What Is Nitrophenol?
Nitrophenol is an organic compound where a nitro group is bonded to phenol. It exists in three forms, distinguished by the nitro group’s position: 2-nitrophenol (o-nitrophenol), 3-nitrophenol (m-nitrophenol), and 4-nitrophenol (p-nitrophenol). Among these, 4-nitrophenol is the most commonly utilized. It is known to irritate the skin and mucous membranes, necessitating caution to avoid prolonged exposure.
Uses of Nitrophenol
1. Synthesis of Phenetidine and Acetophenetidine
4-nitrophenol is employed in synthesizing phenetidine and acetophenetidine. Phenetidine, once used in the artificial sweetener dultin’s production, is now utilized as an intermediate for azo dyes and pharmaceuticals. Meanwhile, acetophenetidine, or phenacetin, serves as an antipyretic and analgesic. However, its usage has declined due to the risk of kidney damage from long-term, high-dose consumption.
2. PH Indicator
4-nitrophenol transitions from colorless in acidic solutions to yellow in alkaline solutions, making it useful as a pH indicator. This change occurs when the hydroxyl group on nitrophenol donates an H+ ion to the base OH-, forming the nitrophenol anion, which absorbs violet light and reflects yellow, turning the solution yellow.
3. Fungicides/Insecticides
Due to its strong protein coagulation and denaturation capabilities, nitrophenol is used as a fungicide and insecticide. Its effective penetration into bacteria also makes it suitable for wood preservation, combating wood-decaying fungi.
Properties of Nitrophenol
1. Acidity
Nitrophenol, with its nitro group (-NO2) attachment to phenol, exhibits relatively strong acidity. Phenols are more acidic than alcohols due to the stability of their conjugate base, which is enhanced by the electron-withdrawing effect of the nitro group, making nitrophenol more acidic than phenol itself.
2. Solubility
The polarity introduced by the nitro group enhances nitrophenol’s solubility in water and various organic solvents, as well as in alkaline solutions, due to its ease of ionization and the strong electron-withdrawing properties of the nitro group.
3. Crystal Polymorphism
4-nitrophenol exhibits two polymorphic forms in its crystalline state: the α form, a colorless columnar crystal unstable at room temperature but stable to sunlight, and the β form, a yellow columnar crystal that is stable at room temperature but turns red upon sunlight exposure.
Structure of Nitrophenol
Nitrophenol’s structure involves replacing one hydrogen atom on the benzene ring of phenol with a nitro group (-NO2), leading to three isomers based on the nitro group’s position relative to the hydroxyl group (ortho, meta, para). These isomers, namely 2-nitrophenol, 3-nitrophenol, and 4-nitrophenol, exhibit distinct properties such as melting points, boiling points, and solubility in water, influencing their reactivity and applications.
Other Information about Nitrophenol
Synthesis of Nitrophenol
Nitration of phenol with dilute nitric acid produces a mixture of 2-nitrophenol and 4-nitrophenol, which can be separated by distillation due to their different boiling points. Additionally, 3-nitrophenol can be synthesized from p-nitrochlorobenzene through hydrolysis and acidification or by diazotizing m-nitroaniline and hydrolyzing the diazonium group.