https://en.wikipedia.org/wiki/Organosulfur_compounds
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Organosulfur compounds are organic compounds that contain sulfur. They are often associated with foul odors, but many of the sweetest compounds known are organosulfur derivatives, e.g., saccharin. Nature abounds with organosulfur compounds—sulfur is essential for life. Of the 20 common amino acids, two (cysteine and methionine) are organosulfur compounds, and the antibiotics penicillin and sulfa drugs both contain sulfur. While sulfur-containing antibiotics save many lives, sulfur mustard is a deadly chemical warfare agent. Fossil fuels, coal, petroleum, and natural gas, which are derived from ancient organisms, necessarily contain organosulfur compounds, the removal of which is a major focus of oil refineries.
Sulfur shares the chalcogen group with oxygen, selenium, and tellurium, and it is expected that organosulfur compounds have similarities with carbon–oxygen, carbon–selenium, and carbon–tellurium compounds.
Naturally occurring organosulfur compounds
Not all organosulfur compounds are foul-smelling pollutants. Penicillin and cephalosporin are life-saving antibiotics, derived from fungi. Gliotoxin is a sulfur-containing mycotoxin produced by several species of fungi under investigation as an antiviral agent. Compounds like allicin and ajoene are responsible for the odor of garlic, and lenthionine contributes to the flavor of shiitake mushrooms. Volatile organosulfur compounds also contribute subtle flavor characteristics to wine, nuts, cheddar cheese, chocolate, coffee, and tropical fruit flavors. Many of these natural products also have important medicinal properties such as preventing platelet aggregation or fighting cancer.
In pollution
Most organic sulfur compounds in the environment are naturally occurring, as a consequence of the fact that sulfur is essential for life and two amino acids (cysteine and methionine) contain this element.
Some organosulfur compounds in the environment, are generated as minor by-products of industrial processes such as the manufacture of plastics and tires.
Selected smell-producing processes are organosulfur compounds produced by the coking of coal designed to drive out sulfurous compounds and other volatile impurities in order to produce 'clean carbon' (coke), which is primarily used for steel production.
In fossil fuels
Odours occur as well in chemical processing of coal or crude oil into precursor chemicals (feedstocks) for downstream industrial uses (e.g. plastics or pharmaceutical production) and the ubiquitous needs of petroleum distillation for gasolines, diesel, and other grades of fuel oils production.
Organosulfur compounds might be understood as aromatic contaminants that need to be removed from natural gas before commercial uses, from exhaust stacks and exhaust vents before discharge. In this latter context, organosulfur compounds may be said to account for the pollutants in sulfurous acid rain, or equivalently, said to be pollutants within most common fossil fuels, especially coal.
The most common organosulfur compound present in all petroleum fractions is thiophene (C4H4S), a cyclic and aromatic liquid. In addition, the heavy fractions of oil contain benzothiophene (C8H6S, thianaphtene) and dibenzothiophene. Most of the last compounds are solids and smell like naphthalene. Many methylated, dimethyl, diethyl benzothiophene derivatives are present in diesel and fuel oils which make fuel oils very difficult to clean.
All these heterocyclic sulfides account for 200–500 ppm of natural fuel, the heavily substituted dibenzothiophenes remain after HDS and account for 10–20 ppm. These molecules are also found in coals and they must be eliminated before consumption.
Reduced molybdenum together with nickel is currently used to eliminate thiophenes from petroleum (HDS) due to its great affinity towards sulfur. In addition tungsten together with nickel and cobalt is used for hydrodesulfurization (HDS) in large refineries. The adsorption mechanism of thiophene to transition metals is proposed to occur through the π system, where the organosulfur compound lies almost parallel to the metal surface. Many researchers focus their efforts in optimizing the oxidation state of the transition metals for HDS, like Cu(I) and Ag(II) which together with Pd(0) have proven to be more specific for π bonding with thiophenes of all kinds.
Basis for odor
Humans and other animals have an exquisitely sensitive sense of smell toward the odor of low-valent organosulfur compounds such as thiols, sulfides, and disulfides. Malodorous volatile thiols are protein-degradation products found in putrid food, so sensitive identification of these compounds is crucial to avoiding intoxication. Low-valent volatile sulfur compounds are also found in areas where oxygen levels in the air are low, posing a risk of suffocation. It has been found that copper is required for the highly sensitive detection of certain volatile thiols and related organosulfur compounds by olfactory receptors in mice. Whether humans, too, require copper for sensitive detection of thiols is not yet known.
