Basic Terminology of Stereochemistry
(IUPAC Recommendations 1996)

D and E

Continued from terms starting with B and C


D, L, DL; d, l , dl ; [Delta] (Delta), [Lambda] (Lambda); Deracemisation; Desymmetrisation; Desymmetrisation Step; Diastereoisomerisation; Diastereoisomerism; Diastereoisomers (Diastereomers); Diastereoisomer Excess/Diastereoisomeric Excess; Diastereomeric Ratio; Diastereomers; Diastereoselectivity; Diastereomorphism; Diastereotopic; Dihedral Angle; Dissymmetry; E, Z; Eclipsed, Eclipsing; Eclipsing Conformation; Eclipsing Strain; Element of Chirality; Enantioconvergence; Enantioenriched; Enantiomer; Enantiomer Excess/Enantiomeric Excess; Enantiomeric Ratio; Enantiomerically Enriched/Enantioenriched; Enantiomerically Pure/Enantiopure; Enantiomeric Purity; Enantiomerisation; Enantiomerism; Enantiomorph; Enantiopure; Enantioselectivity; Enantiotopic; endo, exo, syn, anti; ent; Envelope Conformation; Epimerisation; Epimers; Equatorial; erythro, threo; exo ; External Compensation

D, L, DL See Fischer-Rosanoff convention.

d, l, dl (usage strongly discouraged)

Obsolete alternatives for respectively the prefixes (+)- and (-)- [used to designate the sign of optical rotation of enantiomers under specified condition (and hence to distinguish the enantiomers)] and ([plus minus])- for a racemate.

[Delta] (Delta), [Lambda] (Lambda)

Designations of stereoisomers of tris(didentate) metal complexes and other octahedral complexes. See helicity, also IUPAC, Nomenclature of Inorganic Chemistry, 3rd ed. 1990, Blackwell Scientific Publications, I-, I-

Deracemisation See asymmetric transformation.


The modification of an object which results in the loss of one or more symmetry elements, such as those which preclude chirality (mirror plane, centre of inversion, rotation-reflection axis), as in the conversion of a prochiral molecular entity into a chiral one.

Desymmetrisation Step

The removal of the smallest possible number of symmetry elements from a molecule with (ref. 1) or without (ref. 2) the restriction to pointwise substitution.

1. K. Mislow and J. Siegel, J. Am. Chem. Soc. 106, 3319-3328 (1984)

2. E.A. Halevi, J. Chem. Res. (S) 206-207 (1985)


The interconversion of diasteoisomers.


Stereoisomerism other than enantiomerism. Diastereoisomers (or diastereomers) are stereoisomers not related as mirror images. Diastereoisomers are characterised by differences in physical properties, and by some differences in chemical behaviour towards achiral as well as chiral reagents.

Diastereoisomers (Diastereomers) See diastereoisomerism.

Diastereoisomer Excess/Diastereoisomeric Excess

This is defined by analogy with enantiomer excess, as D1 - D2 [and the percent diastereoisomer excess as 100(D1 - D2)], where the mole fractions of two diastereoisomers in a mixture or the fractional yields of two diastereoisomers formed in a reaction are D1 and D2 (D1 + D2 = 1). The term is not applicable if more than two diastereoisomers are present. Frequently this term is abbreviated to d.e. See stereoselectivity; diastereoisomerism.

Diastereomeric Ratio

This is defined by analogy with enantiomeric ratio as the ratio of the percentage of one diastereoisomer in a mixture to that of the other.

Diastereomers See diastereoisomerism.

Diastereoselectivity See stereoselectivity.


The relationship between objects (or models) analogous to that between diastereoisomeric molecular entities.


Constitutionally equivalent atoms or groups of a molecule which are not symmetry related. Replacement of one of two diastereotopic atoms or groups results in the formation of one of a pair of diastereoisomers. In the example below the two hydrogen atoms of the methylene group C-3 are diastereotopic. See also prochirality; enantiotopic; heterotopic.

Dihedral Angle

The angle between two intersecting planes on a third plane normal to the intersection of the two planes. See torsion angle.


Obsolescent synonym for chirality.

E, Z

The approved stereodescriptors of stereoisomeric alkenes R1R2C=CR3R4 (R1 [not equal] R2, R3 [not equal] R4; neither R1 nor R2 need be different from R3 or R4), cumulenes R1R2C[=C=C]n=CR3R4 and related systems e.g. R1R2C=NOH, HON=C{[CH2]n}2C=NOH. The group of highest CIP priority attached to one of the terminal doubly bonded atoms of the alkene, oxime, etc. or cumulene (i.e. R1 or R2) is compared with the group of highest precedence attached to the other (i.e. R3 or R4). The stereoisomer is designated as Z (zusammen = together) if the groups lie on the same side of a reference plane passing through the double bond and perpendicular to the plane containing the bonds linking the groups to the double-bonded atoms; the other stereoisomer is designated as E (entgegen = opposite). The descriptors may be applied to structures with a fractional bond order between one and two; and to double bonds involving elements other than carbon. They are not used to describe ring substitution relationships. See also cis-trans isomers.

Eclipsed, Eclipsing

Two atoms or groups attached to adjacent atoms are said to be eclipsed if the torsion angle between the three bonds is zero (or approximately so). See bisecting conformation, eclipsing strain.

Eclipsing Conformation See bisecting conformation.

Eclipsing Strain

Intramolecular strain present due to non-bonding interactions between two eclipsed atoms or groups. It is, for example, one of the interactions responsible for the existence of a rotational barrier and hence restricted rotation about the C-C bond in ethane. It has also been called Pitzer strain, torsional strain, and bond opposition strain.

Element of Chirality See chirality element.

Enantioconvergence See stereoconvergence.

Enantioenriched See enantiomerically enriched


One of a pair of molecular entities which are mirror images of each other and non-superposable. See also enantiomorph.

Enantiomer Excess/Enantiomeric Excess

For a mixture of (+)- and (-)-enantiomers, with composition given as the mole or weight fractions F(+) and F(-) (where F(+) + F(-) = 1) the enantiomer excess is defined as |F(+) - F(-)| (and the percent enantiomer excess by 100|F(+) - F(-)| ). Frequently this term is abbreviated as e.e. See optical purity.

Enantiomeric Ratio

The ratio of the percentage of one enantiomer in a mixture to that of the other e.g. 70(+) : 30(-).

Enantiomerically Enriched/Enantioenriched

A sample of a chiral substance whose enantiomeric ratio is greater than 50:50 but less than 100:0

Enantiomerically Pure/Enantiopure

A sample all of whose molecules have (within limits of detection) the same chirality sense. Use of homochiral as a synonym is strongly discouraged.

Enantiomeric Purity See enantiomer excess.


The interconversion of enantiomers. See racemisation.


The isomerism of enantiomers.


One of a pair of chiral objects or models that are non-superposable mirror images of each other. The adjective enantiomorphic is also applied to mirror-image related groups within a molecular entity.

Enantiopure See enantiomerically pure.

Enantioselectivity See stereoselectivity.


Constitutionally identical atoms or groups in molecules which are related by symmetry elements of the second kind only (mirror plane, inversion centre or rotation-reflection axis). For example the two groups c in a grouping Cabcc are enantiotopic. Replacement of one of a pair of enantiotopic groups forms one of a pair of enantiomers. Analogously, if complexation or addition to one of the two faces defined by a double bond or other molecular plane gives rise to a chiral species, the two faces are called enantiotopic. See also prochiral; diastereotopic.

endo, exo, syn, anti

Descriptors of the relative orientation of groups attached to non-bridgehead atoms in a bicyclo[x.y.z]alkane (x [greater than or equal] y > z > 0). If the group is orientated towards the highest numbered bridge (z bridge, e.g. C-7 in example below) it is given the description exo; if it is orientated away from the highest numbered bridge it is given the description endo. If the group is attached to the highest numbered bridge and is orientated towards the lowest numbered bridge (x bridge, e.g. C-2 in example below) it is given the description syn; if the group is orientated away from the lowest numbered bridge it is given the description anti. See also syn, and anti.


A prefix used to indicate the enantiomer for natural products and related molecules where the trivial name only refers to one enantiomer, e.g. ent-kaurene is the enantiomer of kaurene.

Envelope Conformation

The conformation (of symmetry group Cs) of a five-membered ring in which four atoms are coplanar, and one atom (the flap) projects out of the plane.


Interconversion of epimers.


Diastereoisomers that have the opposite configuration at only one of two or more tetrahedral stereogenic centres present in the respective molecular entities.


1. For the use of this term in the context of cyclohexane see axial, equatorial.

2. For the use of this term in the context of a bipyramidal structure see apical, basal, equatorial.

erythro, threo

Descriptors of the diastereoisomers of an acyclic structure or partial structure having two stereogenic centres. This notation is derived from carbohydrate nomenclature; 'Tentative Rules for Carbohydrate Nomenclature', Eur. J. Biochem. 21, 455-477 (1971). The extension of this system has given rise to conflicting interpretations of these prefixes. It is recommended that for such cases the l,u or R*,S* system should be used.

exo See endo, exo, syn, anti.

External Compensation (usage strongly discouraged)

The absence of optical activity in a racemate.

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