PHENOLPHTHALEIN  , in organic

chemistry, a compound derived from phthalophenone, or diphenyl phthalide (formula I.), the anhydride of triphenyl-carbinol-ortho-carboxylic acid, which is obtained by condensing phthalyl chloride with benzene in the presence of aluminium chloride . The phthaleins are formed from this anhydride by the entrance of hydroxyl or amino groups into the two phenyl residues, and are prepared by condensing phenols with phthalic anhydride, phenol itself giving rise to phenolphthalein (formula II.) together with a small quantity of fluorane (formula III.), whilst resorcin under similar conditions yields fluorescein (q.v.) . The phthaleins on reduction yield phthalines, which are derivatives of triphenylmethane carboxylic acid; these reduction products are colourless and may be regarded as the leuco-compounds of the phthaleins, hus phenolphthalein itself gives phenolphthaline (formula IV.) . Dehydrating agents usually convert the phenolphthalines into anthraquinone derivatives .