The Benzene is a planar, six-membered cyclic ring structure with alternating double bonds that are responsible for its aromaticity. The Benzene is a simplest aromatic hydrocarbon made up of only carbon and hydrogen without any other substituents. The molecular formula of Benzene is C6H6.
On removal of one hydrogen of the benzene, a phenyl (Ph-) group is obtained that is written as C6H5-. If the lost hydrogen is replaced with Chloro, it becomes phenyl chloride (or chlorobenzene). On using phenyl as a side chain for propane, we get 1-phenylpropane. Replacement of two hydrogens of methane with phenyl group gives diphenylmethane and so on.
But when one hydrogen of the benzene ring is replaced with a hydroxy functional group, the molecule obtained is a combination of phenyl and alcohol called as the phenol. In other words, a phenol is the hydroxy derivative of benzene having the molecular formula C6H5-OH.
Adding a one carbon methylene (-CH2-) linker to the phenyl, C6H5-CH2- or Ph-CH2- gives a benzyl group (Bn).
Bn= C6H5-CH2- or Ph-CH2-
The carbon linker can be monosubstituted (C6H5-CHR-) or disubstituted (C6H5-CR2-). If the functional group present at the end of the alkyl chain is an amine, the molecule is called benzylamine (Bn-NH2). Other examples of different functional groups are benzyl alcohol (Bn-OH), benzyl bromide (Bn-Br), benzyl methyl ether (Bn-O-CH3), etc.
So far all the molecules encountered were the benzene derivatives. But the presence of an aromatic ring in a molecule is, in general, is written as an Ar-. The aromatic ring can be pure benzene, substituted benzene, naphthalene, heteroaromatic ring (thienyl, indolyl), etc. Few examples of aryl iodides are-
In summary, do note that both phenol and benzene are whole complete structures whereas aryl, phenyl, benzyl are groups or side chain substituents forming just a part of the entire molecule.
Shown below are all the groups in one hypothetical structure-