Ethers, Sulfides, Epoxides, and Sulfur Functional Groups

Sulfur Functional Groups

Sulfur functional groups are analogous to their oxygen counterparts. A thiol (SH) is analogous to an alcohol, a sulfide (RSR) is analogous to an ether, a disulfide (RSSR) is analogous to a peroxide, and a sulfoxide (RS(O)R) is analogous to a ketone.

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 (SH{-}{\rm {SH}}) through CS{\rm {C{-}S}} bonds, with RSH{\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 SH{-}{\rm {SH}} group. The name of this compound is hexane-3-thiol.

If the SH{-}{\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 CH {\rm {C{-}H}}\; bond in an organic compound. For example, in 3-mercaptopropanol, the SH{-}{\rm {SH}} group is not top priority. Therefore, the SH{-}{\rm {SH}} group in 3-mercaptopropanol is a substituent. When the SH{-}{\rm {SH}} group is a substituent, the group is called a mercapto group instead of a thiol.

Naming Thiols

When a compound contains a thiol and an alcohol, the thiol is named as a prefix and called mercapto, such as 3-mercaptopropanol (SH(CH2)3OH). When a compound contains only a thiol, the compound is named as a thiol, such as propane-1,3-dithiol (SH(CH2)3SH).
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 SH{\rm {S{-}H}} bond of thiols is less polar than the OH{\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 pKaa of 11, while alcohols have a pKa of 16. Therefore, thiols dissolve in an aqueous base, such as hydroxide.

Converting a Thiol to an Alkanethiolate Ion

A thiol is converted to an alkanethiolate by the addition of a base such as hydroxide (OH-).
A sulfide, or thioether (RSR), is an organic molecule containing a sulfur bonded to two alkyl or aryl groups through CS{\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 CS{\rm {C{-}S}} bonds. Disulfides are sulfur compounds that are analogous to peroxides (RSSR instead of ROOR). The SS {\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 SS{\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.
Cystine ((SCH2CH(NH2)CO2H)2) is an example of a biologically important disulfide. Two units of the amino acid cysteine combine into cystine, 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 CS{\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.


Alliin (C6H11NO3S) is a compound that has a sulfoxide functional group (along with an alkene, amine, and carboxylic acid). Alliin is a component of fresh 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 CS{\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.


Examples of sulfones include sulfolane ((CH2)4SO2) and dapsone (C12H12N2O2S). Sulfolane is an important industrial solvent. Dapsone is a widely used antibiotic.