Types Of Stereoisomers

When molecules composed of the same constituents have the same structural formulas but differ only with respect to the spatial arrangement of certain atoms or groups of atoms, they are defined as stereoisomers. Stereoisomers can be optical isomers or geometrical isomers. Optical isomers are members of a set of stereoisomers, at least two of which are optically active or chiral; geometrical isomers are members of a set of stereoisomers that contains no optically active members. If the relationship between optical isomers is one of nonsuperimposable mirror images, the isomers are defined as enantiomers. Molecules having at least one pair of enantiomers are considered chiral. Optical isomers not related to each other as enantiomers are diastereomers.

A. Enantiomers

Enantiomeric compounds in which the asymmetric center is a tetravalent carbon, as in Fig, 2, represent the largest class of chiral molecules. The tetrahedral orientation of the bonds to a tetravalent carbon is such that when four nonidentical ligands are present, the mirror image of the molecule is nonsuperimposable, and the molecule is enantiomeric and chiral. When two of the ligands are identical, the mirror image is superimposable, and the molecule possesses a plane of symmetry and is achiral.

Molecules that do not possess an asymmetric center may still have nonsuperimposable mirror images and exist as enantiomers. These molecules contain a chiral plane or chiral axis and are dissymmetric with respect to either that plane or axis. The structures of the enantiomers of the sedative-hypnotic methaquaione are presented in Fig. 4, In this molecule there is a chiral axis between the nitrogen atom (N-I) and phenyl ring (C-l). The dissymmetry of the two forms of the molecule is a result of hindered rotation around this axis, which is due to steric interactions between methyl groups (M-l and M-2), Other axially dissymmetric molecules include allene, biaryls, alkylidenecyclohexanes, and spiranes. Planar dissymmetric molecules are exemplified by molecules such as tra « s-cy cloa lkene s.

B. Diastereomers

Diastereomers are optical isomers that are not related as an object and its mirror image. Unlike enantiomers, the physical and chemical properties of diastereomers can differ and it is not unusual for them to have different melting and boiling points, refractive indices, solubilities, etc. Their optical rotations can differ in both sign and magnitude.

The most common diastereomeric molecule is one that contains two asymmetric carbons. This situation is illustrated by the compounds

Figure 4 The enantiomers of methaquaione. The molecule is dissymmetric about the (N-l)—(C-l) axis.

Figure 4 The enantiomers of methaquaione. The molecule is dissymmetric about the (N-l)—(C-l) axis.

H CH3

HO NHCH3 ephedrine

HO CH3

H NHCH3 pseudoephedrine

Figure 5 The structures of the diastereomeric molecules ephedrine and pseudoephedrine (only one enantiomer of each substance is presented).

ephedrine and pseudoephedrine (Fig, 5). In these molecules, the asymmetric carbons 2 and 2' are identical, whereas carbons 1 and 1' are mirror images. The different relationships between carbons 1 and 1' and between carbons 2 and 2' result in a non-mirror-image relationship between ephedrine and pseudoephedrine. It is noteworthy that each diastereomer (ephedrine and pseudoephedrine) exists as a member of an enantiomeric pair, that is, d- and ¡-ephedrine and d- and /-pseudoephedrine, respectively. Thus, diastereomeric molecules with two asymmetric centers are most often represented by four stereoisomers.

C. Geometrical Isomers

Molecules that contain a carbon-carbon double bond, alkenes, and similar double-bonded systems, C=N, for example, can exist as stereoisomers. Because each of these sets of stereoisomers contains no optically active members, these compounds are classified as geometrical isomers. This situation is illustrated by 2-butene (Fig. 6).

Figure 6 The isomers of 2-butene.

cis-or Z-isomer trons-or E-isomer

Figure 6 The isomers of 2-butene.

In this molecule, the two methyl groups can be found on the same or opposite sides of the double bond. When they are on the same side, the molecule is defined as the as or 2 isomer; when they are on the opposite sides, the trans or E designation is used. The descriptor Z comes from the German zusammen, which means together; E comes from the German entgegen, which means opposite.

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