Stereochemical terminology

Stereochemistry has engendered a sometimes confusing terminology, with several terms that are frequently misused. Here we provide definitions of the most common terms. This collection is based in large measure on a much more extensive listing in the following book Eliel, E. L., Wilen, S. H., and Mander, L. N. (1994). Stereochemistry of Organic Compounds,John Wiley Sons, New York. 

Absolute configuration. A designation of the position or order of arrangement of the ligands of a stereogenic unit in reference to an agreed upon stereochemical standard. 

Achiral. Not chiral. A necessary and sufficient criterion for achirality in a rigid molecule is the presence of any improper symmetry element (S , including a and i). 

Anomers. Diastereomers of glycosides or related cyclic forms of sugars that are specifically epimers at the anomeric carbon (C of an aldose, or C2, C3, etc., of a ketose). 

Modern usage is to describe relative configuration of two stereogenic centers along a chain. The chain is drawn in zigzag form, and if two substituents are on opposite sides of the plane of the paper, they are designated anti. See also syn , antiperiplanar , and anticlinal . 

The elemental composition of a molecule can be unambiguously described by its molecular formula. In inorganic chemistry, the molecular formula is often sufficient in order to identify the species in question, because there is usually only one possible arrangement of the constituent atoms that are combined in that particular elemental ratio. Thus, for example, S8 and Sn are two possible molecular 

In contrast, in organic chemistry, the situation is different for all but the very simplest of molecules. This is because, for any given elemental ratio of the constituent atoms, there might be many possible spatial arrangements that those atoms could assume relative to one another. So, even though the 

The problem may be easily illustrated by considering the molecules that have the molecular formula, C4H10O. There are eight isomers that have this molecular formula. Seven of these isomers 

The first four compounds are alcohols, butan-l-ol, 2-methylpropan-l-ol, butan-2-ol, and 1,1-dimethylethanol while the last three compounds are ethers, diethyl ether, /.so-propylmethyl ether, and propylmethyl ether. However, there is a further ambiguity that has not been revealed by the line notation. For butan-2-ol there are actually two possible structures that differ only in the way the groups are arranged relative to one another in three dimensions, i.e. there are two stereoisomers. 

In order to distinguish between these two possibilities, further information must be included in the name of the compound, i.e. stereochemical terminology is needed. 

---

STEREOCHEMICAL TERMINOLOGY, lUPAC RECOMMENDATIONS ANTI PORT UNIPORT SYMPORT... 

STEREOCHEMICAL TERMINOLOGY, lUPAC RECOMMENDATIONS Apical and equatorial ligand interchange, PSEUDOROTATION APICOPHILIOITY APOACTIVATOR Apolar side-chain interactions, HYDROPHOBIC EFFECT Apparent equilibrium constant,... 

STEREOCHEMICAL TERMINOLOGY, lUPAC RECOMMENDATIONS Enantiomerically enriched/Enantioenriched, STEREOCHEMICAL TERMINOLOGY, lUPAC RECOMMENDATIONS Enantiomerically pure/Enantiopure,... 

STEREOCHEMICAL TERMINOLOGY, lUPAC RECOMMENDATIONS ENANTIOSELECTIVE REACTION ASYMMETRIC INDUCTION ENCOUNTER COMPLEX ENCOUNTER-CONTROLLED RATE DIFFUSION CONTROL FOR BIMOLECULAR COLLISIONS ENDERGONIC PROCESS ENDO-a (or j8)-N-ACETYLGALACTOSAMI-NIDASE... 

---