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Carbon Halides
Andrew R. Barron
This work is produced by The Connexions Project and licensed under the Creative Commons Attribution License †
There are two general classes of carbon halides.
- Homoleptic halides, e.g., CCl 4 , CCl 2 F 2 , C 6 Cl 6 , etc.
- Carbonyl halides, e.g., Cl 2 C=O.
A summary of some simple carbon halides is given in Table 1.
Compound Mp ( ◦C) Bp ( ◦C) Remarks CF 4 -185 -128 Very stable gas CCl 4 -23 76 Colorless liquid, stable CBr 4 93 190 Pale yellow solid, de- composes upon boiling CI 4 171 - Bright red solid, decom- poses prior to boiling, sublimed at low pressure F 2 C=O -114 -83 Decomposed by H 2 O Cl 2 C=O -118 8 Phosgene, highly toxic Br 2 C=O - 65 Fumes in air
Table 1: Physical properties of simple halogen compounds of carbon.
1 Carbon tetrahalides
The carbon tetrahalides are generally prepared by the direct (thermal) reaction of carbon with the appro- priate halogen, (1); however, speci c syntheses are possible for each derivative.
In addition to the direct reaction of uorine with carbon, CF 4 can be prepared from SiC, (2). The SiF 4 side product is removed by passing the reaction mixture through NaOH solution, in which SiF 4 reacts to form
∗Version 1.4: Jan 22, 2010 2:35 pm US/Central †http://creativecommons.org/licenses/by/3.0/
silicate. The dierence in reactivity of SiF 4 and CF 4 is attributable to the lack of an energetically accessible ve-coordinate intermediate required for the associative mechanism.
Carbon tetrabromide can be obtained by bromination of CH 4 with HBr or Br 2 , or by the reaction of CCl 4 with AlBr 3 , (3). Carbon tetraiodide (CI4) can be made by the Lewis acid catalyzed halogen exchange reaction, (4).
CF 4 is very stable. In fact, it is so stable that it does not even react with molten sodium. In contrast to CF 4 , carbon tetrachloride (CCl 4 ) reacts readily with alkali metals (K and Na) or other strong reducing agents (e.g., F 2 , Al, Ba, Be, and Zn). While CCl 4 is thermodynamically unstable with respect to hydrolysis, it is kinetically stable, and thus nds extensive use as a solvent. Photolysis can result in the transfer of a chloride radical to various substrates. It is also used in the conversion of metal oxides to the chlorides. Carbon tetrabromide (CBr 4 ) is insoluble in water and other polar solvents, but soluble in benzene. Carbon tetraiodide (CI 4 ) decomposes thermally, (5).
The decreasing stability of CX 4 , from uorine to iodine, is directly related to the C-X bond energy (Table 2).
C-X Bond energy (kJ/mol) C-F 485 C-Cl 327 C-Br 285 C-I 213
Table 2: Bond energies for carbon-halide bonds.
1.1 Hazards
Despite its use as a solvent CCl 4 has signi cant hazardous eects. Inhalation of carbon tetrachloride vapor can cause headaches, mental confusion, depression, fatigue, loss of appetite, nausea, vomiting, and coma. The symptoms can take many hours to appear. The vapor and liquid irritate the eyes, and internal irritation, nausea, and vomiting are caused when taken orally. Chronic eects from prolonged inhalation include bronchitis and jaundice, while skin exposure can cause dermatitis. Carbon tetrabromide is toxic by inhalation, and the vapor is narcotic if taken in high concentrations. As with CCl 4 , CBr 4 can react explosively with alkali metals.
3 Mixed halides
Mixed halides are an important class of halocarbon compound. They are synthesized by halide exchange, (7). The high cost of SbF 3 means that the reaction is generally run with an excess of the chloride.
The ordinary name for mixed carbon halide is halon or Freon, although Freon is actually a Du Pont trade- mark. A list of selected Freon compounds are given in Table 4. Halons are non-toxic, non- ammable, and have no odor. However, it is their very lack of reactivity that has caused a problem.
Freon Formula Uses 12 CCl 2 F 2 Refrigerant 11 CCl 3 F Refrigerant 114 ClF 2 C-CClF 2 Refrigerant 113 Cl 3 C-CF 3 Solvent 13B1 CBrF 3 Fire extinguisher 1211 CBrClF 2 Fire extinguisher
Table 4: Selected Freons and their applications.
3.1 Environmental impact of chloro uorcarbon compounds (CFCs)
Chloro uorcarbon compounds (CFCs) are very stable and are not degraded in the environment. As a consequence they are transported to the stratosphere where they decomposed upon photolysis, (8). The resulting chloride radical is a catalyst for the decomposition of ozone, (9), as well as a catalyst for the reaction of ozone with molecular oxygen, (10).
The widespread use of CFCs as refrigerants and propellants meant that by 1986 there were 2.5 billion pounds of CFC being liberated to the atmosphere. This was equivalent to 1 / 2. lb per person on the planet. Since the ozone layer provides the vital protection to life on the Earth's surface from high energy UV radiation the release of CFC (along with other chemicals) caused a dramatic change in the ozone layer, including the increase in the polar hole in the ozone layer. As a result of the EU called for a complete ban of CFCs (which was followed by other countries). In their place new chemicals with similar refrigerant properties were developed. These compounds contained C-H bonds (e.g., C 2 HCl 2 F 3 and C 2 H 3 Cl 2 F) that are readily broken in the lower atmosphere, thus limiting the transport to the stratosphere.
4 Carbonyl halides
All the carbonyl halides (X 2 C=O, X = F, Cl, Br, I) are known (Table 1). Phosgene (Cl 2 C=O) was rst synthesized by John Davy (Figure 2) in 1812 by exposing a mixture of carbon monoxide and chlorine to sunlight, (11). He named it phosgene from the Greek, phos (light) and gene (born), in reference to use of light to promote the reaction. The uoride is also prepared by the reaction of carbon monoxide with the halogen, while the bromide is prepared by the partial hydrolysis of CBr 4 with sulfuric acid.
Figure 2: British doctor and chemist John Davy FRS (1790 1868) was also the brother of the noted chemist Sir Humphry Davy.
The synthesis of isocyanates from alkyl or aryl amines illustrates the electrophilic character of phosgene and its ability to introduce the equivalent of "CO2+", (12). This reaction is conducted in the presence of a base such as pyridine that absorbs the hydrogen chloride. Phosgene may also be used to produce acyl chlorides from carboxylic acids, (13). However, thionyl chloride is more commonly and more safely used in this reaction.
4.1 Phosgene as a weapon of war
Phosgene is a toxic gas with the smell of new-mown hay and was used in chemical warfare during the First World War (Figure 3) where it was a more potent weapon than chlorine. While chlorine was potentially
Table 5: Casualties from gas attacks during the First World War (including chlorine, phosgene, and mustard gas). British Empire includes troops from United Kingdom, Australia, Canada, India, New Zealand, and South Africa.
