Among aniline and anilinium ion, in which auxochromic effect is more pronounced and why?

The auxochromic effect is more pronounced in the anilinium ion compared to aniline. This is because the anilinium ion has a positive charge, which makes it more electron deficient and therefore more reactive towards other molecules. This increased reactivity allows the anilinium ion to form stronger interactions with other molecules, leading to a more pronounced auxochromic effect. Additionally, the positive charge on the anilinium ion can also lead to stronger interactions with solvent molecules, further enhancing the auxochromic effect.

Aniline shows a more pronounced auxochromic effect compared to anilinium ion. This is because:

Resonance in Aniline:

Aniline has a lone pair of electrons on the nitrogen atom that can participate in resonance with the benzene ring. This resonance extends the π-electron cloud, increasing the conjugation and wavelength of absorbed light.

Loss of Resonance in Anilinium Ion:

In anilinium ion, the nitrogen atom is protonated, which removes the lone pair of electrons. As a result, the resonance between the nitrogen and benzene ring is lost, decreasing the conjugation and wavelength of absorbed light.

Increase in Charge Separation:

The protonation of aniline to form anilinium ion results in charge separation between the positively charged nitrogen atom and the negatively charged benzene ring. This charge separation reduces the electron density in the benzene ring, further decreasing the conjugation and wavelength of absorbed light.

Therefore, due to the loss of resonance, increase in charge separation, and reduced conjugation, the auxochromic effect is less pronounced in anilinium ion compared to aniline.