Chloroform (CHCl3), also known as trichloromethane, is an organic and highly volatile compound that is produced both naturally and industrially. Previously used as an anaesthetic during surgery, and now as an industrial solvent in manufacturing chemicals, chloroform has a wide variety of applications. This article will explore the many aspects of chloroform, from how it is used and how it works, to its chemical structure, history and safety issues.
A Brief History of Chloroform
- 1830: A German pharmacist named Moldenhawer is credited as the first person to synthesise chloroform, though he mistakenly identified the compound as chloric ether. His method involved mixing chlorinated lime, i.e. calcium hypochlorite, with ethanol.
- 1834: After experiments carried out by Justus von Liebig and Eugène Soubeiran led to the identification of this new chemical compound, Jean-Baptiste Dumas, a French Chemist, assigned it an empirical formula and named the compound chloroform.
- 1842: The effectiveness of this new compound as a strong anaesthetic was demonstrated by Robert Mortimer Glover, who proved its efficacy by experimenting on laboratory animals.
- 1847: James Y Simpson became the first person to use chloroform on humans during surgery, causing it to become the world’s leading anaesthetic for the next century.
Chloroform’s Chemical Formula & Structure
A relatively simple organic compound with the formula CHCl3, chloroform is made up of a central carbon atom that is attached to three chlorine atoms and one hydrogen atom. This means that the chloroform molecule has a tetrahedral geometry:
- Chloroform’s tetrahedral structure belongs to the C3v symmetry group, which is characterised by a three-fold axis with three symmetrical, vertical planes.
- According to the Valence Shell Electron Pair Repulsion Theory (VSEPR), these geometrical symmetries are created by electrostatic forces between electrons.
- These forces make the surrounding atoms as far away from each other as possible, leading to a molecule’s geometrical symmetries and specific angles.
- In the case of the chloroform molecule, the repulsive forces between the surrounding hydrogen and chlorine atoms make the structure a perfect tetrahedral.
At the centre of chloroform’s tetrahedral molecule is a carbon atom. Since this is surrounded by three chlorine atoms and one hydrogen atom, a covalent intramolecular bond is formed, making the molecule slightly polar. This is what allows chloroform to be soluble in water.
The structure of the chloroform molecule is determined by these chemical bonds. In turn, the structure determines how the molecule reacts with other chemicals, as well as how it interacts with biological functions, like the firing of neurotransmitters.
Chemical Names for Chloroform
Just like many other chemicals, chloroform is known by several names, each one corresponding to its chemical composition, use or classification. As a precursor to many refrigerants, chloroform is sometimes known as Refrigerant-20, R-20 or by the brand name Freon-20. Meanwhile, it is referred to by its UN number (UN 1888) in industrial classifications to allow for easy indexing.
The other names for chloroform are all derivations of its chemical composition, including:
- Methane trichloride
- Methyl trichloride
- Methenyl trichloride
However, it is most accurately known by its IUPAC name: trichloromethane (TCM), which is similarly descriptive of its composition and structure.
What is Chloroform Used For?
Long before chloroform was widely used in industrial applications, its primary use was as a powerful anaesthetic during surgery. This is because when inhaled or ingested, it has anxiolytic, euphoriant and sedative effects.
However, the use of chloroform as an anaesthetic soon fell out of practice once its toxic effects on the body became known. The serious health hazards caused by this compound include kidney, liver and neurological damage. Its toxic effects on the central nervous system and organ function of the body meant that chloroform was entirely replaced as an anaesthetic by the 1970s.
When it comes to industries, trichloromethane is still a widely used compound with two main applications:
- Solvent: One of the most common industrial applications of chloroform as a solvent is in the manufacturing of pesticides. It is also used as a solvent in the industrial processing of fats, oil, rubber, lacquers, alkaloids and resins.
- Reagent: Chloroform is a good reagent because it is a source of dichlorocarbene. Sodium hydroxide reacts with chloroform and, with the help of a catalyst, produces dichlorocarbene. As a reagent, it helps produce many precursor chemicals that have various applications.
Chloroform is also industrially produced on a large scale as a precursor to polytetrafluoroethylene (PTFE), which has several applications from Teflon coatings to non-corrosive pipework. This chemical compound is also used as a precursor for various types of refrigerants, which are crucial in air conditioning systems. Among its many other uses, chloroform is an effective cleaning agent and even grain fumigant.
How Does Chloroform Work?
The large-scale industrial production of chloroform takes place through several stages of chlorination. Either chloromethane (CH3Cl) or methane (CH4) are used for this process. Chlorine is gradually added at 400–500°C, triggering free radical halogenation. The series of chemical reactions are summarised as follows:
CH4 + Cl2 → CH3Cl + HCl
CH3Cl + Cl2 → CH2Cl2 + HCl
CH2Cl2 + Cl2 → CHCl3 + HCl
The chemical reactions of chloroform as a reagent and solvent used in industrial applications are well understood. They can be easily written as balanced chemical equations or described in physical or catalytic actions.
However, the way chloroform works as an anaesthetic is uncertain. While we know that other anaesthetic agents, like ether and alcohol, can be mixed with chloroform to produce A.C.E, used historically for general anaesthesia, the biochemical action of chloroform as an anaesthetic is not fully understood. It could even involve a wide range of functions and mechanisms.
One potential method of action scientists have considered is that chloroform affects the movement of potassium ions travelling through channels in the nerve cells, and changes the way the body responds to nerves. It has also been suggested that this substance can stop you from feeling pain by disrupting cell membranes. Still, chloroform’s method of action in this area is widely unknown.
Chloroform in the Environment
Chloroform is both naturally-occurring and industrially produced. In fact, of the 660,000 metric tonnes of this substance that is estimated to pass through our environment every year, 90% of it is derived from natural sources. Some of these natural sources include several species of seaweed, fungi, and natural abiotic processes in soils.
Given this high global flux rate, it is reassuring to know that chloroform easily evaporates. Then, in this gaseous state, it begins to degrade into other compounds. The compounds produced by the degradation of chloroform include:
- Formyl chloride
- Carbon monoxide
- Carbon dioxide
- Hydrogen chloride
The degradation rate of chloroform varies depending on where it is dispersed. In the air, it has a half-life of between 55 and 620 days. Chloroform’s biodegradation in water, on the other hand, can vary massively, from a half-life of 1.5 days to over 1000 years, depending on the depth of water and whether it is under aerobic or anaerobic conditions. Fortunately, however, chloroform is not very bioaccumulative, and so its impact on the environment is minimised.
How to Identify Chloroform
Chloroform is an organic compound that is colourless, with an odour similar to ether compounds. There are two main ways to identify chloroform:
- Physical Characteristics: The first way you can identify chloroform is through its physical characteristics, like its strong odour, volatility, and freezing point. However, this method is not very accurate since the chemical may also physically resemble other compounds. Its odour, for example, is very similar to ether, another colourless and volatile chemical.
- Chemical Analysis: The most accurate way of identifying chloroform is through chemical analysis. These can be done by using chemical testing kits in the field, or by getting samples for laboratory testing. This method is particularly successful at identifying whether chloroform is present in the environment, which is important to ecological studies and wildlife protection.
As a toxic substance that has a lot of criminal uses, the toxicology laboratory test for chloroform is also crucial in forensics. There are several laboratory methods for identifying unknown chemical samples, including chromatography, spectroscopy, mass spectrometry, and X-ray diffraction. The downside is that most of these testing methods are complicated and some versions are not portable.
One simple and easy way of specifically testing for chloroform in aqueous and non-aqueous solutions is the use of optical thin film sensors, which make use of a modified Fujiwara reaction. This method works by detecting trace amounts of chloroform using visible light.
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