Concept:Picric acid is 2,4,6-trinitrophenol, formed by introducing three nitro groups into the phenol ring.Explanation:Phenol reacts with dilute nitric acid to give only mononitration products, i.e. ortho- and para-nitrophenols.Concentrated nitric acid alone oxidises phenol and produces tarry products instead of picric acid.Therefore, a nitrating mixture is required for complete nitration of phenol.A mixture of concentrated nitric acid and concentrated sulphuric acid is used for this conversion.Concentrated sulphuric acid helps generate the nitronium ion (NO2+​) and prevents oxidation of phenol.The nitration reaction is:C6​H5​OH+3HNO3​conc. H2​SO4​​C6​H2​(OH)(NO2​)3​+3H2​OThe product C6​H2​(OH)(NO2​)3​ is picric acid (2,4,6-trinitrophenol).
