Chloroform
The molecular lifesaver
Chloroform [shown left] is formally called trichloromethane and possesses the chemical formula CHCl3. This is a colourless, sweet smelling liquid which easily forms a vapour (Daintith, 2004). The molecule is traditionally called 'chloroform' due to its molecular similarity to formic acid (CHOOH) (Microsoft, 1998).
Although
not readily flammable, chloroform can decompose forming harmful products
such as hydrogen chloride and phosgene.
The former
is found in hydrochloric acid while the latter was extensively used as a lethal war
gas (Stratmann, 2003). Since
its discovery, chloroform has possessed a manifold of applications: mainly as an
anaesthetic. This article aims to summarise the history and chemistry of this important molecule. The main
industrial route used today is more complicated. This involves the action of
iron and acid on carbon tetrachloride
(Microsoft,
1998).
Clinical use
The
medicinal use of chloroform was pioneered in 1847 (Microsoft, 1998)
by the Scottish physician Sir
James Young Simpson (1811-1870)[shown below]. He used chloroform as a general
anaesthetic. This induced the loss of
consciousness necessary for painless surgery. Chloroform was non-flammable and relatively rapid at producing
anaesthesia. These advantages allowed chloroform to
replace ether (C4H10O) as the most commonly used anaesthetic. The
effects of chloroform inhalation became more serious as the dose was increased.
These effects were divided into 5 stages: 1.
The patient became insensible but retained consciousness. 2.
The patient entered a lethargic state in which some pain could be felt. 3.
The patient was physically incapable and could feel no pain. 4.
The patient exhibited strenuous breathing and complete muscle relaxation. 5.
The patient suffered an (often fatal) paralysis of the chest muscles. Stage 3 was recommended for most
surgical procedures. Contrary to popular belief, it was very difficult to
chloroform a patient to that extent. A skilled anaesthetist could take 5 minutes
to render a patient suitable for surgery. Despite
being an effective anaesthetic, chloroform had several disadvantages. The quantity
of chloroform required to differentiate stage 3 from stage 5 (above) was small.
Great skill was required to administer chloroform safely as the fatal dose was
only ~30ml.
Even if the patient survived the operation, 'delayed chloroform
poisoning' could lead to problems such as liver damage (Stratmann, 2003). Chloroform is now
regarded as a possible cause of cancer (ASTDR, 1997). The clinical use of chloroform decreased with the discovery of safer
general anaesthetics such as Halothane and Desflurane (Stratmann, 2003).
Exposure today There
are several ways in which chloroform may be encountered in everyday life: 1.
Drinking
water or beverages made using water containing chloroform.
2. Breathing indoor or outdoor air containing chloroform.
3.
Eating food containing chloroform.
4. Skin contact with chemicals containing chloroform.
(ATSDR, 1997)
The consumption of chlorinated drinking water represents the main everyday source of chloroform. The chlorine is added to kill unwanted bacteria but is often accompanied by extremely small concentrations of chloroform (as an impurity) (Stratmann, 2003).
Current use
Although chloroform is no longer the foremost anaesthetic, it remains the molecule of choice in a variety of modern applications. This section provides a summary of two of the most common.
The reaction between chloroform and sodium hydroxide forms an interesting molecule called dichlorocarbene [shown left]. The carbon atom in this molecule is electron deficient (because it possesses only 6 outer electrons). Such deficiency makes the molecule extremely reactive. Dichlorocarbene can be used to prepare very strained compounds such as cyclopropane [shown right] (Clayden et al, 2001).
The non-polar nature of the chloroform molecule makes it a useful solvent for non-polar molecules such as sulphur and iodine (Microsoft, 1998). The general rule is that "like will dissolve in like" (Murrell et al, 1994). An analogue of chloroform, called deuterochloroform, is a common solvent in a form of chemical analysis called NMR spectroscopy. Chloroform (CHCl3) and deuterochloroform (CDCl3) are chemically indistinguishable but that latter molecule is heavier. This is because deuterochloroform (CDCl3) possesses a deuterium atom (D) instead of a hydrogen atom (H). Deuterochloroform is a useful solvent because it is invisible to the NMR machine. This ensures that only the intended sample, not the solvent, is analysed (Clayden et al, 2001).
Property |
Data |
Reference |
CAS number |
[67-66-3] |
Stratmann, 2003 |
SI number | 1888 | Stratmann, 2003 |
Molecular
mass |
119.37704
gmol-1 |
Calculated using Clayden et al, 2001 |
Melting
point |
-63.5 0C |
Daintith, 2004 |
Boiling
point |
61.70C |
Daintith |
Density |
1.4832 gcm-3 |
Stratmann, 2003 |
Water solubility at 25 °C |
7.2-9.3 gdm-3 |
Watts et al, 2004 |
Vapour pressure at 20 °C |
|
Watts et al, 2004 |
Henry’s law constant at 20 °C |
304 Pa·m3/mol |
Watts et al, 2004 |
Stephen Belding is a 2nd year undergraduate studying Chemistry at The University of Oxford.
Agency For Toxic Substances And Disease Registry (ATSDR) (1997) Toxicological Profile for Chloroform, Agency For Toxic Substances And Disease Registry. Visited August 2006. URL: http://www.atsdr.cdc.gov/tfacts6.html
Clayden, J, Greeves, N, Warren, S and Wothers, P (2001) Organic Chemistry, Oxford University Press, New York.
Daintith, J (2004) A Dictionary of Chemistry, Oxford University Press, UK.
Microsoft (1998) Microsoft Encarta Encyclopedia 99, Microsoft.
Murrell, J, N, Jenkins, A, D (1994) Properties of Liquids and Solutions, John Wiley & Sons, UK.
Ritchie, R (2003) Revise AS & A2 Chemistry, Letts, London.
Stratmann, L (2003) Chloroform The Quest For Oblivion, Sutton Publishing, UK.
Watts, P, Long, G, Meek, M, E (2004) Concise International Chemical Assessment Document 58 CHLOROFORM, Chemical Safety Information from Intergovernmental Organizations. Visited August 2006. URL: http://www.inchem.org/documents/cicads/cicads/cicad58.htm
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