CHAPTER XIIINITRO DERIVATIVES OF PHENOL
GENERAL INFORMATION
AMONG nitrophenols 2,4,6-trinitrophenol - picric acid - is of the greatest
importance. The fact that this compound was first obtained as long ago as the mid-
eighteenth century can be explained by the exceptional ease with which nitro groups
can be introduced into the aromatic ring, due to the presence of the phenol group.
Three nitro groups can be introduced by nitrating with nitric acid of a relatively
low concentration, while concentrated acid enables four and even five nitro groups
to be introduced.
The presence of nitro groups enhances the acidic properties of the phenol group.
This is why the trinitro derivatives are also called acids, e.g. trinitrophenol is known
as picric acid and trinitroresorcinol as styphnic acid. These compounds readily
form salts with metals or bases. Polynitro derivatives of phenols also form addition
compounds with hydrocarbons, e.g. naphthalene.
According to Hantzsch, nitrophenols exist in two forms: a phenolic, colourlessform (I) and a quinonoid coloured one (IIa and IIb). This occurs with picric acid:
I IIa IIb
Due to the presence of a phenolic group nitrophenols are soluble in water and
their acidic character enables them to dissolve in aqueous solutions of bases with
the formation of the corresponding salts. Solutions of the salts are usually brightly
coloured due to the presence of form II. Strong acids precipitate nitrophenols from
their solutions as colourless products.
The ability of nitrophenols to appear in the coloured quinoid form has been
utilized in acidimetry, where some of them are used as indicators, e.g. m- and
p- nitrophenols, 2,4-, 2,6- and 2,5-dinitrophenols, nitrohydroquinone etc.
Of all the nitrophenols, picric acid has been known the longest. The first mention