I am wondering if there is a difference in the maximum absorbed wavelength of light if nitrobenzene is added to an acidic or basic solution.

I've figured that resonance plays a part in some structures like phenol, where the electron density in the benzene ring increases when in basic solution and therefore the energy required to go from a $\pi$ to $\pi^*$ orbital decreases and thus $\lambda_{\text{max}}$ increases. But could resonance play a role in nitrobenzene too? And if it does, how?


Nitrobenzene does protonate at the nitrogroup in concentrated acid. Obviously, then the group becomes even more electron-withdrawing, and a bathochromic shift is observed. W. F. Forbes provided an excellent article for You in 1958, freely available now: https://cdnsciencepub.com/doi/pdf/10.1139/v58-200

The "B-band" (CT band from the ring onto the nitrogroup) of nitrobenzene is at 252 nm in hexane, at ca. 265–266 nm in neutral water, at 266 nm both in 1M NaOH and 0.1M HCl but at 287.5 nm in concentrated H2SO4.

I am not sure about the intensities Forbes et al. report, as newer sources (but not that rich in experimental material) cite the transition at 252 nm (in non-polar solvents) to have intensity about 10 times lower. Maybe another scale was in common use back then.


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