Nitrophenol

Nitrophenol refers to any phenol molecule in which a nitro group (-NO2) is attached to the benzene ring. The three principal isomers are 2-nitrophenol (ortho), 3-nitrophenol (meta), and 4-nitrophenol (para). They are typically colorless to yellow crystalline solids and act as weak acids, with phenolic hydrogen readily dissociating in water to give phenoxide and a relatively acidic solution. The exact acidity (pKa) values vary by isomer, generally falling in the approximate range of 7 to 9.

Production and properties vary by isomer, but nitrophenols are usually generated industrially by nitration of phenol or substituted phenols, which yields mixtures rich in the ortho and para isomers. Separation and purification are achieved by distillation or crystallization. Chemically, nitrophenols are reactive intermediates in organic synthesis and can undergo reductions to yield corresponding aminophenols, which are important in dye and pharmaceutical manufacture.

Applications are broad. Nitrophenols serve as intermediates in the synthesis of azo dyes, polyurethanes, and various agrochemicals. In analytical chemistry, 4-nitrophenol and related compounds are used as reagents and substrates; for example, 4-nitrophenyl phosphate is a common chromogenic substrate for alkaline phosphatase assays. Reductive conversion of nitro groups to amino groups affords aminophenols, including 4-aminophenol, a precursor to paracetamol (acetaminophen).

Safety and environmental aspects are important considerations. Nitrophenols are toxic and can be irritants to skin and eyes; they pose risks to aquatic life and require careful handling and waste treatment. They are typically managed under strict regulatory controls due to their health and environmental hazards.