4-Nitrophenol

4-Nitrophenol

Names
Preferred IUPAC name 4-Nitrophenol
Other names p-Nitrophenol (no longer recommended) para-Nitrophenol 4-Hydroxynitrobenzene PNP
Identifiers
CAS Number	100-02-7 Y
3D model (Jmol)	Interactive image Interactive image
ChEBI	CHEBI:16836 Y
ChEMBL	ChEMBL14130 Y
ChemSpider	955 Y
DrugBank	DB04417 Y
ECHA InfoCard	100.002.556
KEGG	C00870 Y
PubChem	980
UNII	Y92ZL45L4R Y
InChI InChI=1S/C6H5NO3/c8-6-3-1-5(2-4-6)7(9)10/h1-4,8H Y Key: BTJIUGUIPKRLHP-UHFFFAOYSA-N Y InChI=1/C6H5NO3/c8-6-3-1-5(2-4-6)7(9)10/h1-4,8H Key: BTJIUGUIPKRLHP-UHFFFAOYAP
SMILES O=[N+]([O-])c1ccc(O)cc1 c1cc(ccc1[N+](=O)[O-])O
Properties
Chemical formula	C6H5NO3
Molar mass	139.11 g·mol−1
Appearance	Colorless or yellow pillars
Melting point	113 to 114 °C (235 to 237 °F; 386 to 387 K)
Boiling point	279 °C (534 °F; 552 K)
Solubility in water	10 g/L (15 °C) 11.6 g/L (20 °C) 16 g/L (25 °C)
Acidity (pKa)	7.15 (in water),
Magnetic susceptibility (χ)	-69.5·10−6 cm3/mol
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y  (what is YN ?)
Infobox references

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.

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.

In solution, 4-nitrophenol has a dissociation constant (pK_a) of 7.15 at 25 °C.

A solution of 4-nitrophenol appears colourless 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.

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.

Accurate measurement of enzyme activity requires that the 4-nitrophenol product is fully deprotonated, existing as 4-nitrophenolate, given the weak absorbance of 4-nitrophenol at 405 nm (see above). A common mistake in measuring enzyme activity using these substrates is to perform the assays at neutral or acidic pH without considering that only part of the chromophoric product is ionised. The problem can be overcome by stopping the reaction with NaOH or other strong base, which converts all product into 4-nitrophenoxide; final pH must be > ca. 9.2 to ensure more than 99% of the product is ionised. Alternatively enzyme activity can be measured at 348 nm, the isosbestic point for 4-nitrophenol/4-nitrophenoxide (see above).