Phosgene (also known as carbonyl chloride, COCl₂) is a highly toxic gas or refrigerated liquid that was used as a chemical weapon in World War I. It has no color, but is detectable in air by its odor, which resembles moldy hay. It is a manufactured chemical, but small amounts occur naturally from the breakdown of chlorinated compounds. Phosgene can also result from the combustion of chlorine-containing organic compounds.

Phosgene is a particularly insidious poison, as phosgene exposure often has no initial symptoms. Symptoms usually appear within 24 hours, but can take up to 72 hours to manifest. The gas combines with water in the tissues of the respiratory tract to form carbon dioxide and hydrochloric acid. The acid then dissolves the membranes in the lungs. Fluid fills the lungs, and death results from a combination of blood loss, shock, and respiratory failure. Unlike nerve agents, phosgene must be inhaled to cause harm and cannot be absorbed through the skin.

Production and use

Industrially, phosgene is produced by passing purified carbon monoxide and chlorine gas through a bed of highly porous carbon, which acts as a catalyst. The chemical equation for their reaction is

CO + Cl₂ → COCl₂

The reaction is exothermic, so the reactor must be continually cooled to carry away the heat it produces. Typically, the reaction is carried out between 50°C and 150°C. Above 200°C, phosgene decomposes back into carbon monoxide and chlorine.

Because of the safety issues with storing and transporting it, phosgene is almost always produced and consumed within the same chemical plant. Phosgene is used chiefly in the production of polymers including polyurethanes, polycarbonates, and polyureas. It is also used to produce isocyanates and acid chlorides as intermediates in the dye, pesticide, and pharmaceutical industries. It is possible to use phosgene to isolate certain metals including aluminium and uranium from their ores, but these methods are not widely used.

In the laboratory, the use of the gaseous phosgene has since long been replaced by diphosgene, which is a liquid at room temperature, or triphosgene, a crystalline substance.

History

Phosgene was synthesized by the chemist John Davy (1790-1868) in 1812. It was first used as a weapon by the French, under the direction of French chemist Victor Grignard in 1915. Later, the Germans, under the direction of German chemist Fritz Haber added small quantities of it to chlorine to increase the latter's toxicity. Soon after, use of pure phosgene was begun. Phosgene was responsible for most of the about 100,000 gas-caused deaths during World War I.