o-nitrophenol is a compound that has numerous applications in the chemical industry. It
is notably used in the synthesis of dyes and as an intermediate in the production of
pigments, rubber and preservatives. o-nitrophenol can be prepared directly from phenol
in a reaction known as "nitration", a type of electrophilic aromatic substitution reaction.
Phenol reacts with hot concentrated nitric acid to give nitrophenol. This sluggish reaction
is hazardous because a hot mixture of concentrated nitric acid with any oxidizable
material might explode. A safer and more convenient procedure uses a mixture of nitric
acid and sulfuric acid. Sulfuric acid is a catalyst, allowing nitration to take place more
rapidly and at lower temperatures.
Sulfuric acid reacts with nitric acid to form the nitronium ion (+NO2), a powerful
electrophile. As resonance contributors of phenol indicate, the aromatic ring in phenol
possesses electron rich areas in the o and p position, thus producing o-nitrophenol and
p-nitrophenol in an electrophilic substitution reaction with +NO2.