Among the following groups, what is the correct priority ordering in the R,S system?
A) (i) (ii) (iii) (iv)
B) (ii) (i) (iv) (iii)
C) (iv) (ii) (i) (iii)
D) (ii) (iv) (i) (iii)

Question

Among the following groups, what is the correct priority ordering in the R,S system? 
A) (i) > (ii) > (iii) > (iv) 
B) (ii) > (i) > (iv) > (iii) 
C) (iv) > (ii) > (i) > (iii) 
D) (ii) > (iv) > (i) > (iii)

Quizplus · Accepted Answer

In the R,S system, priority is assigned based on the atomic number of the substituent group directly attached to the chiral center. In compound (i), the chiral center is attached to a carbon with a chlorine atom (atomic number 17), while in compound (ii), the chiral center is attached to a carbon with a bromine atom (atomic number 35). Therefore, compound (ii) has higher priority than compound (i). In compound (iv), the chiral center is attached to a carbon with an oxygen atom (atomic number 8), which has lower priority than both the chlorine and bromine atoms. Thus, compound (ii) > compound (iv). Finally, in compound (iii), both substituent groups attached to the chiral center are carbon atoms, so priority is determined by the atomic number of the substituent attached to the second carbon atom. In this case, that substituent is a methyl group (atomic number 6), which has lower priority than all three of the substituents in the other compounds. Therefore, the correct priority ordering is compound (ii) > compound (iv) > compound (i)> compound (iii).

Among the Following Groups, What Is the Correct Priority Ordering