Pattern Specifications

To define the structure of an enantiomer it is necessary to be able to draw the structure of the molecule. This procedure can be very unwieldy and tedious if the structural information is not presented in a concise and comprehensible manner. The symbols (R) and (S) (rectus and sinister) have been proposed to describe the structure of an enantiomer, individually, by R. S. Cahn, C. Ingold and V. Prelog and the format that they suggested has now been formally accepted. To define the structure of an isomer two rational procedures must be carried out in separate steps.

1/ Employing a set of sequence rules (which will be described later) a sequence priority is assigned to the atoms or groups around the chiral atom. If there are only atoms attached to the chiral atom (e.g. H,Cl,Br,I) the priority will be in order of their atomic number (i.e. the priority sequence will be given by I, Br, Cl, H)

Figure 13. Atomic Priority Sequences

2/ The molecule is visualized in such a manner that the lowest priority atom points away from the viewer into the paper. If each atom or group is labeled in order of its priority sequence, if the decrease in priority order is clockwise the substance is defined as (R) and if the decrease in priority order is anti clockwise the substance is defined as (S) as shown in figure 13.

If both the conformation and the rotation are known then the compound can be specifically defined (e.g. (S)-(+)-sec-butyl chloride). A racemic mixture of sec. butyl chloride would be designated as (R.S)-sec butyl chloride. The designation (R) and (S) should not be used in conjunction with (+) and ( ) unless the rotation has been experimentally verified.

In most cases, chemical groups are attached to the chiral atom, not single monovalent atoms and thus, some further rules are necessary to determine the priority sequence of which are three.

Rule 1

If all the atoms directly attached to the chiral atom are different, then the priority sequence is determined by the atomic number of each element as shown in figure 14.

Priority Sequence I-Cl-S-H Priority Sequence Br-C-D-H

Figure 14. Priority Sequence of Atoms