Trichloromethane, also known as chloroform [67-66-3], is an organic compound with the chemical formula CHCl3. It is a clear, colorless liquid with a sweet yet pungent odor. Historically, it was used in anesthesia and as a solvent, but due to its toxicity, it has been substituted by safer products.
Table of Contents
1. Physical Properties of Trichloromethane
Trichloromethane is a colorless, dense liquid with a characteristic sweet odor reminiscent of dichloromethane. Under standard conditions, it is non-flammable, and its vapors do not form explosive mixtures with air.
Historically, trichloromethane was used as a solvent for a wide range of organic materials, including alkaloids, fats, oils, resins, waxes, gums, rubber, and paraffin. However, due to its toxicity, its use as a solvent is gradually decreasing in favor of dichloromethane, which possesses similar solvent properties in most cases.
Additionally, trichloromethane effectively dissolves iodine and sulfur and readily mixes with numerous organic solvents.
Trichloromethane forms azeotropic mixtures with other compounds like methanol, 2,3-dimethyl butane, 1,5-hexadiene, formic acid, ethanol, hexane, 2-propanol, acetone, 2-butanone, and others.
Important physical properties of trichloromethane are listed in the following table:
| Property | Value |
|---|---|
| Molecular weight | 119.38 g/mol |
| Boiling point at 1 bar | 61.1 °C |
| Melting point | −63.6 °C |
| Vapor pressure at 20 °C | 212 mbar |
| Enthalpy of vaporization | 29.64 kJ/mol |
| Enthalpy of fusion at mp | 9.5 kJ/mol |
| Density of liquid at 20 °C | 1483.2 kg/m3 |
| Density of vapor at bp | 4.372 kg/m3 |
| Cubic expansion coefficient of liquid (0–40 °C) | 0.00129 K−1 |
| Enthalpy of formation of vapor at 25 °C, 1 bar | −103.14 kJ/mol |
| Gibbs free energy of formation of vapor at 25 °C, 1 bar | −70.34 kJ/mol |
| Specific heat capacity of vapor at 25 °C, 1 bar | 0.550 kJ kg−1 K−1 |
| Enthalpy of formation of liquid at 25 °C | −134.47 kJ/mol |
| Gibbs free energy of formation of liquid at 25 °C | −73.66 kJ/mol |
| Specific heat capacity of liquid at 25 °C | 0.953 kJ kg−1 K−1 |
| Critical temperature (Tc) | 263 °C |
| Critical pressure | 53.8 atm |
| Critical volume | 0.2407 mL/mol |
| Critical compressibility factor | 0.2972 |
| Thermal conductivity of vapor | 0.00787 W K−1 m−1 |
| Thermal conductivity of liquid at 20 °C | 0.130 W K−1 m−1 |
| Surface tension at 20 °C | 27.1 × 10−3 N/m |
| Viscosity of liquid at 20°C | 0.570 cP |
| Dipole moment | 1.00 D |
| Refractive index of liquid at 25 °C | 1.4455 |
| Dielectric constant of vapor at 20 °C | 1.00 |
| Dielectric constant of liquid at 20 °C | 4.79 |
| Partition coefficient air/water at 20 °C | 0.12 |
| Partition coefficient n-octanol/water at 20 °C as log Pow | 1.97 |
2. Chemical Properties of Trichloromethane
Trichloromethane is non-flammable but decomposes in flames or on hot surfaces, releasing hydrogen chloride (HCl).
In the presence of air, photochemical cleavage of chloroform by peroxides forms phosgene and HCl, and iron catalyzes oxidative degradation in the dark. To reduce autoxidation and HCl generation, stabilizers can be used.
Aqueous alkali hydrolysis of trichloromethane yields formic acid, and with alcoholates, it forms orthoformate esters (the commercial route for trimethyl and triethyl orthoformate).
Phenolates react with chloroform to form salicylaldehydes (Reimer-Thiemann reaction). Under Friedel-Crafts conditions, it reacts with benzene to generate triphenylmethane.
A key reaction is that of trichloromethane with hydrogen fluoride and antimony pentahalides to produce monochlorodifluoromethane (HCFC-22), a PTFE precursor.
Chloroform is the main source of deuterochloroform (CDCl3), an important solvent for NMR spectroscopy, via deuterium exchange with D2O.
Trichloromethane reacts violently with solid alkali hydroxides and amides to generate dichlorocarbene, a highly reactive and toxic intermediate, and it forms shock-sensitive and explosive mixtures with alkali and alkaline-earth metals.
The reaction of trichloromethane with amines in alcoholic alkaline solutions produces isonitriles via dichlorocarbene, known as the carbylamine reaction, which is used as a test for amines (e.g., aniline).
Chloroform reacts with bromine to form bromochloromethanes (CCl3Br, CCl2Br2, and CClBr3) through bromination and chlorine-bromine exchange.
3. Production of Trichloromethane
Modern industrial production of dichloromethane and trichloromethane primarily relies on direct chlorination of methane and monochloromethane using chlorine. This process also generates tetrachloromethane as a byproduct. The reaction initiation can be achieved through various routes:
- High-temperature gas-phase activation offers efficient conversion but requires high energy input.
- High-temperature liquid-phase without initiator is suitable for large-scale production but lacks selectivity, leading to a mixture of chlorinated methanes.
- Low-temperature liquid-phase with initiator improves selectivity towards dichloromethane and trichloromethane but necessitates additional reaction control measures.
- Photochemical activation provides cleaner reaction profiles but has limited industrial application due to scalability challenges.
3.1. Gas-Phase Thermal Chlorination
Gas-phase thermal chlorination has become the primary method for industrial production of dichloromethane and trichloromethane after overcoming earlier technical challenges like explosions and carbon liberation.
The chlorination of methane and its derivatives (monochloromethane) are exothermic reactions, with Gibbs free energy (ΔrG) significantly negative.
- CH4 + Cl2 → CH3Cl + HCl ⇒ ΔrG = -106.9 kJ∕mol
- CH3Cl + Cl2 → CH2Cl2 + HCl ⇒ ΔrG = -102.2 kJ∕mol
- CH2Cl2 + Cl2 → CHCl3 + HCl ⇒ ΔrG = -96.4 kJ∕mol
- CHCl3 + Cl2 → CCl4 + HCl ⇒ ΔrG = -78.7 kJ∕mol
The chlorination process follows a radical substitution mechanism, initiated by thermal dissociation of chlorine into radicals at high temperatures. Chain propagation reactions involving these radicals lead to successive hydrogen atom substitutions in methane molecules.
While monochlorination is the primary goal, polysubstitution also occurs, leading to dichloromethane, chloroform, and heavier chloromethanes. Chain termination involves the recombination of various radical species, including chloroalkane-chlorine, chlorine-chlorine, and methane-methane. Impurities like oxygen in chlorine can also terminate chains.
Reactor design plays a crucial role, as a high wall surface area relative to volume promotes chlorine adsorption and chain termination. The thermal reaction exhibits second-order kinetics, with conversion rate dependent on time, chlorine and methane partial pressures, and reactant chlorination degree.
Relative reaction rate constants are relatively independent of temperature and pressure within the relevant industrial range. Quantitative models can predict product distribution from monochloromethane chlorination and primary product pyrolysis.
a) Loop reactor; b) Process gas cooler; c) Quench; d) Gas/liquid separator; e) HCl absorption; f) Neutralization system; g) Sulfuric acid drying column; h) Compressor; i) First condensation step; j) Second condenser; k) Condensate buffer vessel; l1–l4) Distillation columns for CH3Cl, CH2Cl2, CHCl3 and CCl4
3.2. Liquid-Phase Chlorination
High-pressure, liquid-phase chlorination of monochloromethane offers an alternative to the conventional gas-phase approach. Tokuyama Soda Co. developed this method and later improved by Dow Chemical Co..
This method operates at moderate temperatures (40–175 °C) and pressures (6.9–55 bar), maintaining the reaction mixture in the liquid phase.
The reaction proceeds with or without the presence of radical-producing initiators like peroxides, azobisnitriles, or azodiisobutyronitrile. While initiators significantly increase the reaction rate, they introduce drawbacks:
- Cost: Initiators add additional expense to the process.
- Byproducts: Initiator decomposition products may contaminate the final product and necessitate removal from the reactor bottoms.
- Water Formation: Certain initiators may promote unwanted water formation as a side product.
Therefore, the choice of initiator requires careful consideration of the desired reaction rate, cost efficiency, and product purity.
Compared to gas-phase chlorination, the liquid-phase process offers potential advantages such as:
- Improved Selectivity: Liquid-phase conditions may favor the targeted chlorination products, minimizing undesired byproducts.
- Enhanced Control: Pressure and temperature control provide greater flexibility in customizing the reaction for specific product distributions.
- Potential Safety Advantages: Operating under pressure can potentially reduce explosion risks associated with gas-phase chlorination.
Further research and development are necessary to fully optimize the liquid-phase chlorination process and explore its potential applications.
3.3. Other Processes
Oxychlorination stands out as a potential method for producing chlorinated methanes with complete chlorine consomation and no HCl byproduct. This eliminates waste generation and increases resource efficiency.
However, low methane reactivity necessitates high reaction temperatures, leading to undesirable side products (combustion products) and methane loss. Pilot-plant studies using fluidized-bed technology failed to overcome this challenge.
The Transcat process by Lummus Co. offers a more promising approach. It uses a molten salt mixture of copper(II) chloride and potassium chloride for two-step chlorination and oxychlorination of methane with chlorine and air.
In this process nearly all by-products are recovered and recycled. However, catalyst volatility at reaction temperatures reduces activity and necessitates downstream corrosion control.
Dow Inc. proposed a more stable system based on an LaOCl catalyst, enabling single-step oxychlorination with high selectivity for chlorinated hydrocarbons. This approach potentially addresses the challenges of the Transcat process.
Optimizing reaction conditions, exploring alternative catalysts, and improving reactor design are important for scaling up these promising oxychlorination technologies and realizing their full potential for sustainable chlorinated methane production.
4. Uses of Trichloromethane
Trichloromethane is primary used in the production of hydrochlorofluorocarbon (HCFC) monochlorodifluoromethane (HCFC-22, R-22). HCFC-22 undergoes thermal dehydrofluorination to yield tetrafluoroethylene (TFE), the monomer for various fluoropolymers and fluororubbers.
These materials possess exceptional thermal and chemical stability, making them valuable in diverse applications:
- Chemical and pharmaceutical industry: Corrosion-resistant linings for steel pipes and reactors.
- Electronics and medical equipment: High-performance components requiring chemical and temperature resilience.
- Cookware: Nonstick coatings for frying pans and other utensils.
- Textiles: Microporous membranes for waterproof, breathable fabrics like Gore-Tex.
- Roofing materials: Specialized membranes for superior weather resistance.
- Lubricants: Sprays and greases with good heat and chemical tolerance.
- Gliding materials: Low-friction surfaces for various applications.
The most prominent fluoropolymer derived from TFE is polytetrafluoroethylene (PTFE), marketed under names like Hostaflon, Teflon, and Polymist.
While once a major refrigerant, HCFC-22 usage is increasingly restricted due to its ozone-depleting properties. Under the Montreal Protocol, industrialized countries phased out HCFC-22 use years ago, and developing countries are gradually following suit with a complete phase-out by 2030. However, limited HCFC-22 consumption will be permitted for servicing existing equipment until 2040.
Trichloromethane is also used as a solvent in various organic reactions and industrial processes.
5. Toxicology of Trichloromethane
Trichloromethane exhibits moderate toxicity from single exposure, but repeated exposure poses a significant risk of severe health consequences. Its use as an anesthetic has been discontinued primarily due to delayed liver toxicity and the availability of safer alternatives.
Ingestion is unlikely to be problematic unless large amounts are consumed accidentally or deliberately. However, trichloromethane readily absorbs through the skin and eyes, necessitating precautions to prevent exposure.
Chronic toxicity is a major concern, requiring strict control measures to avoid liver and kidney damage, the primary adverse effects of excessive exposure.
While fetotoxic in animals, trichloromethane’s teratogenicity is weak, if present at all. Genotoxicity studies, both in vitro and in vivo, have yielded largely negative results, although some suggest potential clastogenic activity.
Trichloromethane is classified as “possibly carcinogenic to humans (Group 2B)” by IARC due to its ability to induce liver and kidney tumors in rats and mice, with sex and strain dependence. This carcinogenicity is believed to be non-genotoxic, arising from chronic tissue damage rather than direct genetic alterations. Inhalation carcinogenicity studies are lacking.
8-hour Occupational Exposure Limits (OELs) for trichloromethane generally range between 0.5 and 3 ppm.
Reference
- Chloromethanes; Ullmann’s Encyclopedia of Industrial Chemistry. – https://onlinelibrary.wiley.com/doi/10.1002/14356007.a06_233.pub4
