Methanethiol
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Methanethiol (also known as methyl mercaptan) is a colorless gas with a smell like rotten cabbage. It is a natural substance found in the blood, brain, and other tissues of people and animals. It is released from animal feces. It occurs naturally in certain foods, such as some nuts and cheese. The chemical formula for methanethiol is CH3SH; it is classified as a thiol.
Methanethiol is released from decaying organic matter in marshes and is present in the natural gas of certain regions in the United States, in coal tar, and in some crude oils.
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 concentations than sulfate (~0.3 nM vs. 28 mM). Bacteria in oxic and anoxic environments can also convert methanethiol to dimethylsulfide (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 anoxic sediments.
Methanethiol is a weak acid, with a pKa of ~10.4. This acidic property makes it reactive with dissolved metals in aqueous solutions. The environmental chemistry of these interactions in seawater or fresh water environments such as lakes has yet to be fully investigated.
The United States 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.
Uses
Methanethiol is primarily used as an odorant to add warning properties to propane and natural gas used as fuel. Propane by itself is odorless, as are many sources of natural gas. Methanethiol is added for safety reasons so that gas leaks are readily noted by anyone present due to the odor. Methanethiol vaporizes readily and is odiforous enough that only small quantities need be used.
For propane, liquid methanethiol is mixed with liquid propane by the refinery. Methanethiol remains in solution in the desired concentration as the propane is piped and trucked through the distribution system until it is stored in the tank from which it will be used. Propane tanks are usually configured to withdraw vapor rather than liquid, and since the boiling point of methanethiol is somewhat higher than that of propane (and since its vapor pressure is lower), methanethiol becomes more concentrated in the tank as vapor is withdrawn. For this reason, even minute leaks in a piping system connected to a nearly-empty tank produce the characteristic odor, especially if much of the propane was withdrawn at low temperatures.
In natural gas systems, methanethiol is injected into the distribution piping, usually within several miles or less of the point of use. Injection equipment and a tank of methanethiol can be seen at the master meter point outside many cities. Methanethiol tanks are similar in construction to tanks used for propane and anhydrous ammonia.
Because of its reactivity, methanethiol can be removed from the gas before it reaches the point of use, or after leaking out of piping. It can react with or be adsorbed by metallic pipe materials, particularly when corroded, and will bind to soil particles. Explosions have occurred where underground gas leaks went unnoticed because methanethiol was filtered out by the soil.
Methanethiol is also manufactured for use in the plastics industry, as a precursor in the manufacture of pesticides, and as a jet fuel additive. It is also released as a decay product of wood in pulp mills.