O

Mirror image of original molecule

Chiral molecule:

Rotated molecule cannot be superimposed on its mirror image

Mirror image of original molecule

FIGURE 1-19 Molecular asymmetry: chiral and achiral molecules.

(a) When a carbon atom has four different substituent groups (A, B, X, Y), they can be arranged in two ways that represent nonsuperim-posable mirror images of each other (enantiomers). This asymmetric carbon atom is called a chiral atom or chiral center. (b) When a tetra-hedral carbon has only three dissimilar groups (i.e., the same group occurs twice), only one configuration is possible and the molecule is symmetric, or achiral. In this case the molecule is superimposable on its mirror image: the molecule on the left can be rotated counterclockwise (when looking down the vertical bond from A to C) to create the molecule in the mirror.

Given the importance of stereochemistry in reactions between biomolecules (see below), biochemists must name and represent the structure of each bio-molecule so that its stereochemistry is unambiguous. For compounds with more than one chiral center, the most useful system of nomenclature is the RS system. In this system, each group attached to a chiral carbon is assigned a 'priority. The priorities of some common substituents are

—OCH2 > —OH > —NH2 > —COOH > —CHO>

For naming in the RS system, the chiral atom is viewed with the group of lowest priority (4 in the diagram on the next page) pointing away from the viewer. If the priority of the other three groups (1 to 3) decreases in clockwise order, the configuration is (R) (Latin rectus, "right"); if in counterclockwise order, the configuration

Enantiomers (mirror images)
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