Basic Terminology of Stereochemistry
(IUPAC Recommendations 1996)

R and S

Continued from terms starting with N to Q


r, s ; R, S; Rp, Sp; R*, S*; rac; Racemate; Racemic; Racemic Compound; Racemic Conglomerate; Racemic Mixture; Racemisation; Re, Si; rel; Relative Configuration; Resolution; Restricted Rotation; Retention of configuration; Ring Reversal (or ring inversion); Rosanoff Convention; Rotamer; Rotational Barrier; Rotatory Power; RS; s-cis, s-trans ; S; Sawhorse Projection; sc; Secondary Structure; Si; Sequence Rules; Skew; sp; Space Formula; Spontaneous Resolution; SR; Staggered Conformation; Stereochemical Formula (Stereoformula); Stereoconvergence; Stereodescriptor; Stereoelectronic; Stereoelectronic Control; Stereoformula; Stereogenic Unit (or Stereogen or Stereoelement); Stereoheterotopic; Stereoisomerism; Stereoisomerism, Planar; Stereoisomers; Stereomutation; Stereoselective synthesis; Stereoselectivity; Stereospecificity; Steric Strain; Superposability; Syn; Synclinal; Synperiplanar

r, s

Stereodescriptors of pseudo-asymmetric atom. For references see under R, S.

R, S

The approved designations (devised by Cahn, Ingold and Prelog) of absolute configuration at four-coordinate (quadriligant) and six-coordinate (sexiligant) stereogenic centres. For the application of these stereodescriptors in systematic chemical nomenclature see R.S. Cahn, C.K. Ingold and V. Prelog, Angew. Chem. 78, 413-447 (1966), Angew. Chem. Internat. Ed. Eng. 5, 385-415, 511 (1966); and V. Prelog and G. Helmchen, Angew. Chem. 94, 614-631 (1982), Angew. Chem. Internat. Ed. Eng. 21, 567-583 (1982).

Rp, Sp See planar chirality.

R*, S* See relative configuration.

rac See racemate.


An equimolar mixture of a pair of enantiomers. It does not exhibit optical activity. The chemical name or formula of a racemate is distinguished from those of the enantiomers by the prefix ([plus minus])- or rac- (or racem-) or by the symbols RS and SR.


Pertaining to a racemate.

Racemic Compound

A crystalline racemate in which the two enantiomers are present in equal amounts in a well defined arrangement within the lattice of a homogeneous crystalline addition compound.

Racemic Conglomerate

An equimolar mechanical mixture of crystals each one of which contains only one of the two enantiomers present in a racemate. The process of its formation on crystallisation of a racemate is called spontaneous resolution, since pure or nearly pure enantiomers can often be obtained from the conglomerate by sorting.

Racemic Mixture (usage strongly discouraged)

The term is confusing since it has been used as a synonym for both racemate and racemic conglomerate.


The production of a racemate from a chiral starting material in which one enantiomer is present in excess.

Re, Si

A stereoheterotopic face of a trigonal atom is designated Re if the ligands of the trigonal atom appear in a clockwise sense in order of CIP priority when viewed from that side of the face. The opposite arrangement is termed Si. See also enantiotopic, and illustrated under prochirality.

rel See relative configuration.

Relative Configuration

1. The configuration of any stereogenic (asymmetric) centre with respect to any other stereogenic centre contained within the same molecular entity. Unlike absolute configuration, relative configuration is reflection-invariant. Relative configuration, distinguishing diastereoisomers, may be denoted by the configurational descriptors R*, R* (or l) and R*, S* (or u) meaning, respectively, that the two centres have identical or opposite configurations. For molecules with more than two asymmetric centres the prefix rel- may be used in front of the name of one enantiomer where R and S have been used. If any centres have known absolute configuration then only R* and S* can be used for the relative configuration. See also [alpha] (alpha), [beta] (beta) (1) and (3).

2. Two different molecules Xabcd and Xabce, may be said to have the same relative configurations if e takes the position of d in the tetrahedral arrangement of ligands around X (i.e. the pyramidal fragments Xabc are superposable). By the same token the enantiomer of Xabce may be said to have the opposite relative configuration to Xabcd. The terms may be applied to chiral molecular entities with central atoms other than carbon but are limited to cases where the two related molecules differ in a single ligand.

Both definitions can be generalised to include stereogenic units other than asymmetric centres.


The separation of a racemate into the component enantiomers. See also kinetic resolution, racemic conglomerate.

Restricted Rotation See free rotation, hindered rotation, restricted rotation.

Retention of configuration See Walden inversion.

Ring Reversal (or ring inversion)

The interconversion by rotation about single bonds (coupled with angle deformation in the transition state) of cyclic conformers having equivalent ring shapes (e.g. chair [arrow] chair) though not necessarily equivalent spatial positions of substituents (e.g. equatorial [arrow] axial)

Rosanoff Convention See Fischer-Rosanoff convention.


One of a set of conformers arising from restricted rotation about one single bond.

Rotational Barrier

In a rotation of groups about a bond, the potential energy barrier between two adjacent minima of the molecular entity as a function of the torsion angle.

Rotatory Power

The quantitative measure of optical activity.

RS See racemate.

s-cis, s-trans

The spatial arrangement of two conjugated double bonds about the intervening single bond is described as s- cis if synperiplanar and s-trans if antiperiplanar. This term should not be applied to other systems such as N-alkyl amides (use E/Z or sp/ap).

S See R, S.

Sawhorse Projection

A perspective formula indicating the spatial arrangement of bonds on two adjacent carbon atoms. The bond between the two atoms is represented by a diagonal line, the left-hand bottom end of which locates the atom nearer the observer and the right-hand top end the atom that is further away. In general a Newman or zig-zag projection is preferred.

sc See torsion angle.

Secondary Structure

The conformational arrangement ([alpha]-helix, [beta]-pleated sheet, etc.) of the backbone segments of a macromolecule such as a polypeptide chain of a protein without regard to the conformation of the side chains or the relationship to other segments. See also primary structure, tertiary structure, quaternary structure.

Si See Re, Si.

Sequence Rules See priority.

Skew See torsion angle and chair, boat, twist.

sp See torsion angle.

Space Formula

Synonym for stereochemical formula.

Spontaneous Resolution See racemic conglomerate.

SR See racemate.

Staggered Conformation

The conformation of groups attached to two adjacent atoms is said to be staggered if the torsion angles are such that the groups are as far away as possible from an eclipsed arrangement.

Stereochemical Formula (Stereoformula)

A three-dimensional view of a molecule either as such or in a projection. See projection formula.


The predominant formation of the same stereoisomer or stereoisomer mixture of a reaction product when two different stereoisomers of the reactant are used in the same reaction. When that product involved in the reaction is one enantiomer the result has been called enantioconvergence.


A prefix to specify configuration (absolute or relative) or conformation. For example R,S; r,s; P,M; Re,Si; E,Z; ap,sp; etc.


Pertaining to the dependence of the properties (especially the energy) of a molecular entity in a particular electronic state (or of a transition state) on relative nuclear geometry. The electronic ground state is usually considered, but the term can apply to excited states as well. Stereoelectronic effects arise from the different alignment of electronic orbitals in different arrangements of nuclear geometry.

['Glossary of Terms Used in Physical Organic Chemistry', Pure Appl. Chem. 66, 1077-1184 (1994)].

Stereoelectronic Control

Control of the nature of the products of a chemical reaction (or of its rate) by stereoelectronic factors. The term is usually applied in the framework of an orbital approximation. The variations of molecular orbital energies with relative nuclear geometry (along a reaction coordinate) are then seen as consequences of variations in basis-orbital overlaps.

['Glossary of Terms Used in Physical Organic Chemistry', Pure Appl. Chem. 66, 1077-1184 (1994)].

Stereoformula See projection formula.

Stereogenic Unit (or Stereogen or Stereoelement)

A grouping within a molecular entity that may be considered a focus of stereoisomerism. At least one of these must be present in every enantiomer (though the presence of stereogenic units does not conversely require the corresponding chemical species to be chiral). Three basic types are recognised for molecular entities involving atoms having not more than four substituents:

(a) A grouping of atoms consisting of a central atom and distinguishable ligands, such that the interchange of any two of the substituents leads to a stereoisomer. An asymmetric atom (chirality centre) is the traditional example of this stereogenic unit.

(b) A chain of four non-coplanar atoms (or rigid groups) in a stable conformation, such that an imaginary or real (restricted) rotation (with a change of sign of the torsion angle) about the central bond leads to a stereoisomer.

(c) A grouping of atoms consisting of a double bond with substituents which give rise to cis-trans isomerism.


Either enantiotopic or diastereotopic. (In contrast the term constitutionally heterotopic has been used to describe identical groups located in constitutionally non-equivalent positions.) See also pro-R, pro-S.


Isomerism due to differences in the spatial arrangement of atoms without any differences in connectivity or bond multiplicity between the isomers.

Stereoisomerism, Planar See planar chirality.


Isomers that possess identical constitution, but which differ in the arrangement of their atoms in space. See enantiomer, diastereoisomer, cis-trans isomers.


A change of configuration at a stereogenic unit brought about by physical or chemical means. See epimerisation.

Stereoselective synthesis

A chemical reaction (or reaction sequence) in which one or more new elements of chirality are formed in a substrate molecule and which produces the stereoisomeric (enantiomeric or diastereoisomeric) products in unequal amounts. Traditionally called asymmetric synthesis. See also stereoselectivity.


The preferential formation in a chemical reaction of one stereoisomer over another. When the stereoisomers are enantiomers, the phenomenon is called enantioselectivity and is quantitatively expressed by the enantiomer excess; when they are diastereoisomers, it is called diastereoselectivity and is quantitatively expressed by the diastereoisomer excess.


A reaction is termed stereospecific if starting materials differing only in their configuration are converted into stereoisomeric products. According to this definition, a stereospecific process is necessarily stereoselective but not all stereoselective processes are stereospecific.

['Glossary of Terms Used in Physical Organic Chemistry', Pure Appl. Chem. 66, 1077-1184 (1994)].

Steric Strain

1. See strain [See 'Glossary of terms used in Physical Organic Chemistry', Pure Appl. Chem. 66, 1077-1184 (1994)].

2. See angle strain.

3. See eclipsing strain.

4. See transannular strain.


The ability to bring two particular stereochemical formulae (or models) into coincidence (or to be exactly superposable in space, and for the corresponding molecular entities or objects to become exact replicas of each other) by no more than translation and rigid rotation.


1. See torsion angle.

2. See endo, exo, syn, anti.

3. See also 'Glossary of Terms Used in Physical Organic Chemistry', Pure Appl. Chem. 66, 1077-1184 (1994) for use as a term to describe suprafacial addition or elimination reactions.

4. It was formerly used to describe the stereochemistry of oximes and related systems (see E,Z).

Synclinal See torsion angle.

Synperiplanar See torsion angle.

Continue with terms starting with T to Z.

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