Aromatic nomenclature, when to use numbers or prefixes?

Question

When you're naming aromatic nomenclature compounds, when do use numbers, such as 2,4-diiodonitrobenzene and when do you use prefixes such as p-xylenene? Thanks! I hope that makes some sense

Answer 1

1,4-dimethylbenzene and p-xylene are both equally correct nomenclatures. You could also say 4-methyl toluene if you wanted. I personally prefer the first example because it allows me to visualize the molecule immediately.

As for 2,4-diiodonitrobenzene, this is indicating three substituents. You have the nitro group, and two iodo groups - one para to the nitro, and one ortho to the nitro.

Answer 2

Recurring to ortho, meta, para constrains the description to two subsitutents, and more often seen for simple "tradtional"compounds; like your example of p-xylene, or something like o-cresol.

Using numbers allows more flexibility, not only in regard of the number of subsitutents and their relative position to each other, but in reference to a hetero-aromatic ring system, too.

Addendum:

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