(R)and (S)Assignments To distinguish between the two enantiomers,one is called the(S)enantiomer,and the other the(R)enantiomer The(R)and(S)assignments are designated via the Cahn-Ingold-Prelog Convention. Each chiral center is designated either(R)or(S) There are two steps to assigning(R)or(S)to an enantiomer. 1)Assignment of"priority"to the groups bound to the asymmetric carbon 2)Using the“priority'”to decide on(R)or(S). 1)Assignment of Priority We look at the atoms directly bound to chiral carbon (a)Atoms with higher atomic numbers receive higher priorities. I>S>0>N>3℃>12C>Li>3H(T>2H(D)>H (b)In the case of the same atoms being bound directly to the chiral carbon,we go to the next atoms along the chain. -CH2Br>-CHCl2>-C(CH3)3>-CH(CH3)CH2F>-CH(CH3)2>-CH2CH; (c)Double and triple bonds are treated as if each bond were to a separate atom.(Imagine that each it bond is broken and that the atoms at both ends were duplicated). Ch05 Stereochemistry (landscape) Page 7 Ch05 Stereochemistry (landscape) Page 7 (R) and (S) Assignments To distinguish between the two enantiomers, one is called the (S) enantiomer, and the other the (R) enantiomer. The (R) and (S) assignments are designated via the Cahn-Ingold-Prelog Convention. Each chiral center is designated either (R) or (S). There are two steps to assigning (R) or (S) to an enantiomer: 1) Assignment of “priority” to the groups bound to the asymmetric carbon 2) Using the “priority” to decide on (R) or (S). 1) Assignment of Priority We look at the atoms directly bound to chiral carbon. (a) Atoms with higher atomic numbers receive higher priorities. I > S > O > N > 13C > 12C > Li > 3H (T) > 2H (D) > H (b) In the case of the same atoms being bound directly to the chiral carbon, we go to the next atoms along the chain. -CH2Br > -CHCl2 > -C(CH3)3 > -CH(CH3)CH2F > -CH(CH3)2 > -CH2CH3 (c) Double and triple bonds are treated as if each bond were to a separate atom. (Imagine that each  bond is broken and that the atoms at both ends were duplicated)