uv-visible absorption spectra of aromatic compounds - phenols

The uv and visible absorption spectrum of phenol and 3-nitrophenol

Image adapted from https://www.nist.gov/ (solvent ?)

Phenol shows a λ_max of 275 nm in the ultraviolet region.

Phenol shows no absorption in the visible region, so it is colourless (solid or in solution)

 

Image adapted from https://www.nist.gov/ (solvent ?)

3-nitrophenol shows a λ_max of 275 nm in the ultraviolet region (same as phenol)

But, unlike phenol, 3-nitrophenol shows a 2nd λ_max at 340 nm, a peak still in the uv region, but this broad absorption band stretches to 450 nm into the visible region.

Therefore you are getting absorption of visible light photons in blue region, as a consequence 3-nitrophenol is a pale yellow as a solid or dissolved in a colourless organic solvent.

Note the addition of another different substituent group in the benzene ring alters the energy levels of the pi electrons, lowering them sufficiently to allow excitation with lower energy uv photons.

Methylbenzene (spectrum below) is colourless and the methyl group does not extend the conjugated system of pi electrons of the benzene ring.

However, in the case of 3-nitrophenol, the nitro group does extend the conjugated pi electron system.

The nitro group, O=N=O with its pi electrons, is directly attached to the benzene ring.

 

need a more colourful phenol

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