See image — Practical Organic Chemistry and Purification Chemistry Question

💡 Solution & Explanation

Step 1 - Concept: Phenolphthalein in alkaline medium undergoes ring opening of the lactone (isobenzofuranone) ring by hydroxide ion, and deprotonation of one or both phenolic OH groups. This generates a highly conjugated dianion/monoanion responsible for the pink/magenta color. Step 2 - Mechanism: When phenolphthalein (the lactone form) reacts with OH⁻, the lactone C-O bond is cleaved, opening the five-membered ring to give a carboxylate (COO⁻) on the benzene ring. Simultaneously, deprotonation of one phenolic OH produces a phenoxide (O⁻), which causes that ring to adopt a quinoid (cyclohexadienylidene with C=O) structure. The other phenol ring retains its OH. This creates an extended conjugated system: carboxylate-benzene-C=C(central)-quinoid ring and a hydroxyphenyl group. Step 3 - Structure of complex A: The correct structure has (i) a benzene ring bearing an ortho-COO⁻ group, (ii) a central exocyclic sp2 carbon (=C<) connected to this ring, (iii) one quinoid ring (cyclohexadienylidene with para C=O, i.e., phenoxide deprotonated), and (iv) one para-hydroxyphenyl ring still bearing OH. This matches option (b), which shows the carboxylate on the phthalate-derived ring, a quinoid ring with para ketone oxygen (phenoxide), and a para-OH phenyl group, all in a planar, extended conjugated arrangement. Step 4 - Why other options fail: Option (a) shows two quinoid/modified rings or incorrect connectivity that does not match the known phenolphthalein dianion structure. Option (c) shows similar fragments but with incorrect regiochemistry or connectivity compared to the actual ring-opened phenolphthalein anion. Option (d) is eliminated because a well-known structure exists. Step 5 - The known pink form of phenolphthalein in base is the dianion where the lactone opens to carboxylate and one phenol deprotonates to give a quinoid structure, exactly as depicted in option (b). Therefore, the correct answer is B.