Phenolphthalein is formed by condensation of phthalic anhydride and OH. Which structure shows colour in basic medium?

JEE Main Chemistry question

Phenolphthalein is formed by condensation of phthalic anhydride and . Which structure shows colour in basic medium?

(A)The neutral lactone (closed-ring) form, with both phenolic OH groups intact
(B)The ring-opened dianion/quinonoid structure with extended conjugation, giving the pink colourCorrect
(C)A structure retaining the lactone ring with only one phenoxide formed
(D)All of the above

Answer: (B)

Solution

In basic medium, NaOH opens phenolphthalein's lactone ring and deprotonates both phenolic groups, generating an extensively conjugated, resonance-stabilised quinonoid dianion; this extended

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Question ID VK-148631