paratolyl

Paratolyl, also known as p-tolyl, is the aryl substituent derived from toluene by removing a hydrogen from the para position of the benzene ring relative to the methyl substituent. It corresponds to the 4-methylphenyl group and is commonly denoted as p-tolyl or 4-methylphenyl. In modern nomenclature, p-tolyl is preferred over the historical spelling paratolyl. Paratolyl is one of three tolyl isomers, the others being o-tolyl (2-methylphenyl) and m-tolyl (3-methylphenyl). As a substituent, p-tolyl is attached to the rest of a molecule at the 1-position, while the ring bears a methyl group at the 4-position.

In chemistry, the p-tolyl group is a widely used building block in organic synthesis. It appears in pharmaceuticals, dyes, and polymers, and is commonly introduced via cross-coupling reactions using p-tolylboronic acids or p-tolyl halides. The presence of the para-methyl substituent influences reactivity and steric properties relative to the o- and m-tolyl isomers.

As a structural moiety, paratolyl is considered a C7H7 substituent when viewed as a radical. It is not a standalone substance with a fixed set of properties; rather, it describes a fragment that can be incorporated into many different molecules. See also tolyl, o-tolyl, and m-tolyl.