[Jayelle]

Hi all, I'd greatly appreciate some assistance with my unknown identification. I've attached my spectra to this post.

So far, I know that the molecular weight is 151 and that there is a benzene ring (indicated by peak in mass spectra at 77). I also think that there is a carboxylic acid present since there were peaks at 1698 and 3147 cm^-1 in the IR. I've also made some other notes on the spectra, but I'm not entirely certain if they are accurate or not. I believe the formula should be C₈H₉NO₂, but I'm having trouble fitting the pieces together and drawing the actual structure.

Any help you could offer would be amazing!

Thanks in advance!

[OrgXemProf]

Hi, Jayelle:

The information that you provided suggests that you're on the right track. Your elemental formula, C₈H₉NO₂, appears to be supported by the given spectral data.

It nevertheless may be instructive to re-examine some aspects of spectral interpretation:

1. Mass spectrum: An intense fragment ion appears at m/z 107. Any ideas?

2. Proton NMR spectrum: If the unknown compound is a carboxylic acid, where in the proton NMR spectrum would you expect to find the CO₂H resonance signal?

The proton NMR spectrum also displays a two-proton absorption signal in the region delta 6.0-6.5. Any ideas?

3. Carbon-13 NMR spectrum: You indicated that the five-proton multiplet situated at delta 6.7-7.2 in the proton NMR spectrum of the unknown compound might correspond to aromatic (benzenoid) protons. If so, where would you expect to find the aromatic carbon atom resonance signals in the corresponding ¹³C NMR spectrum? 

4. IR Spectrum: Strong absorption signal at 3494 cm^-1 is ascribed to "3^o N-H"; however, tertiary amines don't possess an N-H bond.

With regard to very intense absorption signal at 1698 cm^-1: In addition to CO₂H, can you think of another functional group that might display intense absorption in this spectral region? Where in the carbon-13 NMR spectrum would you expect to find an absorption signal that corresponds to this "other functional group"?

5. After you have arrived at a final structure for your unknown compound you might visit the following website to simulate the (approximate) appearance of its NMR spectrum and also the appearance of its mass spectral parent ion region:

http://web.chemdoodle.com/demos/simulate-nmr-and-ms

6. And finally: After you have arrived at a final structure for the unknown compound: Can you explain why the two-proton absorption at delta 6.0-6.5 in the proton NMR spectrum of this compound appears as two separate peaks (not a doublet!!) rather than as a single peak that integrates for 2 H?