Question

Explain why sulfanilic acid and N,N-dimethylaniline absorb UV light while methyl orange absorbs visible light.

Answer 1

When ultraviolet visible light is observed by a molecule the energy goes into increasing the energy levels of valence or outer shell electrons. in other words when light is absorbed by the molecules these electrons which normally recite in the ground state are pushed up to higher energy level or to an excited state. If the energy level of the excited state is lot higher than that of the ground state a large amount of light energy is required to push the electrons up to the higher level and the correspondingly higher energy of light is needed to this. If the excited state is closer in energy to the ground state is correspondingly lower energy of light is needed.

4 molecules having conjugated system of electrons the ground state and excited state of the electrons are closer in energy than for non conjugated system this means the lower energy light is needed to excite electrons in conjugated system which means that lower energy light is absorbed by conjugated systems. Sulfanilic acid & N,N-Dimethylamine contains a aromatic ring where's only aromatic ring conjugation occurred & required higher energy for transition to excited state. While in methyl Orange, the C=N bond is conjugatedwith multiple bonds in the phenyl group. The absorption of light in the visible region which is responsible for the colour of certain compounds involves a lower energy transition as compared with the absorption of light in the ultraviolet region.