Question

How to give a priority in halogen organic compounds?

Answer 1

Functional groups priority: halogens NEVER take priority. They are always substituents.Locant numbering priority: there are a number of rules
but in principle the functional group takes priority (i.e. never a halogen). In absence of other functional groups double and triple bonds would take priority. In absence of unsaturation, the first point of difference rule is valid and if that is still undecided you would need to check the alphabet (Of the name not the symbol: therefore it might be different depending on the language, but in English it is Br>Cl>F> I)

Priority in ordering of the name:that’s alphabetically. In the structures above bromo is mentioned before chloro in each caseWhen you want to assign priority in R/S or E/Z nomenclature following Cahn-Ingold-Prelog rules: the higher atomic number get priority.
 I > Br > Cl > F