Regios

Regios is a term not widely used as a standalone concept in standard chemistry references. In practice, regios appears most often as a prefix or plural form in discussions of regioisomerism and regioselectivity. The root 'regio-' comes from Latin regio, meaning 'region' or 'area,' and is applied to describe the location of chemical changes or substituents on a molecule.

Regioisomers are constitutional isomers that differ in the position of a substituent or functional group within the same carbon skeleton. Regioselectivity describes a reaction's tendency to produce one regioisomer over others. For example, electrophilic addition of HBr to propene in the presence of peroxides can give different regioisomer distributions compared with standard ionic conditions (Markovnikov vs anti-Markovnikov outcomes). Aromatic nitration of toluene typically yields mainly ortho- and para-nitro derivatives rather than meta, illustrating regioselectivity in substituted arenes.

Although regios is sometimes encountered in informal or shorthand usage, the precise terminology is regioisomer, regioselectivity, and regiochemical. The term serves to distinguish not just which atoms are altered but where the changes occur within a molecule. See also: Regioisomer, Regioselectivity, Regiochemistry.