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. It belongs to the class of organic compounds known as nitrophenols. Nitrophenols are compounds containing a nitrophenol moiety, which consists of a benzene ring bearing both a hydroxyl group and a nitro group on two different ring carbon atoms. 4-Nitrophenol shows two polymorphs in the 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 exists as a mixture of these two forms. 4-Nitrophenol can be used as a pH indicator and as an intermediate in the synthesis of paracetamol. Itis also 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. 4-Nitrophenol is a potentially toxic compound: it can cause eyes, skin, and respiratory tract irritations. It may also cause inflammation of those parts. It has a delayed interaction with blood and forms methaemoglobin which is responsible for methemoglobinemia -which is characterized by tissue hypoxia, as methemoglobin cannot bind oxygen-, potentially causing cyanosis, confusion, and unconsciousness. When ingested, it causes abdominal pain and vomiting. Prolonged contact with skin may cause allergic response. Genotoxicity and carcinogenicity of 4-nitrophenol are not known. The LD50 in mice is 282 mg/kg and in rats is 202 mg/kg. Outside of the human body, 4-Nitrophenol has been detected, but not quantified in cow milk. Conjugates are more polar than the parent compounds and therefore are easier to excrete in the urine.