How to choose the base compound ? And start counting from that ? IS there is a prioty list ?

41 minutes ago, HbWhi5F said:

How to choose the base compound ? And start counting from that ? IS there is a prioty list ?

I don’t understand the question. Can you provide some background and more detail?

@exchemist

The functional group written on the end is considered as the "base" group I think

3 hours ago, HbWhi5F said:

@exchemist

The functional group written on the end is considered as the "base" group I think

I am having to guess at your meaning but what I think you may be trying to ask is which substituent is allocated position number one in the numbering convention.

It is normally done by choosing position 1 to give the lowest numbers for the other groups. So in the first example the left hand NO2 group is a poor choice since you then get 1, 3 nitro , 4 chloro, total 8. Similarly if you choose the right hand NO2 as position 1 you get 1, 5 nitro 2 chloro = 8 or (going round the other way) 1,3 nitro 5 chloro = 9, even worse. By choosing Cl as position 1 you get 1 chloro, 2,4 nitro = 7, which minimises the numbers.