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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 (<?..). 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, thus phenolphthalein itself gives phenolphthaline (formula IV.). Dehydrating agents usually convert the phenolphthalines into anthraquinone derivatives.

Phenolphthalein is obtained when phenol and phthalic anhydride are heated with concentrated sulphuric acid. It crystallizes in colourless crusts and is nearly insoluble in water, but dissolves in dilute solutions of the caustic alkalis with a fine red colour, being reprecipitated from these solutions by the addition of mineral acid. It dissolves in concentrated caustic alkalis to a colourless solution which probably contains salts of a non-quinonoid character. This difference in behaviour has led to considerable discussion (see H. Meyer, Monats., 1899, 20, p. 337; R. Meyer, Ber., 1903, 36, p. 2949; A. G. Perkin and Green, Jour. Chem. Soc., 1904, p. 398). On fusion with caustic alkali, phenolphthalein yields benzoic acid and para-dihydroxybenzpphenone, which shows that in the original condensation the phthalic acid residue has taken the para position to the hydroxyl groups of the phenol.

Fluorane is a product of the condensation of the phthalic acid residue in the ortho position to the hydroxyl groups of the phenol, anhydride formation also taking place between these hydroxyl groups. It dissolves in concentrated sulphuric acid with a yellowishgreen fluorescence. The rhodamines, which are closely related to the phthaleins, are formed by the condensation of the alkyl metaaminophenols with phthalic anhydride in the presence of sulphuric acid. Their salts are fine red dyes.

Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)

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