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| Names | |||
|---|---|---|---|
| Preferred IUPAC name Methanethiol | |||
| Other names Methyl mercaptan Mercaptomethane Methiol Thiomethyl alcohol/Thiomethanol Methylthiol | |||
| Identifiers | |||
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3D model (JSmol) | |||
| ChEBI | |||
| ChemSpider |
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| ECHA InfoCard | 100.000.748 | ||
| EC Number |
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| KEGG |
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| RTECS number |
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| UNII | |||
| UN number | 1064 | ||
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| Properties | |||
| CH3SH | |||
| Molar mass | 48.11 g·mol−1 | ||
| Appearance | colorless gas[1] | ||
| Odor | Distinctive, like that of rotten cabbage or eggs | ||
| 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 | |||
| GHS labelling: | |||
    | |||
| Danger | |||
| H220,H331,H410 | |||
| P210,P261,P271,P273,P304+P340,P311,P321,P377,P381,P391,P403,P403+P233,P405,P501 | |||
| NFPA 704 (fire diamond) | |||
| Flash point | −18 °C; 0 °F; 255 K[1] | ||
| 364 °C; 687 °F; 637 K[3] | |||
| Explosive limits | 3.9%–21.8%[1] | ||
| Lethal dose or concentration (LD, LC): | |||
LD50 (median dose) | 60.67 mg/kg (mammal)[2] | ||
LC50 (median concentration) | 3.3 ppm (mouse, 2 hr) 675 ppm (rat, 4 hr)[2] | ||
| NIOSH (US health exposure limits): | |||
PEL (Permissible) | C 10 ppm (20 mg/m3)[1] | ||
REL (Recommended) | C 0.5 ppm (1 mg/m3) [15-minute][1] | ||
IDLH (Immediate danger) | 150 ppm[1] | ||
| Related compounds | |||
Related compounds | Ethanethiol | ||
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
Methanethiol (/ˌmɛθeɪnˈθaɪ.ɒl/METH-ayn-THY-ol), also calledmethyl mercaptan, is anorganosulfur compound with thechemical formulaCH3SH. It is a colorlessflammable gas with a distinctive putrid smell. In small amounts, it is pervasive in nature and found in certain foods, such as somenuts andcheese. It contributes to many odors, including the emissions frompulp mills,bad breath, andflatus. Methanethiol is the simplestthiol and is sometimes abbreviated asMeSH.
The molecule is tetrahedral at the carbon atom, likemethanol. It is aweak acid, with apKa of ~10.4, but is about a hundred thousand times more acidic than methanol. The colorless salt can be obtained by treatment withsodium methoxide:
The thiolate anion insodium methanethiolate is a strongnucleophile.
It can be methylated todimethyl disulfide:
Further oxidation takes thedisulfide to two molecules ofmethanesulfonic acid, which is odorless.Bleach deodorizes methanethiol in this way.
Methanethiol (MeSH) is released as a by-product ofkraft pulping inpulp mills. In kraft pulping,lignin isdepolymerized 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) inlignin, demethylating them to give freephenolate groups (PhO−) and releasing MeSH. Due to alkalinity, MeSH is readily deprotonated (MeSNa), and the formed MeS− ion is also a strong nucleophile, reacting further todimethyl sulfide. The compounds remain in the liquor and are burned in therecovery boiler, where the sulfur is recovered assodium sulfide.[4]
Methanethiol is released from decaying organic matter inmarshes and is present in thenatural gas of certain regions, incoal tar, and in somecrude oils. It occurs in various plants and vegetables, such as radishes.
In surfaceseawater, methanethiol is the primary breakdown product of thealgalmetabolitedimethylsulfoniopropionate (DMSP).Marine bacteria appear to obtain most of the sulfur in their proteins 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).[5] Bacteria in environments both with and without oxygen can also convert methanethiol todimethyl sulfide (DMS), although most DMS in surface seawater is produced by a separate pathway.[6] Both DMS and methanethiol can be used by certain microbes as substrates formethanogenesis in some anaerobic soils.
Methanethiol is a byproduct ofasparagus metabolism.[7] The production of methanethiol inurine after eating asparagus was once thought to be agenetic 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.[8] The chemical components responsible for the change in the odor of urine show as soon as 15 minutes after eating asparagus.[9]
Methanethiol is prepared commercially by the reaction ofmethanol withhydrogen sulfide gas over analuminium oxide catalyst:[10]
Although impractical, it can be prepared by the reaction ofmethyl iodide withthiourea.[11]
Methanethiol is mainly used to produce the essential amino acidmethionine, which is used as a dietary component in poultry and animal feed.[10] Methanethiol is also used in theplastic industry as a moderator forfree-radical polymerizations[10] and as a precursor in the manufacture ofpesticides includingisomalathion.
This chemical is also used in thenatural gas industry as anodorant, as it mixes well withmethane. The characteristic rotting vegetation smell of the mix is widely known by natural gas customers as an indicator of a possiblegas leak, even a minor one.[12]
Thesafety 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.[13] The United StatesOSHA Ceiling Limit is listed as 10 ppm.
In 2001 a rail car fire of 25,000 US gallons (95,000 L) nearTrenton, Michigan left three people dead and nine injured.[14]
On November 15, 2014, atDuPont'sLa Porte, Texas facility, 24,000 lb (11,000 kg) of methyl mercaptan were released and travelled downwind into surrounding areas, killing four and injuring one other.[15][16] In 2023, DuPont pleaded guilty to criminal negligence for its role in the leak. The company was ordered to pay a $12 million fine and donate an additional $4 million to theNational Fish and Wildlife Foundation.[17][18]
On July 14, 2022, an accidental release inCharlotte, North Carolina led to the temporary closure of local government offices.[19]
On April 10, 2024, an accidental release[20] of a higher-than expected level of methyl mercaptan into the natural gas supply was attributed to an "upstream supplier" forColumbia Gas. This release was noticed by residents in at leastRichland,Ashland, andLorain counties in Ohio. Numerous schools cancelled their school days and numerous evacuations took place.
