|Molar mass||48.11 g·mol−1|
|Odor||Garlic or rotten cabbage|
|Melting point||−123 °C (−189 °F; 150 K)|
|Boiling point||5.95 °C (42.71 °F; 279.10 K)|
|EU classification||F+ T N|
|R-phrases||R12, R23, R50/53|
|S-phrases||S16, S25, S33S60, S61|
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
|what is: / ?)(|
Methanethiol // (also known as methyl mercaptan) is an organic compound with the chemical formula CH
3SH (also written as CH
4S). It is a colorless gas with a distinctive putrid smell. It is a natural substance found in the blood and brain of humans and other animals as well as plant tissues. It is disposed of through animal feces. It occurs naturally in certain foods, such as some nuts and cheese. It is also one of the main chemicals responsible for bad breath and the smell of flatus. Methanethiol is classified as a thiol and is sometimes abbreviated as MeSH. It is very flammable.
Structure and properties
The molecule is tetrahedral at carbon, like methanol. It is a weak acid, with a pKa of ~10.4. This acidic property makes it reactive with basic solutions. With bleach, the compound is oxidized readily to dimethyl disulfide:
- 2CH3SH + [O] → CH3SSCH3 + H2O
Further oxidation takes the disulfide to methanesulfonic acid, the latter being odorless.
It is released as a by-product of kraft pulping in pulp mills. In kraft pulping, lignin is depolymerized with nucleophilic attack with the strongly nucleophilic hydrosulfide ion (HS−) in a highly alkaline medium. However, in a side reaction, HS− attacks methoxyl groups (ArOMe) in lignin, demethylating them to give free phenolate groups (ArO−) and releasing MeSH. Due to alkalinity, MeSH is readily deprotonated (MeSNa), and the formed MeS− ion is also a strong nucleophile, reacting further to dimethyl sulfide. The compounds remain in the black liquor and are burned in the recovery boiler, where the sulfide is recovered as sodium sulfide.
Methanethiol is released from decaying organic matter in marshes and is present in the natural gas of certain regions, in coal tar, and in some crude oils. It occurs in various plants and vegetables, such as radishes.
In surface seawater, methanethiol is the primary breakdown product of the algal metabolite dimethylsulfoniopropionate (DMSP). Marine bacteria appear to obtain most of their protein sulfur by the breakdown of DMSP and incorporation of methanethiol, despite the fact that methanethiol is present in seawater at much lower concentrations than sulfate (~0.3 nM vs. 28 mM). Bacteria in oxic and anoxic environments can also convert methanethiol to dimethyl sulfide (DMS), although most DMS in surface seawater is produced by a separate pathway. Both DMS and methanethiol can be used by certain microbes as substrates for methanogenesis in some anaerobic soils.
Methanethiol is a byproduct of the metabolism of asparagus. The ability to produce methanethiol in urine after eating asparagus was once thought to be a genetic trait. More recent research suggests that the peculiar odor is in fact produced by all humans after consuming asparagus, while the ability to detect it (methanethiol being one of many components in "asparagus pee") is in fact the genetic trait. The chemical components responsible for the change in the odor of urine show as soon as 15 minutes after eating asparagus.
- CH3OH + H2S → CH3SH + H2O
Methanethiol is mainly used to produce methionine, which is used as a dietary component in poultry and animal feed. Methanethiol is also used in the plastics industry and as a precursor in the manufacture of pesticides.
Methanethiol is also used for communication in mining operations. Releasing the substance into the ventilation system to alert mine workers of an emergency and is referred to as "releasing the pest".
A material safety data sheet (MSDS) lists methanethiol as a colorless, flammable gas with an extremely strong and repulsive smell. At very high concentrations it is highly toxic and affects the central nervous system. Its penetrating odor provides warning at dangerous concentrations. An odor threshold of 1 ppb has been reported. The United States OSHA Ceiling Limit is listed as 10 ppm.
- Sixta, H.; Potthast, A.; Krotschek, A. W., Chemical Pulping Processes. In Handbook of Pulp, Sixta, H., Ed. Wiley-VCH Verlag GmbH & Co.: Weinheim, 2006; Vol. 1, p 169 (109-510).
- Richer, Decker, Belin, Imbs, Montastruc, Giudicelli: "Odorous urine in man after asparagus", British Journal of Clinical Pharmacology, May 1989
- Lison M, Blondheim SH, Melmed RN. (1980). "A polymorphism of the ability to smell urinary metabolites of asparagus". Br Med J 281 (6256): 1676–8. doi:10.1136/bmj.281.6256.1676. PMC 1715705. PMID 7448566.
- Skinny On : Discovery Channel
- Norell, John; Louthan, Rector P. (1988). "Thiols". Kirk-Othmer Concise Encylclopedia of Chemical Technology (3rd ed.). New York: John Wiley & Sons, Inc. pp. 946–963. ISBN 978-0471801047.
- Reid, E. Emmet (1958). Organic Chemistry of Bivalent Sulfur 1. New York: Chemical Publishing Company, Inc. pp. 32–33, 38.
- NY Post article: "Mercaptan is the stinky odor added to natural gas — which is odorless — so that people can easily detect a leak. Miller said that if any one smells the cinnamon, there may be gas but there is no danger," 6/17/13.
- Devos, M; F. Patte; J. Rouault; P. Lafort; L. J. Van Gemert (1990). Standardized Human Olfactory Thresholds. Oxford: IRL Press. p. 101. ISBN 0199631468.
- Reuters News article: http://www.reuters.com/article/2014/11/16/us-usa-chemicals-death-idUSKCN0J001B20141116