Organosulfur chemistry

Organosulfur chemistry is the scientific study of organic compounds that contain sulfur. These molecules are essential to life and various industrial processes. Sulfur is a member of the chalcogen group, which includes oxygen, selenium, and tellurium. Because of this relationship, organosulfur compounds often behave similarly to carbon-oxygen, carbon-selenium, or carbon-tellurium compounds. While many people associate sulfur with unpleasant odors, these compounds also produce some of the sweetest substances known, such as saccharin.

To understand how these molecules work, scientists look at the strength and length of their chemical bonds. Sulfur atoms are larger than carbon atoms. This size difference means that C−S bonds are longer than C−C bonds. These bonds are also approximately 10% weaker than carbon-carbon bonds. For example, the single bond in methanethiol is 183 pm long. In thiophene, the bond length is 173 pm. The bond dissociation energy for thiomethane is 89 kcal/mol, which is lower than the 100 kcal/mol found in methane. Understanding these specific measurements helps chemists predict how a molecule will react.

Chemists classify these substances into different structural classes based on their functional groups. Sulfides, formerly called thioethers, are characterized by C−S−C bonds. They are often prepared through the alkylation of thiols using agents like alkyl halides or epoxides. Another method is the Ferrario reaction. In this process, phenyl ether is converted to phenoxathiin using elemental sulfur and aluminium chloride.

Thioacetals and thioketals are a subclass of sulfides featuring a C−S−C−S−C bond sequence. These are useful in organic synthesis for protecting carbonyl groups.

Another major class is thiols, which contain the R−SH group. Although they are structurally similar to alcohols, thiols are much more chemically active. They are more nucleophilic, more acidic, and more easily oxidized than alcohols. In fact, their acidity can differ by as much as 5 pKa units. Some aliphatic thiols are even used in nanotechnology because they can form monolayers on gold surfaces. Disulfides, which contain a covalent sulfur-to-sulfur bond (R−S−S−R), are also vital. In biology, they help proteins fold and remain stable. In industry, they are used for the crosslinking of rubber.

Organosulfur compounds appear in many natural and man-made materials. Fossil fuels, including coal, petroleum, and natural gas, contain these compounds because they are derived from ancient organisms. Oil refineries must focus heavily on removing sulfur from these fuels. In the human body, sulfur is vital for life. Of the 20 common amino acids, two are organosulfur compounds: cysteine and methionine. Many medicines also rely on sulfur, such as the antibiotic penicillin and various sulfa drugs. However, sulfur can also be dangerous, as seen with the chemical warfare agent sulfur mustard.

Specific flavors in food also come from organosulfur chemistry. Isothiocyanates are compounds that provide the characteristic pungent flavors in vegetables. You can find them in wasabi, horseradish, mustard, and radish. They are also present in Brussels sprouts, watercress, nasturtiums, and capers. Beyond food, these compounds play roles in complex biological signaling. For instance, S-nitrosothiols can act as donors of nitric oxide, which serves as a signaling molecule in living systems related to vasodilation.

Finally, the field includes many specialized derivatives like sulfoxides, sulfones, and sulfonic acids. Sulfoxides (R−S(O)−R) are the S-oxides of sulfides, while sulfones (R−S(O)2−R) are the S,S-dioxides. Sulfonic acids are strong acids that are often soluble in organic solvents. Some, like trifluoromethanesulfonic acid, are used as important reagents in organic chemistry. Other complex structures include sulfonium salts, which feature a positively charged sulfur atom with three organic substituents.

These diverse structures allow organosulfur chemistry to connect to fields ranging from pharmacology to materials science.