Enantiomers and the Tetrahedral Carbon

Understanding the causes and consequences of molecular handedness is crucial to understanding biological chemistry. The subject can be a bit complex, but the material covered in this chapter nevertheless forms the basis for much of the remainder of the book. 

Molecules that are not identical to their mirror images are kinds of stereoisomers called enantiomers (Greek enantio, meaning opposite ). Enantiomers are related to each other as a right hand is related to a left hand and result whenever a tetrahedral carbon is bonded to four different substituents (one need not be H). For example, lactic acid (2-hydroxypropanoic acid) exists as a pair of enantiomers because there are four different groups (-H, -OH, -CH3, and -CO2H) bonded to the central carbon atom. The enantiomers are called 

Lactic acid a molecule of general formula CHXYZ 

FIGURE 5.2 Attempts at superimposing the mirror-image forms of iactic acid, (a) When the -H and -OH substituents match up, the -COjH and -CH3 substituents don t (b) when -CO2H and -CHj match up, -H and -OH don t. Regardiess of how the moiecuies are oriented, they aren t identicai. 

Molecules of the type CH5X and CH2XY are identical to their mirror images, but a molecule of the type CHXYZ is not. A CHXYZ CH3X 1 H-Vh H r  

Test your knowledge of Key Ideas by using resources in ThomsonNUW or by answering end-of-chapter problems marked with A. 

VIolecules that are not identical to their mirror images, and thus exist in two enantiomeric forms, are said to be chiral (ky-ral, from the Greek cheir, meaning hand ). You can t take a chiral molecule and its enantiomer and place one on the other so that all atoms coincide. 

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Enantiomers and the Tetrahedral Carbon 307 The Reason for Handedness in Molecules Chirality Optical Activity 312 Specific Rotation 313... 

The other example of note is the optically active tartaric acids (Fig. 11). Tartaric add contains two asymmetric carbon atoms. The dextro- and levo-tartaric adds are enantiomers. However, a third isomer is possible in which the two rotations due to the two asymmetric carbon atoms compensate and the molecule is optically inactive as a whole. That is, the molecule contains a plane of symmetry. This form, meso-tartaric acid, was also discovered by Pasteur, differs from the two optically active tartaric adds in being internally compensated, and is not resolvable. Thus, the tetrahedral model for carbon and the asymmetric carbon atom proposed by van t Hoff were able to completely explain the observations of Pasteur relating to the three isomers of tartaric add. 

Atoms other than asymmetric carbons can be chirality centers. When an atom such as nitrogen or phosphorus has four different groups or atoms attached to it and it has a tetrahedral geometry, it is a chirality center. A compound with a chirality center can exist as enantiomers, and the enantiomers can be separated. 

A similar situation holds foi a molecule containing a tetrahedral carbon is shown in (Figure 16). The reaction converting one enantiomer to another, is formally equivalent to the exchange of two sigma-bond electr on pair s, and... 

We have indicated how the enantiomers of 2-butanol differ by drawing their structures 5 and 6 (Section 5-ID) in perspective to show the tetrahedral configuration of substituents at the chiral carbon. This configuration also can be represented by the sawhorse or Newman formulas using any one of the several possible staggered conformations such as 5a and 6a or 5b and 6b ... 

When a molecule is chiral, then it will have two isomeric forms called enantiomers, each of which is the nonsuperimposable mirror image of the other. Enantiomers are distinct stereoisomers because they are compounds that have die same molecular formula and sequence of bonded elements but which differ in tile spatial arrangement of groups in the molecule. If a molecule is chiral, and thus has two enantiomers, it usually (but not always) contains at least one chiral center. In organic compounds a chiral center usually corresponds to an asymmetric tetrahedral carbon atom. 

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