Halomethanes

The halomethanes are a subgroup of the halogenated hydrocarbons which are derived from methane and in which at least one hydrogen atom has been replaced by one of the halogens fluorine , chlorine , bromine or iodine . In organic chemistry , halogenated hydrocarbons are simply abbreviated as R – X , where R stands for a hydrocarbon radical and X for a halogen atom. Depending on the number of halogen atoms in the molecule, a distinction is made between mono- , di- , tri- and tetrahalomethanes .

In theory, 69 different halomethanes are possible, four of which are monohalomethanes and 35 completely halogenated methanes. Some of the compounds are also chiral , i.e. they occur in different spatial arrangements.

Occurrence

Halomethanes are both naturally occurring, especially in marine environments, as well as artificially produced compounds. They are formed biogenically in considerable quantities by marine algae, microorganisms and fungi. In addition, it forms in forest fires and volcanic eruptions. The global emission of chloromethane and iodomethane from marine and terrestrial biomass is estimated at around 5 million tons per year and is thus far higher than the anthropogenic input. Other halomethanes also arise naturally in large quantities.

The compounds are formed in the atmosphere through the breakdown of more highly chlorinated or fluorinated halomethanes.

${\displaystyle \mathrm {CFCl_{3}+h\nu \longrightarrow CFCl_{2}+Cl} }$

Wavelength λ <226 nanometers

presentation

Halomethanes are commonly made by one of three methods. The radical chlorination of methane is suitable for the production of methane chlorides. The main problem with this method is that it also generates hydrogen chloride and mixtures of different products.

By heating chlorine and methane to 400–500 ° C, a gradual radical substitution takes place up to carbon tetrachloride :

${\displaystyle {\ce {CH_{4}\ +Cl_{2}\longrightarrow CH_{3}Cl\ +HCl}}}$

${\displaystyle {\ce {CH_{3}Cl\ +Cl_{2}\longrightarrow CH_{2}Cl_{2}\ +HCl}}}$

${\displaystyle {\ce {CH_{2}Cl_{2}\ +Cl_{2}\longrightarrow CHCl_{3}\ +HCl}}}$

${\displaystyle {\ce {CHCl_{3}\ +Cl_{2}\longrightarrow CCl_{4}\ +HCl}}}$

Methane reacts with chlorine to form hydrogen chloride, first to monochloromethane, and then to dichloromethane , trichloromethane (chloroform) and finally carbon tetrachloride.

The halogenation of methanol is used for the production of monochloromethane, monobromomethane and monoiodomethane.

${\displaystyle {\ce {CH_{3}OH\ +HX\longrightarrow CH_{3}X\ +H_{2}O}}}$

Halogen exchange mainly produces fluorinated derivatives from the chlorides. Inorganic fluorides such as mercury (II) fluoride or cobalt (III) fluoride or electrolysis in liquid hydrogen fluoride are used.

${\displaystyle {\ce {CH_{3}Cl\ +HF\longrightarrow CH_{3}F\ +HCl}}}$

${\displaystyle {\ce {CH_{2}Cl_{2}\ +HF\longrightarrow CH_{2}FCl\ +HCl}}}$

The iodomethanes can also be used as starting compounds.

${\displaystyle {\ce {CI_{4}\ +BrF_{3}\longrightarrow CBr_{2}F_{2}\ +CF_{4}}}}$

Tetrahalomethanes can also be obtained by thermal decomposition of trihaloacetyl halides or metal haloacetates in the presence of the halogens.

properties

Like methane itself, halomethanes are tetrahedral molecules. Since the halogen atoms with each other and with respect to hydrogen are very different in size and charge, the halogen methane molecules deviate depending on their composition from the perfect tetrahedral symmetry and from the properties of methane. Halogen methanes usually have a higher boiling temperature than that of methane, as their molecules interact more strongly with one another due to increasing van der Waals forces . The bond strength increases and the bond length decreases. For example, fluoro- and chloromethane are gaseous at room temperature, iodomethane is liquid, and the completely halogenated tetraiodomethane is already in solid form. Fluoromethanes are a specialty , in which the boiling point can even decrease with increasing degree of fluorination. Dispersion forces play a much smaller role here due to the poor polarizability of fluorine. They are replaced by significantly weaker dipole-dipole forces , which are caused by the high electronegativity difference between fluorine and carbon and which are strongly dependent on the molecular geometry . The boiling point of tetrafluoromethane, at −128 ° C, is below that of trifluoromethane at −82 ° C. The halomethanes are far less volatile and flammable than methane. This is mainly due to the polarizability of halogens with respect to hydrogen , which increases from top to bottom within the halogen group . In general, the reactivity is highest for iodomethanes and lowest for fluoromethanes. The vapor pressure of the compounds decreases with increasing halogenation, with fluorine having a lesser effect. The water solubility decreases with increasing halogenation, but is generally much higher than that of methane. The opposite is true for Henry's law constant .

Examples of halomethanes

ASHRAE number	formula	Name (* = chiral)	Melting temperature
R-14	CF 4	Tetrafluoromethane	−184 ° C
R-10	CCl 4	Carbon tetrachloride	−23 ° C
	CBr 4	Tetrabromomethane	88-90 ° C
	CI 4	Tetraiodomethane	168 ° C
	CBrClFI	Bromochlorofluoroiodomethane *
	CBrCl 2 F	Bromodichlorofluoromethane
	CBrCl 2 I.	Bromodichloroiodomethane
	CBrF 2 I	Bromodifluoroiodomethane
R-12	CCl 2 F 2	Dichlorodifluoromethane	−158 ° C
	CCl 2 I 2	Dichlorodiiodomethane
	CCl 2 FI	Dichlorofluoroiodomethane
	CBrCl 3	Bromotrichloromethane	−6 ° C
	CBrI 3	Bromotriiodomethane
R-12B1	CBrClF 2	Bromochlorodifluoromethane	−159 ° C
	CBrClI 2 Cl	Bromochlorodiiodomethane
	CBrFI 2 Cl	Bromofluorodiiodomethane
	CBr 2 Cl 2	Dibromodichloromethane
R-12B2	CBr 2 F 2	Dibromodifluoromethane	−140 ° C
	CBr 2 I 2	Dibromodiiodomethane
	CBr 2 ClF	Dibromochlorofluoromethane
	CBr 2 ClI	Dibromochloroiodomethane
	CBr 2 FI	Dibromofluoroiodomethane
	CClF 2 I.	Chlorodifluoroiodomethane
	CClFI 2	Chlorofluorodiiodomethane
R-13	CClF 3	Chlorotrifluoromethane	−181 ° C
	CClI 3	Chlorotriiodomethane
R-13B1	CBrF 3	Bromotrifluoromethane	−168 ° C
	CF 2 I 2	Difluorodiiodomethane
	CFI 3	Fluorotriiodomethane
	CBr 3 Cl	Tribromochloromethane
	CBr 3 F	Tribromofluoromethane	−73 ° C
	CBr 3 I.	Tribromoiodomethane
R-11	CCl 3 F	Trichlorofluoromethane	−111 ° C
	CCl 3 I.	Trichloroiodomethane
	CF 3 I.	Trifluoroiodomethane	<−78 ° C
	CHBr 3	Bromoform	9 ° C
	CHBr 2 Cl	Dibromochloromethane	−22 ° C
	CHBr 2 F	Dibromofluoromethane	−78 ° C
	CHBr 2 I.	Dibromoiodomethane
R-20	CHCl 3	chloroform	−63 ° C
R-21	CHCl 2 F	Dichlorofluoromethane	−135 ° C
	CHCl 2 I.	Dichloroiodomethane
R-22	CHClF 2	Chlorodifluoromethane	−157 ° C
	CHClI 2	Chlorodiiodomethane
	CHClFI	Chlorofluoroiodomethane *
	CHBrCl 2	Bromodichloromethane	−57 ° C
R-22B1	CHBrF 2	Bromodifluoromethane	−145 ° C
	CHBrI 2	Bromodiiodomethane
	CHBrClF	Bromochlorofluoromethane *
	CHBrClI	Bromochloroiodomethane *
	CHBrFI	Bromofluoroiodomethane *
	CHF 2 I.	Difluoroiodomethane
	CHFI 2	Fluorodiiodomethane
R-23	CHF 3	Fluoroform	−155 ° C
	CHI 3	Iodoform	123 ° C
	CH 2 I 2	Diiodomethane	6 ° C
	CH 2 Br 2	Dibromomethane	−52 ° C
R-30	CH 2 Cl 2	Dichloromethane	−96 ° C
R-31	CH 2 ClF	Chlorofluoromethane	−133 ° C
	CH 2 ClI	Chloroiodomethane
R-32	CH 2 F 2	Difluoromethane	−136 ° C
	CH 2 BrCl	Bromochloromethane	−86 ° C
	CH 2 BrF	Bromofluoromethane
	CH 2 BrI	Bromiodomethane
	CH 2 FI	Fluoroiodomethane
	CH 3 I.	Iodomethane	−66 ° C
	CH 3 Br	Bromomethane	−93 ° C
R-40	CH 3 Cl	Chloromethane	−97 ° C
R-41	CH 3 F	Fluoromethane	−137 ° C

use

Halogen methanes are mainly used as refrigerants , solvents , propellants , fumigants and as intermediates in the production of other chemical compounds.

For example, dichloromethane and trichloromethane ( chloroform ) are used to extract vegetable oils, decaffeinate and degrease metal parts. In the past, chloroform was also used as an anesthetic, carbon tetrachloride as a solvent and earlier also as a fire extinguishing agent. Trichlorofluoromethane , CFC (CCl ₃ F) serve as refrigerants and propellants .

Some other halomethanes have also been used as refrigerants. Because of the ozone-depleting effect, it was decided in 1987 in the Montreal Protocol with the participation of around 70 nations to phase out the manufacture and use of CFCs and later incorporated into national regulations. In Germany, this was transposed into national law by the Federal Cabinet's “CFC-Halon Prohibition Ordinance” ( halon = halogenated hydrocarbon, which contains fluorine or chlorine as well as bromine) of May 30, 1990.

safety instructions

Halomethanes are quickly absorbed by the body after inhalation or ingestion. Because they can cross the blood-brain barrier , acute poisoning often has numbing effects. Some halomethanes multiply the toxicity of other compounds such as the insecticide chlordecone . Only a few are acutely toxic, but many carry risks with prolonged exposure. Some problematic aspects include carcinogenicity (e.g. iodomethane) and liver damage (e.g. carbon tetrachloride).

Individual evidence

1. ↑ Parthasarathy Namhi: Counting halomethanes. In: Journal of Chemical Education . 64, 1987, p. 678, doi : 10.1021 / ed064p678 .
2. ^ Igor Novak, Dong Bo Li, Anthony W. Potts: Electronic Structure of Chiral Halomethanes. In: Journal of Physical Chemistry A . 106, 2002, p. 465, doi : 10.1021 / jp0116959 .
3. ↑ Rainer Koch: Environmental chemicals, physical-chemical data, toxicities, limit and reference values, environmental behavior . John Wiley & Sons, 2008, ISBN 3-527-62481-3 , pp. 266 ( limited preview in Google Book search).
4. ↑ Anders Grimvall, Ed WB de Leer: Naturally-Produced Organohalogens . Springer Science & Business Media, 1995, ISBN 978-0-7923-3435-4 , pp. 30 ( limited preview in Google Book search).
5. ^ Theory of Planetary Atmospheres An Introduction to Their Physics and Chemistry . Academic Press, 1990, ISBN 0-08-096313-7 , pp. 139 ( limited preview in Google Book search).
6. ↑ ^{a b} Manfred Rossberg, Wilhelm Lendle, Gerhard Pfleiderer, Adolf Tögel, Eberhard Ludwig Dreher, Ernst Langer, Heinz Rassaerts, Peter Kleinschmidt, Heinz Strack, Richard Cook, Uwe Beck, Karl-August Lipper, Theodore R. Torkelson, Eckhard solver , Klaus K. Beutel, Trevor Mann: Chlorinated Hydrocarbons ; in: Ullmann's Encyclopedia of Industrial Chemistry 2006, Wiley-VCH, Weinheim. doi : 10.1002 / 14356007.a06_233.pub2 .
7. ^ ^{A b c} Bernhard Schrader, Paul Rademacher: Short textbook of organic chemistry . Walter de Gruyter, 2009, ISBN 978-3-11-020360-8 , p. 116 ( limited preview in Google Book search).
8. ↑ ^{a b c} Thomas L. Gilchrist: Comprehensive Organic Functional Group Transformations . Elsevier, 1995, ISBN 0-08-042704-9 , pp. 212 ( limited preview in Google Book search).
9. ↑ S. Roszak, WS Koski, JJ Kaufman, K. Balasubramanian: Structures and electron attachment properties of halomethanes (CX (n) Y (m), X = H, F; Y = Cl, Br, I; n = 0, 4; m = 4-n). In: SAR QSAR Environ Res . 11, 2001, pp. 383-396, PMID 11328711 .
10. ^ Günter Siegemund, Werner Schwertfeger, Andrew Feiring, Bruce Smart, Fred Behr, Herward Vogel, Blaine McKusick: Fluorine Compounds, Organic ; in: Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2002. doi : 10.1002 / 14356007.a11_349
11. ↑ John E. McMurry: Organic Chemistry . Cengage Learning, 2015, ISBN 1-305-68646-2 , pp. 289 ( limited preview in Google Book search).
12. ↑ Joachim Buddrus: Fundamentals of organic chemistry . 4th edition. Gruyter, 2011, ISBN 978-3-11-024894-4 , pp. 248 ( full text / preview in Google book search).
13. ↑ William H. Brown, Brent L. Iverson, Eric Anslyn, Christopher S. Foote: Organic Chemistry . Cengage Learning, 2013, ISBN 978-1-133-95284-8 , pp. 308 ( limited preview in Google Book search).
14. ↑ Alasdair H. Neilson: Organic Bromine and Iodine Compounds . Springer Science & Business Media, 2003, ISBN 978-3-540-02777-5 , pp. 308 ( limited preview in Google Book search).
15. ↑ Manfred Sietz: Environmental protection, product quality and corporate success From eco-audit to eco-balance . Springer-Verlag, 2013, ISBN 978-3-642-93580-0 , pp. 293 ( limited preview in Google Book search).
16. ^ Raymond SH Yang: Toxicology of Chemical Mixtures Case Studies, Mechanisms, and Novel Approaches . Elsevier, 2013, ISBN 978-1-4832-8940-3 , pp. 301 ( limited preview in Google Book search).
17. ↑ LW Condie, CL Smallwood, RD Laurie: Comparative renal and hepatotoxicity of halomethanes: bromodichloromethane, bromoform, chloroform, dibromochloromethane and methylene chloride. In: Drug Chem Toxicol . 6, 1983, pp. 563-578, PMID 6653442 .
18. ^ KP Cantor, R. Hoover, TJ Mason, LJ McCabe: Associations of cancer mortality with halomethanes in drinking water. In: J. Natl. Cancer Inst. 61, 1978, pp. 979-985, PMID 702538 .