A C atom is asymmetric, if it has 4 different substituents.

A substituant has priority over another if the atom by which it is attached has a larger atomic number.

In the case where two substituents have the same atomic number, one considers their own substituents and then ranks them in decreasing order of priority by comparing them as integers ( for instance 311 >299 because 3>2 )

Here C has priority over C since the substituent atoms of these two C atoms ranked in decreasing order of priority give: Cl,H,H > H,H,H (like as 411>111)

Deux isomères R - S Appear if a C atom has four different substituents. The substituent with the lowest priority is turned to the back, then - the isomer (enantiomer)R has the substituents in decreasing order of priority in the direction of clockwise rotation. - the isomer (enantiomer) Shas the substituents in decreasing order of priority in the opposite direction of clockwise rotation.