Chloroform (Methyl Trichloride): From Anesthetic Past to Modern Industrial Solvent

Chloroform, CAS 67-66-3, is the trichloromethane homologue of the chlorinated methane series, with molecular formula CHCl₃ and molar mass 119.38 g/mol. At standard atmospheric pressure of 101.3 kPa, the liquid boils at 61.2 °C, freezes at −63.5 °C, and exhibits a density of 1.489 g/cm³ at 20 °C. The vapour pressure of 21.2 kPa at 20 °C makes closed-transfer and vapour-containment equipment mandatory for industrial use. The historical designation “methyl trichloride” originates from the replacement of three hydrogen atoms in the methane structure with chlorine; the modern IUPAC name is trichloromethane.

What Controls Selectivity in the Methane-to-Chloroform Chain?

Industrial production of chloroform is dominated by the photochemical or thermal free-radical chlorination of methane or methyl chloride, followed by distillation. The substitution sequence is:

CH₄ + Cl₂ → CH₃Cl + HCl CH₃Cl + Cl₂ → CH₂Cl₂ + HCl CH₂Cl₂ + Cl₂ → CHCl₃ + HCl CHCl₃ + Cl₂ → CCl₄ + HCl

Selectivity to CHCl₃ is not a thermodynamic endpoint; it is a kinetic compromise between underchlorination to methyl chloride and methylene chloride and overchlorination to carbon tetrachloride. In a continuous multitubular gas-phase reactor, heat removal becomes the limiting scale-up parameter because each substitution step is exothermic by approximately 100 kJ/mol of HCl generated. Hot spots in the catalyst-free tube accelerate polyhalogenation and reduce the yield of the trichloromethane cut. Process control therefore relies on chlorine-to-methane ratio, recycle of lower chlorinated cuts, and quench cooling of the reactor effluent. Published production data for specific plant configurations is limited, but operation with excess methane and sequential recycle of methyl chloride and methylene chloride is the standard route to maximise chloroform yield without shifting to carbon tetrachloride.

Crude chlorination condensate contains water, HCl, chlorinated C₁ compounds, and trace unsaturated byproducts. The first separation stage is a water wash and neutralisation step, followed by a three-column distillation train. Chloroform forms a heterogeneous minimum-boiling azeotrope with water at approximately 56 °C; the overhead condensate splits into a chloroform-rich phase and an aqueous phase. The chloroform-rich phase is decanted into the drying column, while the aqueous phase is refluxed or distilled to recover dissolved chloroform. Final drying to low mg/kg moisture is performed with molecular sieves or anhydrous calcium chloride beds. This unit operation sequencing is well established, and specific equipment details are available from chlorinated solvent technology licensors rather than from standardised public data.

Typical physical property values for solvent-grade chloroform

Property	Value	Condition
Molar mass	119.38 g/mol	—
Boiling point	61.2 °C	101.3 kPa
Freezing point	−63.5 °C	—
Density	1.489 g/cm³	20 °C
Vapour pressure	21.2 kPa	20 °C
Water solubility	8.1 g/L	20 °C
Refractive index n20/D	1.4459	589 nm, 20 °C

Thermal Stabilizers and Solvent-Grade Specifications

Chloroform is not permanently inert. In the presence of oxygen and ultraviolet light, it undergoes slow photochemical oxidation to phosgene and hydrogen chloride. Commercial grades are therefore stabilised, typically with amylene at 0.0005–0.002 mass fraction or ethanol at 0.5–1.0 mass percent for pharmacopoeial and laboratory grades. The stabiliser protects the product during storage in lined steel drums and during reflux operations where trace HCl accelerates corrosion. Solvent-grade chloroform is sold with a minimum assay of 99.9 wt% on a water-free basis, and moisture is controlled by Karl Fischer coulometry. Acidity and phosgene content are limited in pharmacopoeial specifications by wet chemical limit tests. The specific stabiliser must be matched to the application: amylene is preferred for extraction and UV spectroscopy, while ethanol is used where amylene may interfere with downstream chemistry. Unstabilised chloroform must be stored under inert gas and protected from light, but its shelf life is short once the container is opened.

The clinical use of chloroform as an inhalational anesthetic was introduced in 1847 and was abandoned for general anesthesia because of a narrow therapeutic ratio, delayed hepatic necrosis, and ventricular fibrillation. The transition from surgical anesthetic to chemical commodity was complete by the mid-20th century. In modern industrial hygiene terms, occupational exposure is the primary health concern. The compound is absorbed by inhalation, ingestion, and through intact skin; the OSHA ceiling limit is 50 ppm (240 mg/m³), and NIOSH has set a short-term exposure limit of 2 ppm (9.78 mg/m³) for 60 minutes. These values are not equivalent, and the NIOSH STEL reflects the current concern for hepatic and renal effects at much lower concentrations than the older OSHA ceiling. Chloroform is classified as a Category 2 carcinogen under CLP and as a Class 2 residual solvent under ICH Q3C. These classifications impose specific operational controls: closed-loop transfer, local exhaust ventilation with monitoring to below the applicable limit, and impermeable gloves with breakthrough-time data.

Regulatory exposure and residual solvent limits

Metric	Value	Reference
OSHA ceiling limit	50 ppm (240 mg/m³)	29 CFR 1910.1000
NIOSH short-term exposure limit	2 ppm (9.78 mg/m³) for 60 min	NIOSH REL
ICH Q3C permitted daily exposure	0.6 mg/day	ICH Q3C
ICH Q3C concentration limit	60 ppm	ICH Q3C

When Chloroform Replaces Dichloromethane in Extraction and Chromatography

Laboratory and pilot-scale processes use chloroform where dichloromethane fails to provide adequate phase separation or where a higher-density organic phase is required. The density difference between chloroform and water is approximately 0.49 g/cm³, which gives rapid settling in continuous extractors. In liquid-liquid extraction of alkaloids and natural products, chloroform is preferred for its high partition coefficient with free-base alkaloids and its immiscibility with aqueous phases. The same density property creates a disadvantage in downstream solvent recovery: distillation columns must be designed for the higher boiling point and for the tendency of wet chloroform to form acidic hydrolytic byproducts. Equipment for bulk solvent handling is typically glass-lined steel or nickel alloy; carbon steel is acceptable only in rigorously dry, stabilised service. Stainless steel is generally avoided in wet chloroform service because chloride-induced stress-corrosion cracking has been documented in 304 and 316 stainless steel heat exchangers after repeated exposure to condensed aqueous HCl. Published data for specific failure rates in pharmaceutical extraction vessels is limited.

In analytical chemistry, deuterochloroform is the most common solvent for proton NMR spectroscopy. The residual solvent signal of CHCl₃ in CDCl₃ appears at 7.26 ppm relative to tetramethylsilane, and the solvent is supplied in sealed ampoules over silver foil or molecular sieves to prevent phosgene formation. The high solvating power of chloroform also makes it suitable for gel-permeation chromatography of polar polymers and for infrared spectroscopy cells, though its plasticising effect on acrylic flow cells imposes material compatibility restrictions. For pharmaceutical samples, residual chloroform must be controlled below 60 ppm in the final drug product under ICH Q3C unless a higher limit is justified by the permitted daily exposure of 0.6 mg/day and actual daily dose.

The largest industrial demand for chloroform is as chemical intermediate for chlorodifluoromethane, R-22: CHCl₃ + HF → CHClF₂ + HCl. This fluorination is conducted in the liquid phase over antimony pentachloride or in the gas phase over fluorinated alumina catalysts. The chloroform feed to fluorination must be low in water, stabiliser, and unsaturated impurities because these components hydrolyse or poison the catalyst and increase HCl corrosion in the reactor train. R-22 is subsequently pyrolyzed to tetrafluoroethylene at 700–900 °C with steam dilution, and tetrafluoroethylene is polymerised to PTFE and related fluoropolymers. The consumption of chloroform in this chain is governed by fluorine-containing polymer demand and by the Montreal Protocol phase-down of R-22 as a refrigerant, which separates the feedstock value of R-22 from its emissive uses. This regulatory boundary means that solvent-grade chloroform and feedstock-grade chloroform are handled as separate supply chains, with different stabiliser packages and shipping specifications.

Chloroform must not be combined with strong bases and primary or secondary amines in the presence of active halogen acceptors because the formation of dichlorocarbene and, in the carbylamine reaction, isocyanides creates toxic and reactive hazards. Mixtures with sodium hydroxide, ethanol, and water can generate reactive intermediates and exotherms. Chloroform also reacts with oxygen to phosgene. Therefore storage in amber glass or lined steel under inert gas with stabiliser is required. Bulk storage tanks are fitted with pressure-vacuum relief valves and scrubbers because chloroform is not classified as flammable under ordinary conditions, but decomposition products are acidic. All transfer and sampling points should be fitted with closed fittings and dry-break couplings to limit worker exposure below the applicable limit. The final operating boundary is the residual solvent limit in pharmaceutical applications: if the downstream product cannot be dried below 60 ppm, chloroform is replaced with dichloromethane or ethyl acetate.