Briefly explain why 9anthraldehyde is yellow and the adduct

Briefly explain why 9-anthraldehyde is yellow and the adduct is colorless. Explain the differences in their UV-spectra. Compare your UV-visible spectra with that of 9-anthraldehyde.

Solution

9 -anthraldehyde is a conjugated polyene with 4 pi electrons,which tends to absorb light in the visible region ,of longer wavelength and lower energy,like any other conjugated multiple bond system.But its adducts do not have any such conjugated multiple bond arrangement ,so they absorb light in the UV region of the spectrum.

Thus ,9 -anthraldehyde is colored but its adducts are not.

explanation:

During a photon absorption, an electron jumps from HOMO (highest occupied energy orbital) to LUMO(lowest unoccupied molecular orbital).In a conjugated ,system ,the energy gap between HOMO and LUMO is narrow ,very less and the electrons are delocalized,free to move or resonated in the molecular orbitals.So,the jump from HUMO to LUMO takes place at the expense of less energy,a longer wavelength of light is absorbed   that falls in the visible spectrum.This reflects the compound being colored.