Nomenclature of enantiomers: R or S
We first have to classify the 4 different groups attached to the atom C in order of importance, then turn the molecule in such a way that the least important group points backwards.
Prefixes R and S
Definition
If the order of importance of the three most important groups of the asymmetric atom C directed towards us is in the direction of clockwise, then it is an enantiomer R , otherwise an enantiomer S
Exchange of two groups
On the two enantiomers of the previous example, we see that only the OH and CH 3 groups are exchanged.
In general:
If two groups of one enantiomer attached to the asymmetric atom C are exchanged, the other enantiomer is obtained.
Method
Here is a practical method to determine which enantiomer it is:
Exchange of OH and H to bring the least important back!
Determination of the configuration of the new enantiomer
The configuration of the initial enantiomer is the opposite!
Let's try to determine if this substance has the configuration R or S:
How is the hydroxyl group oriented ?
For answers, use (possibly multiple times) the arrows ↑ and ↓ at the top!
Complete please this question before moving on to the next one!
On the asymmetric C atom carrying the hydroxyl group is attached another atom. Which ?
If we exchange this $ H $ with $ OH $, the configuration changes $R\leftrightarrows S$
But then $ H $ (the least important) is in the right position to determine the CIP configuration: Let's go !
$O\gt C(C,H,H)\gt C(H,H,H) $
so R is the configuration after the exchange.
S is the real configuration of the initial molecule !
