4-Nitrophenol

4-Nitrophenol

Names
Preferred IUPAC name 4-Nitrophenol
Other names p-Nitrophenol[1] para-Nitrophenol 4-Hydroxynitrobenzene PNP
Identifiers
CAS Number	100-02-7 Y
3D model ( JSmol )	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 CID	980
UNII	Y92ZL45L4R Y
CompTox Dashboard (EPA)	DTXSID0021834
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	139.110 g·mol−1
Appearance	Colourless to pale yellow crystals
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 verify (what is YN ?) Infobox references

4-Nitrophenol (also called p-nitrophenol or 4-hydroxynitrobenzene) is a

at the opposite position of the hydroxyl group on the benzene ring.

Properties

4-nitro phenol is a slightly yellow, crystalline material, moderately toxic.

It shows two

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.^[2]

Preparation

From phenol

4-Nitrophenol can be prepared by

and 4-nitrophenol.

Uses

pH indicator

4-Nitrophenol (pH indicator)
below pH 5.4		above pH 7.5
5.4	⇌	7.5

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.^[3] 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).^[4] In contrast, 4-nitrophenol has a weak absorbance at 405 nm (ε = 0.2 mM⁻¹ cm⁻¹).^[4] The isosbestic point for 4-nitrophenol/4-nitrophenoxide is at 348 nm, with ε = 5.4 mM⁻¹ cm⁻¹.^[5]

Other uses

• 4-Nitrophenol is an intermediate in the synthesis of
  4-aminophenol, then acetylated with acetic anhydride.^[6]
• 4-Nitrophenol is used as the precursor for the preparation of
  acetophenetidine, indicators, and raw materials for fungicides. Bioaccumulation
  of this compound rarely occurs.
• In
  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.^[7]

4-Nitrophenol is a product of the enzymatic cleavage of several synthetic substrates such as 4-nitrophenyl phosphate (used as a substrate for

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. Complete ionization of the alcohol functional group affects the conjugation of the pi bonds on the compound. A lone pair from the oxygen can be delocalized via conjugation to the benzene ring and nitro group. Since the length of conjugated systems affects the color of organic compounds, this ionization change causes the 4-nitrophenol to turn yellow when fully deprotonated and existing as 4-nitrophenolate.^[8]

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 ionized. The problem can be overcome by stopping the reaction with sodium hydroxide (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.

Toxicity

4-Nitrophenol irritates the eyes, skin, and respiratory tract.

in mice is 282 mg/kg and in rats is 202 mg/kg (p.o.).

See also

• Nitrophenols

References