4-Nitrophenol

4-Nitrophenol is a phenolic compound that has a nitro group at the opposite position of hydroxy group on the benzene ring. Also called p-nitrophenol or 4-hydroxynitrobenzene. Its molecular formula is C6H5NO3 and CAS registry number is 100-02-7.

Properties
4-Nitrophenol shows two polymorphs in crystalline state. The alpha-form is colorless pillars, unstable at room temperature, and stable toward sunlight. The beta-form is yellow pillars, stable at room temperature, and gradually turns red upon irradiation of sunlight. Usually 4-nitrophenol is existing as a mixture of these two forms.

In solution, 4-nitrophenol has a dissociation constant (pKa) of 7.08 at 22 °C. Solution of 4-nitrophenol alone appears colorless or pale yellow, whereas its phenolic salts tend to develop a bright yellow color. This color-changing property makes this compound useful as a pH indicator.

Uses
It is believed that aerobic decomposition of 4-nitrophenol in an environment is not very good. Bioaccumulation of this compound is considered to rarely occur. Mainly, 4-nitrophenol is used as the precursor for the preparation of phenetidine and acetophenetidine, indicator, and raw material of fungicides.

In the peptide synthesis, carboxylate ester derivatives of 4-nitrophenol may served as activated components for a constraction of an amide moieties.

Toxicity
4-Nitrophenol irritates eyes, skin, and respiratory tract; and may cause the inflammation of those parts. It makes delayed intaraction with blood and formes methaemoglobin that is responsible for methemoglobinemia, which may cause cyanosis, confusion, and unconsciousness. When ingested, one will get abdominal pain and vomiting. Prolonged contact with skin may cause allergic response. Genotoxicity and carcinogenicity of 4-nitrophenol are not known. LD50: mouse, p.o. 282 mg/kg; rat, p.o. 202 mg/kg.