What product would you obtain when phenol is treated with i) Benzene diazonium chloride? [1] ii) Phthalic anhydride? [1]

i) When phenol is treated with benzene diazonium chloride, an azo dye is formed. This reaction is known as electrophilic aromatic substitution (specifically, azo coupling). The product obtained is p-hydroxyazobenzene (also called phenol-p-azobenzene), where the diazonium ion acts as an electrophile and couples with the phenol ring at the para position relative to the –OH group.

phenolphenol+C₆H₅N₂+ Cl−NaOH (alkaline medium)p-Hydroxyazobenzene (azo dye)p-Hydroxyazobenzene (azo dye)

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ii) When phenol is treated with phthalic anhydride, it undergoes esterification to form phenolphthalein, a well-known pH indicator. The reaction involves the nucleophilic attack of the phenolic –OH group on the anhydride, followed by ring closure and dehydration.

phenolphenol+C₈H₄O₃phthalic anhydrideHeat, conc. H₂SO₄ (catalyst)Phenolphthalein (pH indicator)Phenolphthalein (pH indicator)