Phosgene

Phosgene is a toxic, colourless gas with the chemical formula COCl₂. Historically significant as both an industrial reagent and a chemical warfare agent, it is a volatile compound that poses serious hazards to human health and the environment. Phosgene’s physical properties—relatively high density, limited odour at low concentrations and reactivity with nucleophiles—render it useful in organic synthesis but hazardous in accidental or intentional releases. The following article summarises phosgene’s chemistry, production, uses, toxicology, detection and control measures, and regulatory context.

Chemical properties and synthesis

Phosgene is a diacyl chloride of carbonic acid; structurally it consists of a central carbonyl group bonded to two chlorine atoms. It is a gas under ambient conditions with a boiling point of 8.2 °C, a vapour density greater than air, and limited solubility in water, where it hydrolyses slowly to yield hydrogen chloride and carbon dioxide. Phosgene is described as having a faint odour reminiscent of newly cut hay or green corn at higher concentrations, but this odour is unreliable as a warning because sensory detection thresholds are close to or above harmful concentrations for some individuals.
Commercial production traditionally proceeds by the reaction of carbon monoxide with chlorine in the presence of activated carbon as a catalyst:
CO + Cl₂ → COCl₂.
The reaction is exothermic and conducted in controlled gas-phase reactors. Owing to phosgene’s hazards, modern industrial practice confines its synthesis and use to closed systems with extensive monitoring and safeguards.

Industrial applications and derivatives

Phosgene is principally employed in the manufacture of isocyanates (for example toluene diisocyanate and methylene diphenyl diisocyanate), polycarbonate resins, agrochemical intermediates, pharmaceuticals and various acid chlorides. Its utility derives from its ability to convert alcohols and amines into corresponding carbonyl derivatives: for instance, phosgene reacts with amines to give isocyanates; with diols it forms carbonate linkages used in polycarbonate production. While alternatives and phosgene-free routes exist for some products, phosgene remains attractive on an industrial scale because of atom efficiency and often superior product profiles.

Toxicology and mechanism of harm

Phosgene is a potent pulmonary irritant and a delayed-action respiratory toxin. Inhalation is the principal route of exposure; the gas reacts with proteins and lipids in the alveolar surfactant and epithelial lining, producing acylation and oxidative injury that disrupts the alveolo-capillary barrier. The pathophysiological sequence typically includes an initial latent period during which symptoms may be mild or absent, followed by progressive non-cardiogenic pulmonary oedema, respiratory distress and hypoxaemia. Severe exposures can be fatal.
Cutaneous or ocular contact with condensed phosgene or concentrated solutions may produce corrosive damage owing to the generation of hydrogen chloride and reactive carbonyl species. Systemic toxicity is largely secondary to pulmonary compromise; nevertheless, secondary inflammation and multi-organ effects may occur in severe cases.

Clinical presentation and management

Clinical features of phosgene exposure begin with irritation of the eyes, nose and throat, cough and chest tightness. Because of the characteristic latent interval (often several hours) between exposure and onset of severe respiratory symptoms, individuals who appear well immediately after exposure may subsequently deteriorate; this delay complicates triage and underscores the need for observation.
Management is primarily supportive. Immediate actions include removal from the exposure source, decontamination (clothing removal and skin washing) and administration of supplementary oxygen. Endotracheal intubation and ventilatory support, including positive end-expiratory pressure (PEEP), may be required for pulmonary oedema. There is no specific widely accepted antidote; treatments such as bronchodilators, corticosteroids and nebulised sodium bicarbonate have been used in various settings, but evidence is limited and largely supportive. Early medical evaluation and observation for at least 24 hours are recommended for significant exposures because deterioration can be delayed.

Detection, monitoring and emergency response

Detection and monitoring of phosgene in industrial and emergency contexts rely on fixed continuous sensors, portable detection tubes, electrochemical detectors and spectrometric techniques. Because ordinary odour detection is unreliable, engineering controls and continuous monitoring are essential in facilities where phosgene is produced, stored or used. Emergency response plans should include rapid area evacuation, sheltering procedures, protective equipment for responders (respirators with appropriate chemical cartridges or self-contained breathing apparatus), and protocols for medical triage and decontamination.
Containment and neutralisation in the event of a release often employ scrubbing systems using alkaline solutions, activated carbon beds and controlled flaring or catalytic destruction where safe and practicable. Incident command must also consider downstream impacts such as contamination of watercourses and impacts on public health.

Safety controls, storage and transport

Safe handling of phosgene requires closed systems, redundant containment, continuous monitoring, automatic shut-offs and robust engineering controls to prevent fugitive emissions. Storage is normally in pressurised cylinders fitted with appropriate safety valves and secondary containment; cylinders are kept chilled to reduce vapour pressure and located in well-ventilated, secure areas. Transport is subject to stringent hazardous-goods regulations that mandate packaging, labelling, emergency documentation and routing restrictions designed to minimise the risk of accidental releases.
Personnel working with phosgene require training in risks and emergency response, use of respiratory protective equipment, gas monitors and procedures for safe transfer and maintenance of equipment to prevent leaks.

Environmental and regulatory considerations

Phosgene degrades in the atmosphere through hydrolysis and photochemical processes but can present acute hazards near release sites. Environmental regulation treats phosgene as an acutely hazardous substance; industrial facilities are generally required to implement risk-management plans, report significant releases and maintain community right-to-know information. Many jurisdictions restrict or severely regulate the use of phosgene and encourage substitution with less hazardous reagents where feasible.
Because of its history as a chemical warfare agent during the First World War, phosgene is also subject to international controls aimed at preventing its misuse. These controls encompass production declarations, limits on stockpiling and obligations for secure handling under international chemical weapons conventions.

Alternatives, substitution and contemporary trends

Industrial chemistry has developed phosgene-free routes to numerous target molecules, including solid-state phosgene equivalents, triphosgene (a crystalline phosgene surrogate that releases phosgene in situ under controlled conditions) and enzymatic or catalytic processes that avoid chlorine-based carbonylation. Substitution is often pursued to reduce the risks associated with handling a volatile, highly toxic gas; nevertheless, economic, technical and product-quality considerations mean that phosgene continues to be used in sectors where established phosgene chemistry offers clear advantages.