According to the modern Quinonoid theory of acid-base indicators, the visual colour change is due to a structural tautomerization. For phenolphthalein, the visual transition from colourless to pink co

Question

According to the modern Quinonoid theory of acid-base indicators, the visual colour change is due to a structural tautomerization. For phenolphthalein, the visual transition from colourless to pink corresponds to a shift from which structure to which structure?

Prashant Kotian · Accepted Answer

Correct answer: B. The Quinonoid theory states that acid-base indicators exist in two tautomeric structural forms: a benzenoid form and a quinonoid form. The benzenoid form is typically lighter in colour or colourless (as is the case for unionized phenolphthalein in acidic medium), while the quinonoid form has an extended conjugated -system making it deeply coloured (pink in basic medium). The visual transition to pink is a shift from Benzenoid to Quinonoid.

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