Stereoheterotopic substituents

The emphasis to this point has been on the stereochemical relationship of one structure to another. However, the principles developed so far are also relevant to the consideration of s)unmetry relationships within a single molecule. Many of the labels for these relationships are based on the suffix -topic, from the Greek for "place." Terms incorporating this suffix apply both to atoms and to spaces in a molecule, although we usually think of them in terms of atoms. Identical atoms that occupy equivalent environments (both in terms of chemical properties and local or molecular S5munetry) are said to be homotopic (i.e., to have the same place). Identical atoms in nonequivalent environments are said to be heterotopic (for different place). Heterotopic substituents can be either constitutionally heterotopic or stereoheterotopic. Stereoheterotopic substituents can be either enantiotopic or diastereotopic. ... 

A stereochemical condition that describes a stereoheterotopic group C in the compound XABC2 containing a prochiral tetrahedral atom X having substituents A, B, C, and C. 

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