Sulfur Functional Groups

The location of sulfur in the periodic table provides an indication of its connection with oxygen. It lies directly below oxygen, which means that sulfur functional groups are analogous to many oxygen-containing compounds, thiols, and other functional groups. A thiol is an organic compound that is derived from hydrogen sulfide (H₂S). Thiols contain an alkyl or aryl group covalently linked to a sulfhydryl group (${-}{\rm {SH}}$) through ${\rm {C{-}S}}$ bonds, with ${\rm {R{-}SH}}$ stoichiometry. Thiols (RSH) are sulfur analogs of alcohols (ROH). Analogs are compounds with similar chemical structure. They are named in the same way as alcohols, but their suffixes are different. Thiols have a suffix of -thiol, while alcohols have the suffix -ol. An alcohol (or hydroxy) is an organic molecule with a hydroxyl group attached to an sp³-hybridized carbon.

The first step in naming a thiol is to determine the IUPAC name for the corresponding alkane. For example, a six-carbon chain with a thiol on carbon 2 would be named by starting with the IUPAC name for six carbons, which is hexane. Next, the suffix -thiol is added to the base name without the removal of the -e on the end of the name. The final -e of the alkane is retained, as it is in the diols, which are organic compounds with two alcohols. So, hexane becomes hexanethiol. The corresponding alcohol would be hexanol. Next, number the carbons so that the highest-priority group is given the lowest number possible. Alcohols have a higher priority than thiols. The locant, or the number that represents the position of the main functional group, bears the ${-}{\rm {SH}}$ group. The name of this compound is hexane-3-thiol.

If the ${-}{\rm {SH}}$ group is not prioritized, it is acting as a substituent. A substituent is an atom or group of atoms (functional group) that replaces a ${\rm {C{-}H}}\;$ bond in an organic compound. For example, in 3-mercaptopropanol, the ${-}{\rm {SH}}$ group is not top priority. Therefore, the ${-}{\rm {SH}}$ group in 3-mercaptopropanol is a substituent. When the ${-}{\rm {SH}}$ group is a substituent, the group is called a mercapto group instead of a thiol. Low-molecular-weight thiols are notable for their strong odor. Ethanethiol has a particularly strong odor and is added to natural gas so humans can detect a gas leak. As the number of carbons in a thiol molecule increases, the odor of the thiol decreases. This trend is a result of lower percentage sulfur content and the increased stability of molecules with more carbon atoms. The aroma of coffee is attributable to 2-(mercaptomethyl)furan, and a skunk's scent fluid contains 3-methyl-1-butanethiol.

The ${\rm {S{-}H}}$ bond of thiols is less polar than the ${\rm {O{-}H}}$ bond of alcohols, and hydrogen bonding is absent in thiols. Methanethiol (CH₃SH) is a gas at room temperature, while methanol (CH₃OH) is a liquid.

Thiols are stronger acids than alcohols, and thiols are converted to their conjugate base (RS^–), an alkanethiolate, by hydroxide. An alkanethiolate is a thiol that has had the hydrogen of the thiol (SH) removed. Thiols have a pKa_a of 11, while alcohols have a pK_a of 16. Therefore, thiols dissolve in an aqueous base, such as hydroxide. A sulfide, or thioether (RSR), is an organic molecule containing a sulfur bonded to two alkyl or aryl groups through ${\rm {C{-}S}}$ bonds. Thioethers are analogous to ethers (ROR) but are less volatile, have a higher melting point, and are less hydrophilic. Thioethers have a strong odor, similar to thiols. Dimethyl sulfide ((CH₃)₂S) is an example of a thioether. Dimethyl sulfide, or DMS, is a solvent used in many organic reactions. A disulfide (RSSR) is an organic molecule containing two bonded sulfur atoms that are each bound to either a hydrogen, alkyl, or aryl group through ${\rm {C{-}S}}$ bonds. Disulfides are sulfur compounds that are analogous to peroxides (RSSR instead of ROOR). The ${\rm {S{-}S}}$ bonds in a disulfide are stronger than hydrogen bonds but weaker than covalent bonds involving C, H, N, and O. Therefore, an ${\rm {S{-}S}}$ covalent bond may be broken while leaving intact the rest of the covalent bonds. Disulfide bonds are important bonds in many compounds. For example, cystine is a dimer formed by linking two cysteine compounds through a disulfide bond. Dimers are organic compounds made of two identical compounds. Cystine provides the cross-linking unit in many proteins. A sulfoxide (RSOR) is an organic molecule containing a sulfur double bonded to an oxygen atom and single bonded twice to alkyl or aryl groups through ${\rm {C{-}S}}$ bonds. Sulfoxides are analogous to a ketone with the carbonyl carbon replaced by a sulfur. Alliin has a sulfur double bonded to an oxygen, which is a sulfoxide. Alliin is a sulfoxide component of fresh garlic. When garlic is chopped or crushed, enzymes released from the garlic cells convert alliin to allicin, which produces the aroma of garlic. A sulfone (RSO₂R) is an organic molecule containing a sulfur double bonded twice to oxygen atoms and single bonded twice to alkyl or aryl groups through ${\rm {C{-}S}}$ bonds. A sulfone is a sulfoxide with an additional double bonded O. Since sulfur is a third period element, there are d orbitals available to have an expanded octet, and sulfur may have 6 bonds. Sulfolane, a sulfone, is an important industrial solvent originally developed by the Shell Oil Company to purify natural gas. Dapsone, or diaminodiphenyl sulfone, is a widely used antibiotic that was discovered in 1937 and was used to treat leprosy beginning in 1945.