Which of the following substituents has the highest priority

Which of the following substituents has the highest priority according to the Cahn-Ingold-Prelog system? O -NH2 O -NHCH3 -CH2NHCH3 O -CH2NH2

Solution

According to the Cahn-Ingold-Prolog,

A substituent with highet atomic number has the higher proirity than the lower atomic number.so both A and B are equal atomic number of N=7.

so we follow the next atom bonding to the Nitrogen.

So the higher proirity is -NHCH3.