube in the water carefully ignite the ethyne bubbles as they reach the air. A loud bang and a smoky flame are produced.

Test for ethyne by adding a bromine solution, or an acidic solution of potassium manganate(VII). In each experiment only a few drops of the test solution are needed. In the case of the bromine solution, the orange colour eventually disappears when ethyne is added. With manganate(VII), the acidified solution goes colourless but sometimes goes through a brown phase. The acidified oxidation of ethyne to ethan-1-2-dioic acid (oxalic acid) is the Baeyer's test for unsaturated organic compounds.

Special tips

Ethyne is highly flammable and only small quantities should be made. Even small amounts of ethyne/air mixtures can produce damaging explosions if the mix is close to stoichiometric. If the gas is made to pass through a small nozzle, then it gets ample air to burn completely.

Teaching goals

Approximately 80% of the ethyne produced annually in the US is used in the synthesis of myriad organic compounds. When burned in oxygen, ethyne produces a flame temperature of over 3300°C, releasing 11,800 J g^-1, making it also useful for welding, cutting, soldering metals, and increasingly for hardening steel.

The acidic hydrogen attached to the carbon-carbon triple bond in ethyne can separate from the rest of the molecule as a hydrogen ion, with the electro-negative carbon retaining both electrons from the broken covalent bond. Ethyne can therefore bond with metals, which distinguishes it from alkenes. For example, passing ethyne gas through ammoniacal copper(I) chloride produces an immediate red precipitate of copper(I) carbide; when bubbled through ammoniacal silver nitrate a white precipitate of silver carbide is produced. You can see this demonstration in Incendiary silver.

Cu₂Cl₂ + C₂H₂→ Cu₂C₂ + 2HCl

2AgNO₃ + C₂H₂→ Ag₂C₂ + 2HNO₃

Sodium amide, NaNH₂, dissolved in liquid ammonia, is a very strong base and though unreactive towards alkanes and alkenes, will remove a proton from ethyne to form a salt:

NaNH₂ + C₂H₂→ HCC^- Na⁺ + NH₃

These salts will react with halogen-oalkanes, providing a useful route to synthesising carbon-carbon bonds:

CH₃CC^- Na⁺ + CH₃CH₂Br → CH₃CH₂CCCH₃ + NaBr

Ethyne reacts with several reagents, forming a variety of different polymers with a range of useful applications.

Do not make more than 0.5 g of copper carbide or silver carbide unless you have a police certificate.