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4-Nitrophenol (also called p-nitrophenol or 4-hydroxynitrobenzene) is a phenolic compound that has a nitro group at the opposite position of the hydroxyl group on the benzene ring.

Uses

4-Nitrophenol can be used as a pH indicator. A solution of 4-nitrophenol appears colorless below pH 5.4 and yellow above pH 7.5. This color-changing property makes this compound useful as a pH indicator. The yellow color of the 4-nitrophenolate form (or 4-nitrophenoxide) is due to a maximum of absorbance at 405 nm (ε = 18.3 to 18.4 mM−1 cm−1 in strong alkali). In contrast, 4-nitrophenol has a weak absorbance at 405 nm (ε = 0.2 mM−1 cm−1). The isosbestic point for 4-nitrophenol/4-nitrophenoxide is at 348 nm, with ε = 5.4 mM−1 cm−1.

Other uses

4-Nitrophenol is an intermediate in the synthesis of paracetamol. It is reduced to 4-aminophenol, then acetylated with acetic anhydride.
4-Nitrophenol is used as the precursor for the preparation of phenetidine and acetophenetidine, indicators, and raw materials for fungicides. Bioaccumulation of this compound rarely occurs.
In peptide synthesis, carboxylate ester derivatives of 4-nitrophenol may serve as activated components for construction of amide moieties.

Uses of derivatives

In the laboratory, it is used to detect the presence of alkaline phosphatase activity by hydrolysis of PNPP. In basic conditions, presence of hydrolytic enzymes will turn reaction vessel yellow.

4-Nitrophenol is a product of the enzymatic cleavage of several synthetic substrates such as 4-nitrophenyl phosphate (used as a substrate for alkaline phosphatase), 4-nitrophenyl acetate (for carbonic anhydrase), 4-nitrophenyl-β-D-glucopyranoside and other sugar derivatives which are used to assay various glycosidase enzymes. Amounts of 4-nitrophenol produced by a particular enzyme in the presence of its corresponding substrate can be measured with a spectrophotometer at or around 405 nm and used as a proxy measurement for the amount of the enzyme activity in the sample.