Reactions of phthalides and phthaleins in concentrated sulphuric acid

Abstract

The triarylmethyl cations produced by protonation of 3,3-diphenylphthalide and the phenolphthaleins are stable in aqueous sulphuric acid solutions but undergo further reaction in dilute oleum solutions. Cryoscopy, conductivity, and spectrophotometry show that 3,3-diphenylphthalide undergoes an irreversible cyclodehydration reaction to form 10-hydroxy-10-phenylanthracen-9-one and that the phthaleins undergo a reversible sulphation reaction resulting in closure of the lactone ring and consequently loss of colour. Phenolphthalein and 10-hydroxy-10-phenylanthracen-9-one also disulphonate in 99·9% H₂SO₄ and in oleums.