pnitrophenolate

P-nitrophenolate is the conjugate base of p-nitrophenol, a para-nitro-substituted phenol. In aqueous solution it forms when p-nitrophenol is deprotonated by a base, such as sodium hydroxide. The resulting anion is yellow and exhibits a strong absorption near 405 nm, reflecting its extended conjugation.

This species is widely used as a chromogenic reporter in biochemical assays. In reactions that release p-nitrophenol, such as the hydrolysis of p-nitrophenyl esters, the product is converted to p-nitrophenolate under basic assay conditions, producing a measurable yellow color that can be followed spectrophotometrically.

Acid–base properties of p-nitrophenol influence the p-nitrophenolate form. The pKa of p-nitrophenol is about 7.1 at 25 °C, so the p-nitrophenolate anion predominates at modestly basic to strongly basic pH. The nitro substituent stabilizes the phenoxide via resonance, enhancing the acidity of the phenolic proton and affecting the color and reactivity of the anion.

Preparation of p-nitrophenolate is typically achieved by treating p-nitrophenol with a base (for example, NaOH) to generate the phenoxide. It can also appear transiently during the alkaline hydrolysis of p-nitrophenyl esters.

Safety considerations apply, as nitroaromatic compounds can be hazardous. Handle with appropriate personal protective equipment and follow proper disposal procedures.

See also: p-nitrophenol; p-nitrophenyl esters.