nonstereogenic

Nonstereogenic is an adjective used in stereochemistry to describe a site in a molecule that cannot give rise to stereoisomerism. In practice, a carbon or other atom is nonstereogenic if, under the rules used to assign stereocenters, it cannot be a stereogenic (asymmetric) center because two or more substituents are identical, or because its arrangement cannot produce two distinct configurations.

Common causes include atoms with identical substituents and high molecular symmetry. For example, a carbon bonded to two hydrogen atoms is not a stereogenic center, since exchanging the two hydrogens does not generate a different molecule. A quaternary carbon bonded to four identical groups, such as a central carbon in neopentane with four methyl groups, is likewise nonstereogenic. Nonstereogenic sites do not create or imply the existence of stereoisomers.

In practice, identifying nonstereogenic centers helps chemists estimate the maximum number of stereoisomers a compound can have. A molecule with n stereogenic centers can, in principle, give up to 2^n stereoisomers (though symmetry can reduce this number in some cases). Nonstereogenic sites do not contribute to that count.

Notes on usage: the term does not universally dictate the overall chirality of a molecule. A compound may be achiral or chiral for reasons unrelated to a given nonstereogenic site, and the presence of nonstereogenic centers does not preclude stereochemical features arising from other parts of the molecule.

See also: stereochemistry, stereocenter, chiral center, achiral, enantiomer.