4tolyl

4tolyl, commonly written as p-tolyl, is the para-substituted methylphenyl aryl group used in organic chemistry as a substituent connected to the rest of a molecule through the benzene ring. It corresponds to the 4-methylphenyl group, indicating a methyl substituent at the 4-position relative to the point of attachment on the ring. The preferred IUPAC name for this substituent is 4-methylphenyl; tolyl is a traditional, non-systematic designation still widely encountered in the literature.

Structure and relation to toluene: The 4-tolyl group consists of a benzene ring bearing both a methyl group and a connection point to another fragment, with the methyl located para to the attachment site. This arrangement gives rise to distinct electronic and steric properties compared with its o- and m-tolyl (2- and 3-tolyl) isomers, influencing reactivity in aromatic substitutions and cross-coupling reactions.

Nomenclature and representations: In systematic nomenclature, the substituent is described as 4-methylphenyl. In common usage, 4-tolyl, p-tolyl, and p-methylphenyl are understood to refer to the same para-methylphenyl group. The group is typically described in context as a substituent, not as a standalone compound.

Applications and synthesis: The 4-tolyl moiety appears in a wide range of aryl compounds, including polymers, pharmaceuticals, dyes, and ligands. It is a frequent target in cross-coupling chemistry, where precursors such as 4-bromotoluene, 4-tolyl boronic acids, or 4-tolyl organometallic reagents are used to form C–C bonds. Its para orientation often imparts distinct steric and electronic effects compared with other tolyl isomers.

See also: o-tolyl and m-tolyl for the other positional isomers of the tolyl group.