Why does phenolphthalein turn white in acidic solution and pink in basic solution?
1) compounds have color when they absorb light at a wavelength that is within the visible light range 2) absorption of light at a particular frequency v occurs when there is an electron in the molecule at an orbital with exactly E=hv energy gap before the next free orbital. 3) In conjugated systems, electrons on the highest occupied pi-bonding orbital (HOMO) can jump up to the lowest vacant pi-antibonding orbital (LUMO) 4) As the length of the conjugated system increases, the gap between HOMO and LUMO decreases: ethene absorbs at 117 nm, butadiene at 217 nm, hexatriene at 238 nm (that’s all UV) 5) in the acidic/neutral form of phenolphthalein, there are three separate conjugated systems: para-C6H4-OH, another para-C6H4-OH, and an ortho-C6H4-C=O. Each of the three conjugated subsystems absorbs light in the UV range, thus visible light passes through unchanged. The molecule is colorless. 6) in basic pH, phenolphthalein has a much larger conjugated system: [O=C6H4=C-C6H4-O]-. This system
