pNitrophenyl

pNitrophenyl, commonly written as p-nitrophenyl, designates the para-nitro-substituted phenyl group, a benzene ring bearing a nitro substituent at the 4-position relative to the point of attachment to the rest of the molecule. The two substituents—nitro and the linking group—are arranged para to each other.

In organic synthesis, the p-nitrophenyl moiety is widely used as a leaving group. Its electron-withdrawing nitro substituent stabilizes the developing negative charge on the phenoxide anion, facilitating cleavage in esters and carbonate derivatives. Consequently, p-nitrophenyl esters and related activated derivatives are common intermediates in peptide and nucleic-acid chemistry, as well as in the preparation of aryl esters.

In biochemistry and enzymology, p-nitrophenyl derivatives are employed in colorimetric assays. Of particular importance is p-nitrophenyl phosphate: hydrolysis releases p-nitrophenol, which, under alkaline conditions, is converted to the yellow p-nitrophenolate anion with strong absorption near 405 nm. This chromogenic response allows straightforward measurement of enzymatic activities such as phosphatases.

Preparation is typically by nitration of phenol derivatives to give the para isomer, or by introduction of nitro groups into a phenyl precursor followed by functionalization to install the linking substituent. Safety notes apply due to the toxicity of nitroaromatics and the irritant nature of phenolic compounds.