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Phosgene
Phosgene is an organic chemical compound with the formula COCl2. It is a toxic, colorless gas; in low concentrations, its musty odor resembles that of freshly cut hay or grass. It can be thought of chemically as the double acyl chloride analog of carbonic acid, or structurally as formaldehyde with the hydrogen atoms replaced by chlorine atoms. In 2013, about 75–80 % of global phosgene was consumed for isocyanates, 18% for polycarbonates and about 5% for other fine chemicals.
Phosgene is extremely poisonous and was used as a chemical weapon during World War I, where it was responsible for 85,000 deaths. It is a highly potent pulmonary irritant and quickly filled enemy trenches due to it being a heavy gas.
It is classified as a Schedule 3 substance under the Chemical Weapons Convention. In addition to its industrial production, small amounts occur from the breakdown and the combustion of organochlorine compounds, such as chloroform.
Phosgene is a planar molecule as predicted by VSEPR theory. The C=O distance is 1.18 Å, the C−Cl distance is 1.74 Å and the Cl−C−Cl angle is 111.8°. Phosgene is a carbon oxohalide and it can be considered one of the simplest acyl chlorides, being formally derived from carbonic acid.[citation needed]
Industrially, phosgene is produced by passing purified carbon monoxide and chlorine gas through a bed of porous activated carbon, which serves as a catalyst:
This reaction is exothermic and is typically performed between 50 and 150 °C. Above 200 °C, phosgene reverts to carbon monoxide and chlorine, Keq(300 K) = 0.05. World production of this compound was estimated to be 2.74 million tonnes in 1989.
Phosgene is fairly simple to produce, but is listed as a Schedule 3 substance under the Chemical Weapons Convention. As such, it is usually considered too dangerous to transport in bulk quantities. Instead, phosgene is usually produced and consumed within the same plant, as part of an "on demand" process. This involves maintaining equivalent rates of production and consumption, which keeps the amount of phosgene in the system at any one time fairly low, reducing the risks in the event of an accident. Some batch production does still take place, but efforts are made to reduce the amount of phosgene stored.
Simple organochlorides slowly convert into phosgene when exposed to ultraviolet (UV) irradiation in the presence of oxygen. Before the discovery of the ozone hole in the late 1970s large quantities of organochlorides were routinely used by industry, which inevitably led to them entering the atmosphere. In the 1970-80s phosgene levels in the troposphere were around 20-30 parts per trillion by volume (peak 60 parts per trillion by volume). These levels had not decreased significantly nearly 30 years later, despite organochloride production becoming restricted under the Montreal Protocol.
Phosgene
Phosgene is an organic chemical compound with the formula COCl2. It is a toxic, colorless gas; in low concentrations, its musty odor resembles that of freshly cut hay or grass. It can be thought of chemically as the double acyl chloride analog of carbonic acid, or structurally as formaldehyde with the hydrogen atoms replaced by chlorine atoms. In 2013, about 75–80 % of global phosgene was consumed for isocyanates, 18% for polycarbonates and about 5% for other fine chemicals.
Phosgene is extremely poisonous and was used as a chemical weapon during World War I, where it was responsible for 85,000 deaths. It is a highly potent pulmonary irritant and quickly filled enemy trenches due to it being a heavy gas.
It is classified as a Schedule 3 substance under the Chemical Weapons Convention. In addition to its industrial production, small amounts occur from the breakdown and the combustion of organochlorine compounds, such as chloroform.
Phosgene is a planar molecule as predicted by VSEPR theory. The C=O distance is 1.18 Å, the C−Cl distance is 1.74 Å and the Cl−C−Cl angle is 111.8°. Phosgene is a carbon oxohalide and it can be considered one of the simplest acyl chlorides, being formally derived from carbonic acid.[citation needed]
Industrially, phosgene is produced by passing purified carbon monoxide and chlorine gas through a bed of porous activated carbon, which serves as a catalyst:
This reaction is exothermic and is typically performed between 50 and 150 °C. Above 200 °C, phosgene reverts to carbon monoxide and chlorine, Keq(300 K) = 0.05. World production of this compound was estimated to be 2.74 million tonnes in 1989.
Phosgene is fairly simple to produce, but is listed as a Schedule 3 substance under the Chemical Weapons Convention. As such, it is usually considered too dangerous to transport in bulk quantities. Instead, phosgene is usually produced and consumed within the same plant, as part of an "on demand" process. This involves maintaining equivalent rates of production and consumption, which keeps the amount of phosgene in the system at any one time fairly low, reducing the risks in the event of an accident. Some batch production does still take place, but efforts are made to reduce the amount of phosgene stored.
Simple organochlorides slowly convert into phosgene when exposed to ultraviolet (UV) irradiation in the presence of oxygen. Before the discovery of the ozone hole in the late 1970s large quantities of organochlorides were routinely used by industry, which inevitably led to them entering the atmosphere. In the 1970-80s phosgene levels in the troposphere were around 20-30 parts per trillion by volume (peak 60 parts per trillion by volume). These levels had not decreased significantly nearly 30 years later, despite organochloride production becoming restricted under the Montreal Protocol.
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