Final answer:
The benzene ring is referred to as a phenyl group when it acts as a substituent. The nomenclature for aromatic compounds with substituents uses numbers or common prefixes (ortho, meta, para) to describe their positions on the ring.
Step-by-step explanation:
The benzene ring is called a phenyl group when it is considered as a substituent on a longer chain of carbon atoms. This occurs when a benzene ring is missing one hydrogen atom, forming a -CH₂ group attached to the main carbon chain. The molecule 2-phenylbutane, for example, has a phenyl group bound to the second carbon atom of a butane chain. The systematic nomenclature of aromatic compounds involves numbering the positions of groups attached to the benzene ring, starting from 1 in the direction that gives the lowest possible numbers for substituents.
When two methyl groups, CH₃, are attached to a benzene ring and positioned adjacent to each other, the compound is called ortho-xylene or o-xylene. If they are opposite each other, it is known as para-xylene or p-xylene. An intermediate arrangement is referred to as meta-xylene or m-xylene. In other examples, an ethyl group (CH₃CH₂) attached to benzene forms ethylbenzene, and substituents like NO₂ and Cl provide names like nitrobenzene and chlorobenzene, respectively.