Identify the colour of compound 'X' in the sequence of the reaction. [figure]
- Violet
- Green
- Red
- Colourless
Answer
(4) Colourless
Solution
Phenol and phthalic anhydride condense (conc. H2SO4) to phenolphthalein, a colourless lactone (central sp3 carbon).
With NaOH the lactone ring opens and both –OH groups ionise; the central carbon becomes sp2 and the dianion has a quinonoid structure with extended conjugation over the whole molecule ⇒ it absorbs visible light ⇒ pink.
In excess (strongly alkaline) NaOH, OH- adds to the central sp2 carbon giving the carbinol trianion X.
In X the central carbon is again sp3; the conjugation between the rings is broken and no quinonoid chromophore is left.
∴ X does not absorb in the visible region – it is colourless (this is why the pink colour fades in very strong alkali).
Answer: (4)