Polyhalogen Compounds: Dichloromethane, Chloroform, Carbon Tetrachloride and Iodoform

Several simple polyhalomethanes are produced and used industrially, and some have important -- occasionally sobering -- histories in medicine, alongside genuine, well-established hazards that govern how (or whether) they are used today.

Dichloromethane ($\text{CH}_2\text{Cl}_2$, methylene chloride). A widely used industrial and laboratory solvent, valued for dissolving a broad range of organic compounds and for its relatively low boiling point ($40\,^\circ\text{C}$), which makes it easy to remove by evaporation. It is used as a paint stripper, as a degreasing solvent, and as an extraction solvent (for instance, in decaffeinating coffee, where residual traces are tightly regulated). Health-wise it acts as a central-nervous-system depressant on inhalation and is metabolised in the body partly to carbon monoxide, and long-term/heavy occupational exposure has raised carcinogenicity concerns, so its use requires good ventilation and exposure limits.

Trichloromethane (chloroform, $\text{CHCl}_3$). Historically significant as one of the first general surgical anaesthetics, introduced into clinical use in the 1840s, though its use for this purpose was abandoned decades later once its toxicity to the liver and heart, and its narrow safety margin, became clear. A further, purely chemical hazard is that chloroform slowly oxidises in the presence of air and light to the highly toxic gas carbonyl chloride (phosgene, $\text{COCl}_2$): $2\,\text{CHCl}_3 + \text{O}_2 \xrightarrow{\text{light}} 2\,\text{COCl}_2 + 2\,\text{HCl}$. To prevent this, chloroform is stored in dark-coloured, completely filled, airtight bottles (minimising exposure to both light and air), and a small amount (about 1%) of ethanol is added, which reacts with any phosgene that does form, converting it into the harmless diethyl carbonate: $\text{COCl}_2 + 2\,\text{C}_2\text{H}_5\text{OH} \to (\text{C}_2\text{H}_5\text{O})_2\text{CO} + 2\,\text{HCl}$.

Tetrachloromethane (carbon tetrachloride, $\text{CCl}_4$). Once widely used as a dry-cleaning and degreasing solvent, and as a fire-extinguishing fluid (marketed as "pyrene") because it is non-flammable, and as an industrial feedstock for manufacturing chlorofluorocarbons (freons). All of these uses have been phased out: it is hepatotoxic and nephrotoxic on exposure, it reacts with moisture or flame to release toxic phosgene (making it dangerous as a fire extinguisher in enclosed spaces), and it is itself a stratospheric ozone-depleting substance controlled under the Montreal Protocol. …