Mercaptans are organic sulfur compounds with a sulfhydryl group bonded to a carbon atom. Methyl mercaptan is the simplest and most well-known, with a strong odor similar to rotten cabbage. It is found in the human body and natural sources such as decayed animal and plant matter. Mercaptans are used as additives to natural gas and in the production of pesticides. Exposure to high concentrations can be dangerous, but the odor is detectable at low concentrations. Ethyl mercaptan is considered less toxic.
Mercaptan is a generic term for a family of organic sulfur compounds that contain a sulfhydryl (SH) group bonded to a carbon atom. They are analogous to alcohols in structure, with the oxygen in the alcohol replaced by sulfur, which belongs to the same group of elements; instead of the hydroxyl (OH) group found in alcohols, mercaptans, also known as thiols, have a sulfhydryl group. Alcohols such as methanol (CH3OH) and ethanol (CH3CH2OH) therefore have thiol equivalents: methanethiol (CH3SH) and ethanethiol (CH3CH2SH). The name is an abbreviation of mercurium captans, Latin for “to capture mercury”, as these compounds combine easily with mercury. Mercaptans are known for their very strong and usually unpleasant odors.
The simplest and most well known of these substances is methyl mercaptan (CH3SH), also known as methanethiol and sometimes simply called mercaptan. It is a colorless, flammable gas that smells like rotten cabbage. This compound is found in small amounts in the human body, being produced from the breakdown of the essential sulfur-containing amino acid methionine, which is found in many foods including eggs, meat, fish, nuts and seeds. The breakdown of methionine occurs in the gut through bacterial action, but various bacteria found in the mouth can also produce methanethiol from this amino acid, causing breath odor.
There are numerous other natural sources of mercaptans. They are formed from the decay of animal and plant matter, and methyl mercaptan is even found in some foods, such as cheese and some nuts. The foul-smelling substance expelled by skunks when they feel threatened contains a number of mercaptans. Many of these compounds are also found in crude oil.
The industrial production of mercaptans is done by combining an alcohol with hydrogen sulphide (H2S), for example, methyl mercaptan is produced by the reaction CH3OH + H2S -> CH3SH + H2O. One of their primary uses is as additives to natural gas; due to their strong odors, small quantities are enough to make gas leaks easily detectable. Methyl mercaptan is used in the production of some pesticides. It is also converted to dimethyl disulfide (DMDS), which is used to remove tar from catalysts used in petroleum refining.
Because methyl mercaptan occurs naturally in the body, it is not thought to be dangerous at very low levels; however, exposure to very high concentrations of can affect the central nervous system, resulting in possible respiratory failure, coma, and death. The odor of this compound is detectable at extremely low concentrations, perhaps as low as one part per billion, so this would normally provide effective warning. There is some evidence, however, that prolonged exposure can desensitize the nose, making odor less detectable. Ethyl mercaptan is considered less toxic.
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