enter image description hereWhich organic compound's NMR and IR are these? I have figured out that the compound is a 1,4 substituted benzene. And the substituents are probably not -COOH and -OH from the spectroscopy. Kindly help me in figuring the other groups out. (Data available: Melting Point ~ 116 °C, gives sooty flame)

  • $\begingroup$ @Soumik Das: The presence of two clean doublets in aromatic reagion of NMR would not agree with your suggestion. It may be 4-nitrophenol, which has $113-114$ $\mathrm{^0C}$ melting point. $\endgroup$ Jan 28 '19 at 16:38
  • $\begingroup$ The very strong signal at ~1350 would fit for C-NO2 $\endgroup$
    – Waylander
    Jan 28 '19 at 21:25
  • $\begingroup$ Why is there no C-H stretch band in the IR spectrum near 3000 $\ce{cm^{-1}}$ ? $\endgroup$
    – S R Maiti
    Jan 28 '19 at 21:41
  • $\begingroup$ @pas — When you present an NMR spectrum, you absolutely need to also give the nucleus, the frequency and the solvent. Nevertheless, I agree with Mathew on the 4-nitrophenol. The chemical shift of OH group can vary A LOT depending on the solvent, the temperature and the pH which, in turn, depends on the concentration. In this case, p-nitrophenol is acidic, so the more concentrated it is in the NMR tube, the more acidic the pH and the more the signal will move to the left. $\endgroup$
    – SteffX
    Jan 29 '19 at 13:20
  • $\begingroup$ @MathewMahindaratne The signals of 1,4-substituted benzenes form a AA'XX' system; so despite their appearance for this sample and measurement they do not qualify as simple doublets. $\endgroup$
    – Buttonwood
    Jul 21 '19 at 21:36

The integrals and shifts in the very simple NMR spectrum are a giveaway that this is a simple compound with two different types of aromatic signal, and another non-aromatic signal from a substituent, at a 2:1 peak integral ratio, hence possibly a para-disubstituted benzene ring with one proton in one substituent, none on the other.

The IR spectrum is practically identical to that in the NIST webbook for p-nitrophenol. Some other clues:

  • Oop Signal at $\pu{800 cm^-1}$: para substitution
  • $\pu{1400-1600cm^-1}$: aromatic cc stretch, matches nicely with typical appearance for para substitution
  • No peak at $\pu{1700cm^-1}$: no carbonyl
  • Weak signals at $\pu{3000cm^-1}$: sp2 stretch, no sp3
  • Signals at $\pu{1350 and 1500cm^-1}$ may be nitro

Note this is a little tricky - a search would show some variation in reported spectra.

The melting point also matches reported values.

This is well in agreement with many of the comments under the original post.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.