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Methanethiol

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Methanethiol

Methanethiol
Methanethiol
Ball-and-stick model of the methanethiol molecule
Space-filling model of the methanethiol molecule
Names
IUPAC name
Methanethiol
Other names
methyl mercaptan
mercaptomethane
thiomethyl alcohol
Identifiers
 Y
ChEBI  Y
ChemSpider  Y
Jmol-3D images Image
KEGG  Y
UNII  Y
Properties
CH4S
Molar mass 48.11 g·mol−1
Appearance colorless gas[1]
Odor Rotten cabbage, flatulence
Density 0.9 g/mL (liquid at 0°C)[1]
Melting point −123 °C (−189 °F; 150 K)
Boiling point 5.95 °C (42.71 °F; 279.10 K)
2%
Solubility alcohol, ether
Vapor pressure 1.7 atm (20°C)[1]
Acidity (pKa) ~10.4
Hazards
Extremely Flammable F+ Toxic T Dangerous for the Environment (Nature) N
R-phrases R50/53
S-phrases S16, S25, S33S60, S61
NFPA 704
4
4
1
Flash point −18 °C; 0 °F; 255 K [1]
Explosive limits 3.9%-21.8%[1]
Lethal dose or concentration (LD, LC):
LD50 (Median dose)
60.67 mg/kg (mammal)[2]
3.3 ppm (mouse, 2 hr)
675 ppm (rat, 4 hr)[2]
US health exposure limits (NIOSH):
PEL (Permissible)
C 10 ppm (20 mg/m3)[1]
REL (Recommended)
C 0.5 ppm (1 mg/m3) [15-minute][1]
150 ppm[1]
Related compounds
Related compounds
Ethanethiol
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
 Y  (: Y/N?)

Methanethiol (also known as methyl chemical formula CH
3
SH
. 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 compounds 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.

Contents

  • Structure and reactions 1
  • Occurrence 2
  • Preparation 3
  • Uses 4
  • Safety 5
  • References 6
  • External links 7

Structure and reactions

The molecule is tetrahedral at carbon, like methanol. It is a weak acid, with a pKa of ~10.4, but is about a million times more acidic than methanol. The colorless salt can be obtained in this way:

CH3SH + CH3ONa → CH3SNa + CH3OH

The resulting thiolate anion is a strong nucleophile.

It can be oxidized to dimethyl disulfide:

2CH3SH + [O] → CH3SSCH3 + H2O

Further oxidation takes the disulfide to methanesulfonic acid, the latter being odorless. Bleach deodorizes methanethiol in this way.

Occurrence

It is released as a by-product of kraft pulping in pulp mills. In kraft pulping, lignin is depolymerized by nucleophilic attack with the strongly nucleophilic hydrosulfide ion (HS) in a highly alkaline medium. However, in a side reaction, HS attacks methoxyl groups (OMe) 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.[3]

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.[4] 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.[5] The chemical components responsible for the change in the odor of urine show as soon as 15 minutes after eating asparagus.[6]

Preparation

Methanethiol is prepared commercially by the reaction of methanol with hydrogen sulfide gas over an acidic solid catalyst, such as alumina:[7]

CH3OH + H2S → CH3SH + H2O

Although impractical, it can be prepared by the reaction of methyl iodide with thiourea.[8]

Uses

Cylinder of methanethiol gas

Methanethiol is mainly used to produce methionine, which is used as a dietary component in poultry and animal feed.[7] Methanethiol is also used in the plastics industry as a moderator for free-radical polymerizations[7] 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".

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.[9]

Safety

A safety data sheet (SDS) 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.[10] The United States OSHA Ceiling Limit is listed as 10 ppm.

Methanethiol has been implicated in the accidental death of four employees and the hospitalization of a fifth at a DuPont facility in La Porte, Texas, near Houston, on 15 November 2014.[11]

References

  1. ^ a b c d e f g h "NIOSH Pocket Guide to Chemical Hazards #0425".  
  2. ^ a b "Methyl mercaptan". Immediately Dangerous to Life and Health.  
  3. ^ 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).
  4. ^ Richer, Decker, Belin, Imbs, Montastruc, Giudicelli: "Odorous urine in man after asparagus", British Journal of Clinical Pharmacology, May 1989
  5. ^ 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.  
  6. ^ Skinny On : Discovery Channel
  7. ^ a b c 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.  
  8. ^ Reid, E. Emmet (1958). Organic Chemistry of Bivalent Sulfur 1. New York: Chemical Publishing Company, Inc. pp. 32–33, 38. 
  9. ^ 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.
  10. ^ Devos, M; F. Patte; J. Rouault; P. Lafort; L. J. Van Gemert (1990). Standardized Human Olfactory Thresholds. Oxford: IRL Press. p. 101.  
  11. ^ Reuters News article: http://www.reuters.com/article/2014/11/16/us-usa-chemicals-death-idUSKCN0J001B20141116

External links

  • NLM Hazardous Substances Databank – Methyl mercaptan
  • CDC - NIOSH Pocket Guide to Chemical Hazards
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