Acetylene

OVERVIEW

Acetylene (uh-SET-ill-ene) is the simplest hydrocarbon, consisting of two carbon atoms joined to each other by a triple bond with their associated hydrogen atoms. It occurs as a colorless gas with a sweet odor when pure, but an unpleasant odor due to the presence of phosphine (PH₃) and/or arsine (AsH₃), with which it is often contaminated. Acetylene is a highly flammable gas that is also somewhat explosive. This property accounts for one of its most important uses, in torches that burn at very high temperatures.

KEY FACTS

OTHER NAMES:

Ethyne; ethine

FORMULA:

CH=CH

ELEMENTS:

Carbon, hydrogen

COMPOUND TYPE:

Alkyne (unsaturated hydrocarbon; organic)

STATE:

Gas

MOLECULAR WEIGHT:

26.04 g/mol

MELTING POINT:

?80.8°C (?113°F)

BOILING POINT:

?84°C (?120°F)

SOLUBILITY:

Slightly soluble in water and alcohol; soluble in acetone and benzene

Acetylene was discovered by the British chemist Edmund Davy (1785–1857) in 1836. Davy obtained the gas accidentally when he combined water with potassium carbide (KCH₂) while attempting to make potassium metal. He noted that the gas burned with a bright flame and thought it might be used as a source of illumination. That application was impractical, however, because of the high cost of potassium carbide. When the German chemist Frederich Wöhler (1800–1882) discovered the far less expensive calcium carbide (CaC₂) in 1862, however, that problem was solved, and the demand for acetylene in lamps and other applications grew rapidly.

HOW IT IS MADE

The original method of making acetylene by the action of water on calcium carbide was abandoned when more efficient methods of synthesis became available. One of those methods, known as the Wulff process, involves the cracking of hydrocarbons from petroleum. In this process, liquid petroleum, which consists of many different kinds of hydrocarbons, is heated with very hot steam, causing the hydrocarbons to break apart ("crack") into smaller compounds. Acetylene is one of these compounds.

A later improvement on this procedure was developed by the German chemist Walter Reppe (1892–1969), sometimes called the Father of Acetylene Chemistry. Since Reppe worked at that time for the BASF chemical corporation, the process is also known as the BASF process of making acetylene. In this procedure, hydrocarbons from petroleum are treated with an oxidizing agent that causes them to break apart to form smaller compounds, one of which, again, is acetylene.

COMMON USES AND POTENTIAL HAZARDS

By far the most important use of acetylene is in the manufacture of other chemicals, such as vinyl chloride, vinylidene chloride, vinyl acetate, acrylonitrile, trichloroethylene and perchloroethylene, and the family of plastics known as the acrylates. (The name vinyl refers to the remnant of an acetylene molecule after one hydrogen atom has been removed: -CH₂=CH-.) About 20 percent of all the acetylene produced is used for torches that produce very hot flames. In one such torch, the oxyacetylene torch, acetylene gas and oxygen are mixed and ignited at the tip of the torch. The combination of gases burns at a temperature of 3,000°C to 3,500°C (5,500°F to 6,300°F). Oxyacetylene torches are used to cut through metal and to weld two metals to each other. They can be used in very cold climates and even under water.

Interesting Facts

Acetylene was used as a source of illumination beginning in the early 1900s. In one type of acetylene lamp, water was allowed to drop on a solid chunk of calcium carbide at a controlled rate. The acetylene produced then passed into an ignition chamber, where it burned with a brilliant white light. Acetylene lamps were used in a variety of settings, such as the source of light for bicycles and early motor cars and as a source of illumination for miners. Some small towns even used acetylene lamps as their major source of town lighting. Acetylene lamps continue to be a popular collectors item among antique dealers today.

Acetylene is both very flammable and explosive. Anyone who works with the compound or uses it in any form should know how to use the device that contains the gas. Acetylene also has the somewhat unusual chemical property of reacting with certain metals, such as copper and silver, to form highly explosive compounds known as acetylides. Lamps, torches, and other devices built to hold and dispense acetylene can not contain any of these metals. High concentrations of the gas also pose a health hazard to humans. It is classified as an asphyxiant, a gas that can produce disorientation, unconsciousness, and death when inhaled to excess.

Words to Know

OXIDATION

A chemical reaction in which oxygen reacts with some other substance or, alternatively, in which some substances loses electrons to another substance, the oxidizing agent.

SYNTHESIS

A chemical reaction in which some desired chemical product is made from simple beginning chemicals, or reactants.

FOR FURTHER INFORMATION

"Acetylene-Properties, Purity and Packaging." http://www.c-f-c.com/specgas_products/acetylene.htm (accessed on September 16, 2005).

"The Environmental Impact of Acetylene Compared to Impact of Propane-Chemtane 2." Chemtane. http://www.chemtane2.com/environmental/cva_env_impact.html (accessed on September 16, 2005).

O'Hara, Kelly. "Chemistry Hall of Fame-1997-Acetylene." York University. http://www.chem.yorku.ca/hall_of_fame/essays97/acetylene/acetylen.htm (accessed on September 16, 2005).

"Oxy-acetylene Welding and Cutting." ESAB. http://www.esabna.com/EUWeb/OXY_handbook/589oxy2_1.htm (accessed on September 16, 2005).

See AlsoPolyvinyl chloride