Mercaptans are used as an additive to natural gas so that it has an odor. In its native state, natural gas is completely odorless. This is quite dangerous, however, since if it was supplied to homes in this state, it would absurdly easy to not notice a gas leak and either suffocate or explode (depending on what happened after you failed to notice). So, it is added to natural gas prior to pumping it into homes (it may be added earlier, in the refining process; I don't know). That way, when you leave the gas on, you smell it!

They are preferred because, as alcohols, they burn quite cleanly and don't leave residue (much).

Mercaptans are a group of chemical compounds similar to alcohols, but with the oxygen in the OH group replaced with sulfur. Because sulfur is just below oxygen in the periodic table, many alcohols have sulfur containing analogs. The modern IUPAC nomenclature for the older name "mercaptans " is alkanethiol or simply thiol. The -SH group of these compounds is called a thiol group or a sulfhydryl group.
       H                      H H S H
       |                      | | | |
     H-C-SH                 H-C-C-C-C-H
       |                      | | | |
       H                      H H H H

   Methanethiol             2-butanethiol

The most characteristic property of thiols are their odor. The human nose can detect the presence of these compounds at levels of about 0.02 ppb (that is parts per billion!). These compounds are found for instance in the spray of skunks, or as breakdown products during the digestion of asparagus. Mercaptans are also added to butane for domestic applications, so that consumers can smell whenever they have a gas leak (butane itself is odorless).

Ethyl mercaptan, CH3CH2SH, is the additive most often used to add odor to natural gas. Mercaptans are particularly suited to giving the colorless, odorless natural gas an odor because of mercaptans' very low threshold of detectability by the human nose. Parts per billion of mercaptans are easily detectable by smell. This ensures that even heavily diluted natural gas will be easily detected.

Mercaptans are known as thiols in modern chemical language. This more accurately describes them as alcohol analogs with sulfur replacing oxygen, and brings the nomenclature together (thioethers are R-S-R', thioesters are R(CO)SR'). No one ever said mercaptoether.

Mer*cap"tan (?), n. [F., fr. NL. mercurius mercury + L. captans, p. pr. of captare to seize, v. intens. fr. capere.] Chem.

Any one of series of compounds, hydrosulphides of alcohol radicals, in composition resembling the alcohols, but containing sulphur in place of oxygen, and hence called also the sulphur alcohols. In general, they are colorless liquids having a strong, repulsive, garlic odor. The name is specifically applied to ethyl mercaptan, C2H5SH. So called from its avidity for mercury, and other metals.


© Webster 1913.

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