Question

Differenciate between para nitrophenol and meta nitrophenol on basis of uv spectroscopy?

Answer 1

4-Nitrophenol has the larger `λ_(max)`.

Explanation:

If we draw the resonance contributors the two isomers, we find that the lone pairs on the `"O"` atom can be delocalized onto the nitro group only in 4-nitrophenol.

This extended delocalization of electrons lowers the energy of the molecular orbitals.

Lower absorption energy means longer wavelength (`λ_text(max))`.

Thus, the UV spectrum of 4-nitrophenol has `λ_text(max) = "320 nm"`.

The UV spectrum of 3-nitrophenol has `λ_text(max) = "270 nm"`.