Phenolphthalein is an acid–base indicator that is colorless below pH 8 and red above pH 8. Explain briefly why the first structure is colorless and the second structure is colored.
Here we have to explain why phenolphthalein of pH<8 is colorless and pH >8 is red.
In the compound A, the central C-atom is sp3, hybridized and this carbon atom is attached to the lactone ring oxygen. Due to the lack of delocalization in the molecule , it is colour less in acidic medium.