The stereocenters of chiral compounds are labeled as R or S. The R for “right hand”, clockwise and L for “left hand”, anti-clockwise is used to name the enantiomers of a chiral compound. The priorities of the atoms directly connected to the stereocenters are given according to their atomic masses. If some of those atoms have the same mass, the next atom connected is concidred. In this case we have just halides. The lower down in a group of the periodic table, the higher the atomic mass. Therefore Iodine has the highest atomic mass and have the highest priority. Hydrogen has the lowest atomic mass and thus the it has the lowest priority as nr 4. The molecule with the lowest priority is places pioting away from the viewer. In this case when following the movement of the red arrow, the movement is left handed, anti clockwise and it is thus L. It is important to view the molecule from the correct side to get the rotation direction correctly.
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