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Methanethiol
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| Names | |||
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| Preferred IUPAC name
Methanethiol | |||
| Other names
Methyl mercaptan
Mercaptomethane Methiol Thiomethyl alcohol/Thiomethanol Methylthiol | |||
| Identifiers | |||
3D model (JSmol)
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| ChEBI | |||
| ChemSpider | |||
| ECHA InfoCard | 100.000.748 | ||
| EC Number |
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| KEGG | |||
PubChem CID
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| RTECS number |
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| UNII | |||
| UN number | 1064 | ||
CompTox Dashboard (EPA)
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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: | |||
   
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| 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)
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60.67 mg/kg (mammal)[2] | ||
LC50 (median concentration)
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3.3 ppm (mouse, 2 hr) 675 ppm (rat, 4 hr)[2] | ||
| NIOSH (US health exposure limits): | |||
PEL (Permissible)
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C 10 ppm (20 mg/m3)[1] | ||
REL (Recommended)
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C 0.5 ppm (1 mg/m3) [15-minute][1] | ||
IDLH (Immediate danger)
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150 ppm[1] | ||
| Related compounds | |||
Related compounds
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Ethanethiol | ||
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Methanethiol (/ˌmɛθeɪnˈθaɪ.ɒl/ METH-ayn-THY-ol), also called methyl mercaptan, is an organosulfur compound with the chemical formula CH3SH. It is a colorless flammable gas with a distinctive putrid smell. In small amounts, it is pervasive in nature and found in certain foods, such as some nuts and cheese. It contributes to many odors, including the emissions from pulp mills, bad breath, and flatus. Methanethiol is the simplest thiol and is sometimes abbreviated as MeSH.
Structure and reactions
[edit]The molecule is tetrahedral at the carbon atom, like methanol. It is a weak acid, with a pKa of ~10.4, but is about a hundred thousand times more acidic than methanol. The colorless salt can be obtained by treatment with sodium methoxide:
- CH3SH + CH3ONa → CH3SNa + CH3OH
The thiolate anion in sodium methanethiolate is a strong nucleophile.
It can be methylated to dimethyl disulfide:
- 2 CH3SH + [O] → CH3SSCH3 + H2O
Further oxidation takes the disulfide to two molecules of methanesulfonic acid, which is odorless. Bleach deodorizes methanethiol in this way.
Occurrence
[edit]Methanethiol (MeSH) 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 (PhO−) 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 liquor and are burned in the recovery boiler, where the sulfur is recovered as sodium sulfide.[4]
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 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 to dimethyl 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 for methanogenesis in some anaerobic soils.
Methanethiol is a byproduct of asparagus metabolism.[7] The production of 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.[8] The chemical components responsible for the change in the odor of urine show as soon as 15 minutes after eating asparagus.[9]
Preparation
[edit]Methanethiol is prepared commercially by the reaction of methanol with hydrogen sulfide gas over an aluminium oxide catalyst:[10]
- CH3OH + H2S → CH3SH + H2O
Although impractical, it can be prepared by the reaction of methyl iodide with thiourea.[11]
Uses
[edit]
Methanethiol is mainly used to produce the essential amino acid methionine, which is used as a dietary component in poultry and animal feed.[10] Methanethiol is also used in the plastic industry as a moderator for free-radical polymerizations[10] and as a precursor in the manufacture of pesticides including isomalathion.
This chemical is also used in the natural gas industry as an odorant, as it mixes well with methane. The characteristic rotting vegetation smell of the mix is widely known by natural gas customers as an indicator of a possible gas leak, even a minor one.[12]
Safety
[edit]The 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.[13] The United States OSHA Ceiling Limit is listed as 10 ppm.
Accidents
[edit]In 2001 a rail car fire of 25,000 US gallons (95,000 L) near Trenton, Michigan left three people dead and nine injured.[14]
On November 15, 2014, at DuPont's La 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 the National Fish and Wildlife Foundation.[17][18]
On July 14, 2022, an accidental release in Charlotte, 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" for Columbia Gas. This release was noticed by residents in at least Richland, Ashland, and Lorain counties in Ohio. Numerous schools cancelled their school days and numerous evacuations took place.
References
[edit]- ^ a b c d e f g h NIOSH Pocket Guide to Chemical Hazards. "#0425". National Institute for Occupational Safety and Health (NIOSH).
- ^ a b "Methyl mercaptan". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
- ^ "Sigma Aldrich Methanethiol SDS". Sigma Aldrich. Millipore Sigma. Retrieved Nov 1, 2022.
- ^ 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).
- ^ Charel Wohl, Julián Villamayor and Martí Galí et al. ,Marine emissions of methanethiol increase aerosol cooling in the Southern Ocean.Sci. Adv.10,eadq2465(2024).DOI:10.1126/sciadv.adq2465
- ^ Kiene, R.P., and Service, S. "Decomposition of DMSP and DMS in Estuarine Waters: dependence on temperature and substrate concentration"," Marine Ecology Progress Series. September, 1991
- ^ 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 Archived 2008-02-29 at the Wayback Machine
- ^ a b c Norell, John; Louthan, Rector P. (1988). "Thiols". Kirk-Othmer Concise Encyclopedia 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. Vol. 1. New York: Chemical Publishing Company, Inc. pp. 32–33, 38.
- ^ SafeGase: About Natural Gas:
- ^ Devos, M; F. Patte; J. Rouault; P. Lafort; L. J. Van Gemert (1990). Standardized Human Olfactory Thresholds. Oxford: IRL Press. p. 101. ISBN 0199631468.
- ^ "Deadly Explosion At Chemical Plant". www.cbsnews.com. 14 July 2001. Retrieved 2022-05-25.
- ^ "DuPont La Porte Facility Toxic Chemical Release | CSB". www.csb.gov. Retrieved 2022-06-02.
- ^ US Chemical Safety Board (30 September 2015). "Animation of Chemical Release at DuPont's La Porte Facility" (Video). Youtube. Retrieved 8 October 2023.
- ^ "Southern District of Texas | DuPont and former employee sentenced for gas release that killed four | United States Department of Justice". www.justice.gov. 2023-04-24. Retrieved 2023-11-13.
- ^ "DuPont ordered to pay $16M in Texas plant leak that killed 4". FOX 5 San Diego. 2023-04-24. Retrieved 2023-04-24.
- ^ Limehouse, Jonathan (2022-07-14). "What is the strong natural gas odor smell in Charlotte NC?". Charlotte Observer. Archived from the original on 2022-07-15. Retrieved 2022-07-14.
- ^ Carr, Dillon (10 April 2024). "Mansfield-area schools back to normal following natural gas scare". Richland Source. Retrieved 2024-04-11.
External links
[edit]
Methanethiol
View on Grokipediafrom Grokipedia
Properties
Physical properties
Methanethiol, with the molecular formula CH₃SH, has a molecular weight of 48.11 g/mol. It appears as a colorless gas at room temperature and standard pressure.[1] The compound exhibits a low boiling point of 5.9 °C (42.6 °F) and a melting point of -123.8 °C (-191 °F), indicating its volatility and tendency to remain gaseous under typical ambient conditions. Its liquid density at the boiling point is 0.89 g/cm³, while its solubility in water is 2.4 g/100 mL at 20 °C, reflecting moderate hydrophilicity compared to nonpolar hydrocarbons but limited by its low molecular weight.[1][7] Methanethiol is highly odorous, detectable at an odor threshold of 0.002 ppm in air, with a characteristic smell resembling rotten cabbage or garlic that arises from its sulfur-containing structure. This sensory property is notable for its structural similarity to methanol (CH₃OH), though the thiol group imparts distinct olfactory traits. As a flammable gas, it has an autoignition temperature of 364 °C and forms explosive mixtures with air in the range of 3.9–21.8% by volume.[7][8] Thermodynamically, methanethiol in the gas phase has a standard enthalpy of formation ΔH_f° = -22.9 kJ/mol and a standard Gibbs free energy of formation ΔG_f° = -31.7 kJ/mol at 298 K, values that underscore its relative stability under standard conditions.[9]Chemical properties and reactivity
Methanethiol exhibits a tetrahedral geometry around the central carbon atom, characteristic of sp³ hybridization, with a C-S bond length of 1.818 Å and an S-H bond length of 1.333 Å. These bond lengths reflect the single-bond nature between carbon and sulfur, longer than typical C-O bonds due to the larger atomic radius of sulfur. The molecule possesses a significant dipole moment of 1.29 D (gas phase), arising from the electronegativity difference and the bent structure at sulfur, which contributes to its polarity despite similar electronegativities of carbon and sulfur.[10][11] The S-H bond in methanethiol is moderately acidic, with a pKa of approximately 10.4, rendering it a weaker acid than corresponding alcohols (pKa ≈ 15–18) but significantly stronger than alkanes (pKa ≈ 50). This acidity stems from the relatively low bond energy of S-H (≈ 365 kJ/mol) compared to O-H (≈ 460 kJ/mol), facilitating deprotonation to form the thiolate anion. The dissociation equilibrium is represented as:
[1]
A key reactivity feature is the oxidation of methanethiol to form dimethyl disulfide, a process common to thiols under mild oxidizing conditions. This occurs via the half-reaction:
The reaction is typically mediated by agents like iodine or air in the presence of catalysts, yielding the symmetric disulfide as the primary product.[12]
The deprotonated thiolate, CH₃S⁻, acts as a potent nucleophile due to the polarizable sulfur lone pairs and soft nucleophilic character, outperforming alkoxides in SN2 displacements. It readily reacts with primary or secondary alkyl halides to produce thioethers, as illustrated by:
where R is an alkyl group and X a leaving group like bromide. This high nucleophilicity makes methanethiolate valuable in organic synthesis for C-S bond formation.
Methanethiol displays notable sensitivity to oxidation, particularly in the presence of atmospheric oxygen or mild oxidants, leading to sequential formation of sulfenic (CH₃SOH), sulfinic (CH₃SO₂H), and sulfonic (CH₃SO₃H) acids depending on conditions. Prolonged exposure can result in complete mineralization to CO₂ and sulfate. Additionally, under thermal or catalytic stress, it undergoes disproportionation to dimethyl sulfide (CH₃)₂S and hydrogen sulfide (H₂S). As a gas at room temperature, this reactivity is enhanced in vapor phase, contributing to its instability in open air.[1]
Spectroscopic characterization confirms these structural features. In infrared spectroscopy, the characteristic S-H stretching absorption occurs at 2580 cm⁻¹, a sharp band diagnostic for thiols and appearing in the 2550–2600 cm⁻¹ range due to the low polarity of the S-H bond. Proton NMR spectroscopy reveals the methyl protons as a singlet at δ 2.1 (3H) and the thiol proton as a singlet at δ 2.05 (1H), with shifts varying slightly by solvent; the SH signal often broadens due to exchange.[13][14]
Occurrence and biosynthesis
Natural occurrence
Methanethiol occurs naturally in various geological and environmental sources. It is present in natural gas deposits, particularly in sour gas from regions like west Texas, where concentrations of thiols including methanethiol can reach up to 200–300 ppm, though typically much lower.[15] It is also found in coal tar and some crude oils, contributing to their sulfur content.[1] In aquatic geological settings, methanethiol is emitted from hydrothermal vents along mid-ocean ridges, with concentrations in high-temperature fluids (>200 °C) around 10⁻⁸ M, increasing in low-temperature mixing zones.[16] Additionally, it has been detected in the interstellar medium near the Sagittarius B2 molecular cloud through millimeter-wave spectroscopy observations.[17] In terrestrial and atmospheric environments, methanethiol is released from decaying organic matter, such as in marshes and swamps, where emissions can reach approximately 6.56 g sulfur per square meter per year in certain salt marsh sediments.[5] Background atmospheric concentrations are typically low, ranging from 0.005–0.1 ppb (5–100 ppt), but levels elevate near sources of organic decay, such as swamps, to several ppb.[18] For instance, ambient air measurements have recorded up to 4 ppb in some locations.[5] Methanethiol contributes to the flavor profiles of certain foods and beverages. In cheeses like cheddar and Beaufort, it is a key volatile sulfur compound associated with desirable nutty and savory notes, with concentrations correlating strongly (r = 0.82) to flavor intensity during aging.[19] It imparts subtle complexity to nuts such as filberts and roasted varieties at low levels.[5] In beverages, methanethiol enhances freshness in coffee aroma, where its rapid decline post-roasting affects perceived quality, and at trace amounts, it adds to the flavor complexity of fresh beer.[20][21] It is also detectable in human breath and flatus due to microbial activity in the gut.[22] Marine environments serve as a significant natural source, with oceanic emissions playing a role in the global sulfur cycle. Recent measurements indicate sea-to-air fluxes of methanethiol ranging from 0.73 to 5.8 μmol m⁻² day⁻¹ over biologically active waters, contributing 11–37% of total volatile marine sulfur emissions depending on latitude.[23][24] Seawater concentrations near productive regions vary from 1–15 nM, driving these fluxes.[25]Biosynthesis and biological role
Methanethiol is produced by various microorganisms, particularly sulfate-reducing bacteria, through the reduction and demethiolation of L-methionine or the degradation of dimethylsulfoniopropionate (DMSP). In sulfate-reducing bacteria, such as those in anoxic environments, L-methionine undergoes demethiolation, yielding methanethiol as a key product alongside other sulfur compounds; the simplified reaction is represented as:
This process facilitates sulfur assimilation and energy generation in these bacteria.[26][27] Similarly, DMSP, an osmolyte produced by marine microorganisms, is demethylated and demethiolated by bacteria, releasing methanethiol as a volatile intermediate that can be further utilized for methionine biosynthesis or released into the environment.[26][28]
In oceanic ecosystems, methanethiol biosynthesis occurs primarily through the cleavage of DMSP by phytoplankton and associated bacteria, contributing significantly to global volatile sulfur emissions. Phytoplankton such as algae produce DMSP as an antioxidant and osmoprotectant, which is then cleaved by DMSP lyases—enzymes like DddP or Alma1 in eukaryotes and prokaryotes—to generate methanethiol alongside dimethyl sulfide (DMS). This pathway accounts for a substantial portion of oceanic sulfur flux, influencing atmospheric chemistry and cloud formation.[29][30] Key DMSP lyases in bacteria, such as those in the Roseobacter clade, preferentially produce methanethiol under certain conditions, enhancing its role in marine sulfur cycling.[31]
Methanethiol serves as an important intermediate in microbial sulfur metabolism, particularly in the degradation and salvage of methionine. In bacteria, it is generated during the methionine salvage pathway, where 5'-methylthioadenosine (MTA) is recycled back to methionine via methanethiol as a sulfur donor, conserving essential sulfur under nutrient-limited conditions.[32] Additionally, methanethiol acts as a signaling molecule in bacterial quorum sensing, regulating community behaviors such as virulence and biofilm formation; for instance, in Vibrio harveyi, its production is modulated by LuxS-dependent quorum sensing systems, influencing the release of other volatiles like DMS.[33]
In human physiology, methanethiol is generated in the liver through the transamination and subsequent demethiolation of L-methionine, primarily as part of sulfur amino acid catabolism. This process occurs in hepatic mitochondria, where methionine is metabolized to yield methanethiol, which is then rapidly oxidized by methanethiol oxidase (MTO, encoded by SELENBP1) to formaldehyde, hydrogen sulfide, and hydrogen peroxide.[34] The compound is excreted primarily via breath and urine, contributing to trace volatile sulfur profiles in healthy individuals.[35] Elevated methanethiol levels occur in metabolic disorders such as SELENBP1 deficiency, leading to increased excretion and associated malodors like halitosis, rather than in trimethylaminuria, which primarily involves trimethylamine accumulation.[35]
From an evolutionary perspective, methanethiol may have played a prebiotic role in early Earth's sulfur chemistry, serving as a precursor for the abiotic synthesis of methionine and other sulfur-containing amino acids under hydrothermal conditions. Experiments simulating primordial ocean environments demonstrate that supercritical carbon dioxide and reducing gases could generate methanethiol, facilitating the formation of organosulfur compounds essential for the emergence of life.[36][37] This positions methanethiol as a bridge between geochemistry and biological sulfur metabolism in prebiotic scenarios.[37]
Synthesis
Industrial production
Methanethiol is primarily produced on an industrial scale through the catalytic reaction of methanol and hydrogen sulfide over alumina-based catalysts, typically γ-alumina promoted with alkali metals such as potassium. The process operates in the vapor phase at temperatures of 350–400 °C, yielding methanethiol via the equation:
This method achieves conversions and selectivities exceeding 90%, making it the dominant commercial route due to the availability of feedstocks and high efficiency in continuous flow reactors.[1][38][39]
An alternative industrial process involves the reaction of dimethyl sulfide with hydrogen sulfide to produce methanethiol:
This route is employed in facilities where dimethyl sulfide is readily available as a byproduct, often using acidic catalysts to control selectivity.[40]
Methanethiol is also recovered as a valuable byproduct from the pulp and paper industry, particularly during the Kraft pulping process where it forms from lignin depolymerization, and from natural gas processing where it occurs naturally or as an impurity in sour gas streams. Recovery involves stripping and absorption techniques to capture and concentrate the compound from waste gases.[1][41]
Following synthesis or recovery, crude methanethiol is purified primarily by fractional distillation under controlled conditions to remove unreacted methanol, hydrogen sulfide, and dimethyl sulfide impurities, achieving purities greater than 99% suitable for commercial use.[42][43]
Laboratory preparation
A classic laboratory method for preparing methanethiol involves the nucleophilic substitution of methyl iodide with thiourea, forming an S-methylisothiouronium iodide intermediate, followed by alkaline hydrolysis to liberate the thiol. The initial step is:
Hydrolysis of the salt with sodium hydroxide then affords methanethiol along with urea and sodium iodide:
This two-step process is widely used in organic laboratories for small-scale synthesis of thiols, offering good yields under mild conditions.[44][45]
Historically, a gas-phase synthesis involved the methylation of hydrogen sulfide with diazomethane, proceeding via carbene insertion:
This approach, employed in early studies, generates the compound directly but is limited by the hazards associated with diazomethane.[46]
Due to its high flammability and low ignition energy, laboratory preparations of methanethiol must be performed under an inert atmosphere, such as nitrogen or argon, using flame-arrested equipment and avoiding open flames or sparks.[47]