nitrophenols

Nitrophenols are organic compounds that consist of a phenol ring bearing a single nitro group. The three structural isomers—ortho-nitrophenol (2-nitrophenol), meta-nitrophenol (3-nitrophenol), and para-nitrophenol (4-nitrophenol)—differ in nitro placement. The nitro substituent is strongly electron-withdrawing, which increases the acidity of the phenolic OH relative to phenol.

Preparation and properties: They are typically produced by nitration of phenol with concentrated nitric acid under controlled conditions, with the hydroxyl group directing electrophilic nitration to o- and p- positions. As a result, mixtures containing o- and p-nitrophenol predominate; the meta isomer is less common. Physical forms range from colorless solids to yellow liquids; solubility depends on the isomer.

Uses: Nitrophenols serve as synthetic intermediates in dyes, pigments, pharmaceuticals, and agrochemicals. In biochemistry, p-nitrophenol derivatives are widely used as chromogenic substrates, and p-nitrophenol itself is employed as a pH indicator with a color change in basic solution.

Safety and environment: Nitrophenols are toxic and can be hazardous to aquatic life. They are relatively persistent and can be reduced to more toxic amines; exposure and waste handling require care and treatment. They may arise as pollutants from industrial processes, requiring appropriate controls.