Thio

Thio is a chemical prefix used in naming to indicate the presence of sulfur, especially when sulfur replaces oxygen in a compound or serves as a key heteroatom. The term derives from the Greek theion, meaning sulfur, and is used across organic and inorganic chemistry to form sulfur-containing analogues of well-known oxygen-containing structures.

In organic chemistry, thio- denotes sulfur analogues of common functional groups. For example, thiols (R–SH) are sulfur analogues of alcohols (R–OH); thioethers or sulfides (R–S–R') are sulfur analogues of ethers (R–O–R'); and thioesters (R–CO–S–R') replace esters (R–CO–O–R'). Thioketones and thioamides replace oxygen in carbonyl-containing groups, producing thiocarbonyl or thioamide functionalities. The concept extends to thioglycosides and other sulfur-containing derivatives used in synthesis and biochemistry.

In bioinorganic and biochemistry contexts, thio is often encountered in cofactors and metabolic intermediates, such as thioesters in fatty acid metabolism (the acetyl group carried by coenzyme A forms a thioester). Sulfur-bound ligands, including thiolates (RS−) and thioethers, are also important in coordination chemistry and materials chemistry, where sulfur donors influence metal–ligand properties.

Distinctions are occasionally made between thio- and related prefixes (such as oxo- for oxygen-containing analogues, or sulf- for certain inorganic sulfur groups). In general, thio signals the substitution of sulfur for oxygen or the central role of sulfur in a molecular framework.