phenyl vs benzene?Watch
I haven't been finding this the easiest thing either!
What you have to do is learn the functional group priority list to see which functional group is the 'main' one.
In the case of that the benzene thing is of higher priority then the molecule is named with the suffix -benzene and the prefix is left to be any other functional group. e.g. chlorobenzene, Nitrobenzene.
In the case the benzene ring is not the group of the highest priority then the molecule is named as having a phenyl group attached with the prefix Phenyl- or phen-.
After a bit of practice you can usually get the hang of it and see that the halogens, alkyl and nitro groups are the ones that are of lower priority and so end in -benzene
and pretty much everything else is of higher priority e.g. amines, alcohols, carboxylic acids and alkene groups.
I've left a priority list for groups below.
Hope this helps,
-Phenyl can always be used (i.e phenyl chloride) but it is not IUPAC to use it when benzene "can" be used (in small substituents). Phenyl is often used when the benzene group is attached to a 6 carbon chain (i.e 2-phenyloctane) or is a part of a more relevant/composite group (phenyl ethanoate as an ester)
There is also the benzyl group, which is a benzene ring where one hydrogen is substituted by a CH2 group with a substituent (i.e benzyl chloride, benzyl alcohol) which then we can name with multiple substitutions as 2,4-difluorobenzyl chloride where the chloride would refer to the substituent on the CH2 group attached to the C1 of benzene.
I will assume you don't need to know about the ortho, meta, para nomenclature, but there's rules for that as well.