UV Absorbance Spectroscopy of Aniline
Aniline is a benzenoid compound. The NH2 group attached to the benzene ring means that there is a lone pair of electrons that can enter into conjugation with the benzene ring resulting in delocalization in the aniline.
Aniline absorbs in the K (220 - 250 nm) and the B (250 - 290 nm) bands exhibited by benzenoid compounds. The K and B bands arise from π to π* transitions as a result of the a group containing multiple bond being attached to the benzene ring. When dissolved in ethanol, λmax for aniline is 230 nm, but in dilute aqueous acid λmax is 203 nm. In the latter case the anilinium cation is formed and the lone pair is no longer available for conjugation with the benzene ring. Consequently, the absorption of the molecule shifts to the lower λmax value and behaves like benzene.
Read more about this topic: Aniline (data Page), Spectral Data