Chloromethane A natural ozone destroyer Simon Cotton University of Birmingham Molecule of the Month July 2024 Also available: JSMol version.

Not another man-made environmental disaster?

Wrong in more than one way.

How so, surely it is like CFCs?

CFCs, such as CF₂Cl₂ (MOTM June 2005 and CFCl₃ MOTM June 2019) all contain fluorine, whereas chloromethane doesn't. Secondly, it is much more reactive than CFCs, so doesn’t hang about in the atmosphere for so long.

OK, so it is like DDT?

No, it is not an insecticide either.

Give me some facts about it, then.

Firstly, chloromethane is simply a methane molecule where one of the hydrogens has been replaced by a chlorine. Most chloromethane is released into the atmosphere from natural sources.

Chloromethane a.k.a. methyl chloride

I thought that all organic chlorine compounds are man-made?

Not so, but it is only recently that their number has been appreciated. Less than a dozen natural organic compounds containing chlorine were known in 1954 – such as chlorotetracycline (see MOTM September 2011), but this number has now risen to over 2500, as scientists have found better ways of detecting them.

The most abundant of these compounds is CH₃Cl, chloromethane, a gas at normal temperatures (its b.p. is -24°C). The great eco-guru James Lovelock detected CH₃Cl in the air over southern England in 1974-1975. It has been estimated that 5 × 10⁶ tonnes of CH₃Cl are released globally into the atmosphere each year, with over 90%, maybe even 99%, being from natural sources.

What natural sources?

Well, until around the mid-1990s, it was assumed that CH₃Cl came primarily from warm oceans. But now it is known that the oceans contribute at most 11% of the world’s chloromethane, and that there are many terrestrial sources.

Namely?

Biomass burning, for a start, forest and brush fires. It is reckoned that there are 200,000 lightning-triggered fires across the world each year. Of course, there are fires created by human activity, but they contribute much less. Wood-rotting fungi also produce chloromethane. Then there are salt marshes, mangroves and peat bogs worldwide, major emitters of CH₃Cl. For example, a halophytic plant that grows abundantly in salt marshes called Batis maritima (pickleweed) produces chloromethane. And it is calculated that tropical plants may emit around half the chloromethane produced worldwide.

Biomass burning [Image: Ramos Keith, U.S. Fish and Wildlife Service, Public domain, via Wikimedia Commons]	Wood-rotting fungi [Image: André Lage Freitas, CC BY-SA 3.0 via Wikimedia Commons]
Pickleweed (Batis maritima) [Image: Forest & Kim Starr, CC BY 3.0 US via Wikimedia Commons]	A peat bog [Image: Peat Bog on Yockenthwaite Moor by John H Darch, CC BY-SA 2.0 via Wikimedia Commons]

Do we know the chemical reactions involved?

People are still working these out. One important source of CH₃Cl at the molecular level is believed to be pectin. This is a biopolymer, a polysaccharide, that is widely found in plant cell walls, and is thought to supply methyl groups, using a S_N2 reaction between chloride ions and a methoxy group in pectin – and also lignin.

Pectin

Are there other sources of chloromethane? One perhaps unexpected source of CH3Cl is volcanic eruptions; it was first noted among emissions in the 1969 eruption at Santiaguito in Guatemala. Since then it has been detected in many others, such as the 1980 eruption of Mount St. Helens, in the state of Washington, USA, which also released CH3Br and CH3I; and more recently at Etna, in Italy. But their contribution is negligible overall.

Mount St Helens in 1982 [Image: Lyn Topinka, Public domain via Wikimedia Commons]

comet 67P/Churyumov–Gerasimenko (67P/C-G) in 2021 [Image: NASA/JPL-Caltech/ZTF, Public domain, via Wikimedia Commons]

And? Then there is chloromethane in outer space. Really? Chloromethane was the first organochlorine compound to be detected in outer space – as both isotopologues (CH335Cl and CH337Cl) - in the coma of comet 67P/Churyumov–Gerasimenko (67P/C-G), and also, some four hundred light years away, within several young star clusters.

OK, what does CH₃Cl do towards breaking up the ozone layer?

The atmosphere contains CH₃Cl at a concentration of some 0.6 ppb, so it contributes far more chlorine than CFCs like CF₂Cl₂. On the other hand, the greater reactivity of CH₃Cl ensures a shorter atmospheric residence time (1.4 years compared with 100 years for CF₂Cl₂). About 20 years ago it was estimated that chloromethane contributed around 15% of ozone-depleting emissions; this proportion is bound to increase as anthropogenic emissions of CFCs decrease.

Chloromethane acts as an ozone depleter in a similar way to CFCs, by acting as a source of chlorine radicals, which catalyse the decomposition of ozone.

CH₃Cl •CH₃ + •Cl

•Cl + O₃ •ClO + O₂

•ClO + O •Cl + O₂

The process regenerates chlorine atoms (radicals), so one chlorine atom can destroy thousands of ozone molecules.

How about global warming?

Chloromethane is not believed to be a significant ‘greenhouse gas’.

Well, that is one bit of good news. So – assuming that you want to - how do you make chloromethane?

One traditional way, first reported by M. Berthelot in 1858, is by the reaction of methane with chlorine, in the presence of ultraviolet light, which generates reactive chlorine atoms by homolytic fission of the Cl-Cl bond. The resulting chlorine atom abstracts a hydrogen atom from a methane molecule, generating a methyl radical (•CH₃), which in its, turn abstracts a chlorine atom from a chlorine molecule. This forms a molecule of chloromethane and in its turn regenerates a chlorine radical, so the cycle can continue.

Overall: CH₄ + Cl₂ CH₃Cl + HCl

Synthesis of CH₃Cl

Apart from the use of elemental chlorine, a very real disadvantage of this method is that further substitution can follow, with the additional formation of CH₂Cl₂, CHCl₃ and CCl₄, causing the need for purification of the product.

But there are better ways, e.g. from methanol by reaction with HCl, possibly generated in situ from the reaction of NaCl with conc. sulfuric acid. This is how chloromethane was originally made in 1835, by pioneering French organic chemists Dumas and Peligot.

Jean Baptiste André Dumas [Image: Public domain via Wikimedia Commons]	Eugène Peligot [Image: Gaspard Félix Tournachon (1820-1910), Public domain, via Wikimedia Commons]

The mechanism involves the protonation of the –OH group, which creates an incipient water group, a much better ‘leaving group’ than hydroxide, OH^-. The intermediate cationic species can then undergo nucleophilic attack by a chloride ion, displacing the water molecule and forming chloromethane.

Alternative synthesis of CH₃Cl

One last thing, does chloromethane have any uses?

It used to be used in refrigerators, as it had similar properties to Freons, but no longer. Another former use was in making the anti-knock agent Pb(CH₃)₄ (similar to Pb(C₂H₅)₄ MOTM Jan 2001), as a fuel additive – again, no more. It’s used to synthesise methylchlorosilanes, such as (CH₃)₂SiCl₂, important in making silicone polymers.

Silicone soup ladles [Image: GOttawaAC, CC BY-SA 3.0 via Wikimedia Commons]	Flexible ice cube trays made of silicone [Image: Gmhofmann, CC BY-SA 3.0 via Wikimedia Commons]	Silicone brush used for basting liquids when cooking [Image: Evan-Amos, Public domain via Wikimedia Commons]

It is also used in methylating –OH groups, for example in making methylcellulose; and in organic synthesis for a range of substances like methanethiol CH₃SH (MOTM May 2017) and methylamines - CH₃NH₂, (CH₃)₂NH and (CH₃)₃N (MOTM August 2004). So it is useful in industrial synthesis, but the industrial production, around a million tons a year, is well below the natural level.

Bibliography

• Chapman and Hall Combined Chemical Dictionary code number: DCL77-X
• J-B. Dumas and E. M. Peligot, Ann. Pharm. 1835, 15, 20 (first synth.)
• J. F. Norris and H. B. Taylor, J. Am. Chem. Soc., 1924, 46, 753- 757 (synth.)
• M. Rossberg, W. Lendle, G. Pfleiderer, A. Tögel, T. R. Torkelson and K.K. Beutel, ‘Chloromethanes’, Ullmann’s Encyclopedia of Industrial Chemistry, 2012, 9, 15-42.
• V. P. Yandrapu and N. R., Kanidarapu, Periodica Polytechnica Chem. Engin., 2022, 66, 341–353 (industrial synth.)

General

• G. W. Gribble (ed.), Natural Production of Organohalogen Compounds, The Handbook of Environmental Chemistry, Volume 3 Part P, Berlin, Springer 2003.
• R. M. Moore in Gribble (ed.), 2003, pp. 85-102. (Marine sources of volatile organohalogens)
• G. W. Gribble, The diversity of naturally occurring organohalogen compounds. Chemosphere, 2003, 52, 289-297.
• G. W. Gribble, Naturally Occurring Organohalogen Compounds, in: Progress in the Chemistry of Organic Natural Products Vol 121, Switzerland, Springer, 2023.

CH₃Cl in the atmosphere

• J. E. Lovelock, Nature, 1975, 256, 193-1974 (detection of CH₃Cl in air over England)

Natural sources of CH₃Cl - Wood rotting fungi

• R. Watling and D. B. Harper, Mycol. Res., 1998, 102, 769–787.
• H. Anke and R. W. S. Weber, Mycologist, 2006, 20, 83-89

Biomass burning

• M. O. Andreae and P. Merlet, Global Biogeochem. Cycles, 2001, 15, 955–966.
• M. A. Cochrane, Nature, 2003, 421, 913-919.

Coastal salt marshes

• X. Ni and L. P. Hager, Proc. Natl. Acad. Sci. USA, 1998, 95, 12866–12871 (CH₃Cl from Batis maritima, a halophytic plant that grows abundantly in salt marshes)
• R. C. Rhew, B. R. Miller and R. F. Weiss, Nature, 2000, 403, 292–295.

Tropical vegetation

• Y. Yokouchi, M. Ikeda, Y. Inuzuka and T. Yukawa, Nature, 2002, 416, 163–165.

Volcanoes

• R. E. Stoiber, D. C. Leggett, T. F Jenkins, R. P. Murrmann and W. I. Rose, Geological Society of America Bulletin, 1971, 82, 2299-2302 (volcanic gas from Santiaguito, Guatemala 1969)
• E. C. Y. Inn, J. F. Vedder, E. P. Concon and D. O’Hara, Science, 1981, 211, 821-823 (Mount St. Helens)
• R. A. Rasmussen, M.A.K. Khalil, R.W. Dalluge, S.A. Penkett and B. Jones, Science, 1982, 215, 665-667 (Mount St Helens).
• F. Tassi, F. Capecchiacci, J. Cabassi, S. Calabrese, O. Vaselli, D. Rouwet , G. Pecoraino and G. Chiodini, J. Geophys. Res., 2012, 117, D17305 (Mount Etna)

Space

• E. C. Fayolle et al., Nature Astronomy, 2017, 1, 703–708 (CH₃Cl in interstellar space and in the coma of comet 67P/Churyumov–Gerasimenko (67P/C-G))

Pectin as a molecular source of chloromethane

• B. R. Thakur, R. K. Singh, A. K. Hand and M. A. Rao, Critical Reviews in Food Science and Nutrition, 1997, 37, 47-73 (chemistry and uses of pectin reviewed)
• J. T. G. Hamilton, W. C. McRoberts, F. Keppler, R. M. Kalin and D. B. Harper, Science, 2003, 301, 206-209. (chloride methylation by plant pectin)
• Y. Sailaukhanuly, Z. Sárossy, L. Carlsen and H. Egsgaard, Chemosphere, 2014, 111, 575–579 (formation of chloromethane by nucleophilic substitution of pectin)
• W. C. McRoberts, F. Keppler, D. B. Harper and J. T. G. Hamilton, Environ. Chem., 2015, 12, 426.

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