Mercaptan is a generic term for a family of organic sulfur compounds that contain a sulfhydril (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 sulfhydril group. Alcohols such as methanol (CH3OH) and ethanol (CH3CH2OH) thus have thiol equivalents: methanethiol (CH3SH) and ethanethiol (CH3CH2SH). The name is an abbreviation for mercurium captans, Latin for “capturing mercury,” as these compounds combine easily with mercury. Mercaptans are notable for their very strong, and usually unpleasant, odors.
The simplest and best-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 by the breakdown of the essential, sulfur-containing, amino acid methionine, which is found in many foods including eggs, meat, fish, nuts and seeds. Methionine breakdown takes place in the intestine through bacterial action, but various bacteria found in the mouth can also produce methanethiol from this amino acid, causing breath odor.
There are a number of other natural sources of mercaptans. They are formed by the decay of animal and plant matter and methyl mercaptan is even present in some foods, such as cheese and some nuts. The foul-smelling substance ejected by skunks when they feel threatened contains a number of mercaptans. Several of these compounds are also found in crude petroleum oil.
The industrial manufacture of mercaptans is carried out by combining an alcohol with hydrogen sulfide (H2S), for example, methyl mercaptan is produced by the reaction CH3OH + H2S -> CH3SH + H2O. One of their main uses is as additives to natural gas; because of their strong smells, only tiny amounts are required to render gas leaks easily detectable. Methyl mercaptan is used in the manufacture of some pesticides. It is also converted to dimethyl disulphide (DMDS), which is used to remove tar from the catalysts used in petroleum refining.
Since methyl mercaptan occurs naturally in the body, it is not thought to be hazardous at very low levels; however, exposure to very high concentrations of can affect the central nervous system, possibly resulting in respiratory failure, coma and death. The odor of this compound is detectable at extremely low concentrations, possibly down to just one part per billion, so this would normally provide an effective warning. There is some evidence, however, that prolonged exposure can desensitize the nose, making the smell less detectable. Ethyl mercaptan is considered to be less toxic.