Nomenclature of phenols

The hydroxyl derivative of benzene -- a benzene ring bearing a single -OH group -- is universally called phenol; its fully systematic IUPAC name is benzenol, though the common, shorter name 'phenol' is itself formally accepted as an IUPAC name too, and is what is used in practice throughout this chapter and elsewhere. When a phenol ring carries a second substituent, the common-name system distinguishes the three possible relative ring positions using the prefixes ortho (adjacent, 1,2-), meta (1,3-) and para (opposite, 1,4-) -- for example o-cresol for 2-methylphenol. The IUPAC system instead abandons these letter prefixes in favour of plain numerical locants (2-, 3-, 4-, and so on) counted around the ring from the -OH-bearing carbon, which is always numbered C-1 -- so the same compound o-cresol is IUPAC-named 2-methylphenol. This same locant convention extends naturally to di- and trihydroxy phenols: catechol (benzene-1,2-diol), resorcinol (benzene-1,3-diol) and hydroquinone/quinol (benzene-1,4-diol) among the dihydroxy phenols, and phloroglucinol (benzene-1,3,5-trio …