Disulfide

In chemistry, a disulfide usually refers to the structural unit composed of a linked pair of sulfur atoms. Disulfide usually refer to a chemical compound that contains a disulfide bond, such as diphenyl disulfide, C6H5S-SC6H5.

Inorganic vs. organic disulfides
The disulfide anion is S22−, or −S–S−. Sulfur is usually assigned to the reduced oxidation number −2, described as S2− and called sulfide. It has the electron configuration of a noble gas (argon). In disulfide, sulfur is only reduced to a state with oxidation number −1. Its configuration then resembles that of a chlorine atom. It thus tends to form a covalent bond with another S− center to form S22− group. Oxygen also behaves similarly, e.g. in peroxides such as H2O2. Examples:
 * Iron disulfide (FeS2), e.g. the mineral pyrite.
 * Disulfur dichloride (S2Cl2), a distillable liquid.

In many cases, each of the sulfur atoms in a disulfide group is covalently bonded to a carbon atom in an organic compound, forming a disulfide bond, sometimes called a disulfide linkage or a disulfide bridge. Examples:
 * The amino acid cystine
 * The vitamin lipoic acid

Misnomers
Disulfide is also used to refer to compounds that contain two sulfide (S2−) centers. The compound carbon disulfide, CS2 is described with the structural formula i.e. S=C=S. This molecule is not a disulfide in the sense that it lacks a S-S bond. Similarly, molybdenum disulfide, MoS2, is not a disulfide in the sense again that its sulfur atoms are not linked.