Q.How will you convert phenol to phenolphthalein ?

Phenolphthalein, the classic acid-base indicator, is synthesised by the acid-catalysed condensation of two molecules of phenol with one molecule of phthalic anhydride.

Reaction: two molecules of phenol condense with one molecule of phthalic anhydride, in the presence of concentrated sulphuric acid (which acts as both catalyst and dehydrating agent), on heating: $2\,C_6H_5OH + C_6H_4(CO)_2O \xrightarrow[\Delta]{conc.\ H_2SO_4} \text{Phenolphthalein} + H_2O$ The product, phenolphthalein, is a well-known acid-base indicator (colourless in acidic/neutral solution, pink/magenta in basic solution above about $pH$ 8.2) widely used in acid-base titrations …

Phenol's hydroxyl group can be either removed entirely (reductive distillation with zinc dust, giving benzene) or replaced by an amino group (heating with ammonia and a Lewis-acid catalyst, giving aniline) — two classic reactions demonstrating the reactivity of the phenolic $-OH$.

Formation of X: when phenol vapour is passed over heated zinc dust, the $C-OH$ bond is reductively cleaved and the hydroxyl group is removed, replaced by hydrogen: $C_6H_5OH + Zn \xrightarrow{\Delta} C_6H_6\ (X,\ \text{benzene}) + ZnO$

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