substituent

As a noun substituent

is substituent.

As an adjective tetrasubstituted is

(organic chemistry) substituted with four substituents.

As a noun substituent

is any atom, group, or radical substituted for another, or entering a molecule in place of some other part which is removed.

As an adjective trisubstituted is

substituted with three (identical or different) substituents.

As nouns the difference between substituent and paraxylene

is that substituent is substituent while paraxylene is an aromatic hydrocarbon consisting of a benzene with two methyl substituents.