The actual three-dimensional arrangement of groups in an asymmetric molecule is called its Absolute Configuration. We can specify the configuration by using the R.S System. The R, S system is also called the Cahn-Ingold-Prelog System (named after its inventors).
In the R,S system, we arrange the four groups attached to the asymmetric carbon in decreasing order of priority (1,2,3,4) by applying the rules given below. We then view the asymmetric center with the lowest priority group (4) on the side opposite the observer (group priorities are shown in parentheses):
In the above example, A has the highest priority (1); B has the next highest (2); C has the next (3), and D has the lowest priority (4); We now go from high to low priority (1 2 3). To do so we move in an anticlockwise direction, and therefore we assign to this structure the S Configuration (S = sinister = left). In the structure given below,
We go from highest to lowest priority by moving in a clockwise direction. We, therefore, assign to this structure the R Configuration (R= rectus =right).
To establish the group priorities we use the following Sequence Rules:
Rule 1. Of the atoms attached directly to the asymmetric carbon atom, the one with the highest atomic number has the highest priority. For Example.
l > Br > Cl > F > O > N > C > H
Rule 2. If the atoms attached to the asymmetric carbon atoms are the same, we determine priority by going to the next atom away from the asymmetric carbon atom. For example,
Ethyl has a higher priority than methyl because the ethyl group has (CHH attached to the first carbon, whereas the methyl carbon has only hydrogens (HHH), and C has priority over H. Isopropyl is of higher priority than ethyl because it has two carbon attached to the first carbon and ethyl has only one. If there is no difference at the second atom in the chain, we go to the next atom and so fort.
Rule 3. A double bond is treated as through each atom of the double bond were bonded to two atoms:
The R and S notations can be used as part of the IUPAC name of an asymmetric molecule, to provide a complete strtuctural description, including configuration. The E and S designations precede the remember of the name, separated from it with a hyphen.
The configuration of compounds with more than one asymmetric centre can also be specified by the R,S system. The configuration of each asymmetric carbon is determine individually, using the same rules as for compounds with one asymmetric carbon. The configuration of all asymmetric centres are then specified before the name of the compound. Identifying each asymmetric carbon by a number before the symbol R or S. For example, The compound 2R, 3S-3chloro-2-pentanol has the R configuration at carbon 2 and the S configuration at carbon 3.
GUIDELINES. Specifying Configuration of an Asymmetric Molecule by R and S Notations. Ordinarily you are given a Fischer projection formula and asked to specify configuration by R or S notation. It is easy to do so if you follow the following step-by-step method. Consider the following example:
Order of priority.
- Br (atomic number 79.9) (Highest)
- Cl (atomic number 35.5)
- CH3 (C, atomic number 12)
- H (atomic number 1) (Lowest)
Step 1. Determine the priority of group attached to the asymmetric carbon. Apply the sequence rules carefully. In the above example, the order of priority is :
Br > Cl > CH3 > H
Step 2. Position the lowest priority group (in this case H) down and away from the observer. This is done by interchanging groups bonded to the asymmetric carbon. Remember that interchanging any pair of groups in a Fischer projection inverts the configuration. Interchanging two pairs of groups retains the original configuration. Thus, the interchange operation must always be done in pairs to avoid a configuration change.
In the above example, a dotted line has been used to highlight a bond between the asymmetric carbon and the lowest priority group (H). The interchange operations have been shown by dotted double-headed curved arrows.
Step 3. Specify the direction of decreasing priority of the three group (1 2 3). Ignore the lowest priority group. If the groups 1,2, and 3 are arranged in clockwise fashion, the configuration is R. If the groups occur in anticlockwise fashion, the configuration is S.
In the above example, the configuration is S because the group 1, 2, and 3 are arranged in anticlockwise fashion (shown by circular erc).