We must first classify the four different groups attached to the C atom in order of importance, then rotate the molecule so that the least important group looks backward.
If the visit in order of importance of the three most important groups of the asymmetric C atom directed towards us is clockwise, then this is an enantiomer R, otherwise an enatiomer S
Of the two enantiomers of the previous example, we see that only the OH and CH 3 are exchanged. In general:
If two groups which are attached to the asymmetrical C atom of one enantiomer are exchanged, the other enantiomer is obtained
Here is a practical method to determine which enantiomer we have:
Exchange of OH and H to bring the less important backward !
Determination of the configuration of the new enantiomer
The configuration of the initial enantiomer is the opposite!