Methanethiol

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Methanethiol
Methanethiol
Ball-and-stick model of the methanethiol molecule Space-filling model of the methanethiol molecule
Identifiers
CAS number 74-93-1 YesY
ChemSpider 855 YesY
UNII 2X8406WW9I YesY
KEGG C00409 YesY
ChEBI CHEBI:16007 YesY
Jmol-3D images Image 1
Properties
Molecular formula CH4S
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)
Solubility in water 2%
Solubility alcohol, ether
Acidity (pKa) ~10.4
Hazards
EU classification Extremely Flammable F+ Toxic T Dangerous for the Environment (Nature) N
R-phrases R12, R23, R50/53
S-phrases S16, S25, S33S60, S61
NFPA 704
Flammability code 4: Will rapidly or completely vaporize at normal atmospheric pressure and temperature, or is readily dispersed in air and will burn readily. Flash point below 23 °C (73 °F). E.g., propane Health code 4: Very short exposure could cause death or major residual injury. E.g., VX gas Reactivity code 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g., calcium Special hazards (white): no codeNFPA 704 four-colored diamond
Related compounds
Related compounds Ethanethiol
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Methanethiol /ˈmɛθnˈθɒl/ (also known as methyl mercaptan) is an organic compound with the chemical formula CH
3
SH
(also written as CH
4
S
). 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. The chemical formula for methanethiol is CH3SH; it is classified as a thiol. It is sometimes abbreviated as MeSH. It is very flammable.

Structure and properties[edit]

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:

CH3SH + [O] → CH3SSCH3 + H2O

Further oxidation takes the disulfide to methanesulfonic acid, the latter being odorless.

Occurrence[edit]

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.[1]

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.[2] 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.[3] The chemical components responsible for the change in the odor of urine show as soon as 15 minutes after eating asparagus.[4]

Preparation[edit]

Methanethiol is prepared commercially by the reaction of methanol with hydrogen sulfide gas over an acidic solid catalyst, such as alumina:[5]

CH3OH + H2S → CH3SH + H2O

Although impractical, it can be prepared by the reaction of methyl iodide with thiourea.[6]

Uses[edit]

Cylinder of methanethiol gas

Methanethiol is mainly used to produce methionine, which is used as a dietary component in poultry and animal feed.[5] Methanethiol is also used in the plastics industry[citation needed] 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"[citation needed].

Since natural gas and propane are colorless and odorless, a small amount of methyl mercaptan or ethyl mercaptan is added to make it easy to detect a gas leak.[7]

Safety[edit]

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.[8] The United States OSHA Ceiling Limit is listed as 10 ppm.

References[edit]

  1. ^ 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).
  2. ^ Richer, Decker, Belin, Imbs, Montastruc, Giudicelli: "Odorous urine in man after asparagus", British Journal of Clinical Pharmacology, May 1989
  3. ^ Lison M, Blondheim SH, Melmed RN. (1980). "A polymorphism of the ability to smell urinary metabolites of asparagus". Br Med J 281 (6256): 1676. doi:10.1136/bmj.281.6256.1676. PMC 1715705. PMID 7448566. 
  4. ^ Skinny On : Discovery Channel
  5. ^ a b 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. 
  6. ^ Reid, E. Emmet (1958). Organic Chemistry of Bivalent Sulfur 1. New York: Chemical Publishing Company, Inc. pp. 32–33,38. 
  7. ^ 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.
  8. ^ Devos, M; F. Patte, J. Rouault, P. Lafort, L. J. Van Gemert (1990). Standardized Human Olfactory Thresholds. Oxford: IRL Press. p. 101. ISBN 0199631468. 

External links[edit]