Matt's+analysis+of+Salicylic+Acid

By Matt Novin Figure 1: Molecule of Salicylic Acid Website data is retrived from: http://www.chemspider.com/Chemical-Structure.331.html Figure 2: Whole HNMR spectrum for Salicylic Acid taken by a Drexel graduate student. Figure 3: Single peak at ~2.5ppm. This is the peak generated by the solvent used for the spectrum – DMSO-d6. Figure 4: These two broad peaks are the functional groups on the benzene ring. Carboxylic acid and hydroxyl groups normally don’t show up with as clear resolution as other hydrogens would, and appear in this spectrum at ~11.5ppm and ~13.7ppm respectively. These groups show up so far down field due to hydrogen bonding. They also are not typically used as concrete proof to analyze molecules because of their unreliability. For instance, the normal range of carboxylic acid is 10-12ppm, and normal spectrums analyze between 0-10ppm. Figure 5: Overview of aromatic hydrogen peaks. Both hydrogens adjacent (ortho) to the functional groups will exhibit doublets from the hydrogens that are meta to the functional groups. Likewise, the hydrogens meta to the functional groups will exhibit triplets because they are adjacent to a hydrogen on either side of their aromatic positioning. All of the peaks are observed roughly between 7-8ppm which is consistent with aromatic hydrogens known chemical shifts. When a hydroxyl group is presence on an aromatic ring, it actually donates electron density into the pi ring of the system at the ortho and para positions. That being said, it is likely the triplet seen at ~6.88ppm is para to the hydroxyl group and the doublet next to it at ~7ppm is the hydrogen adjacent (ortho) to the hydroxyl group. The remaining curves are distributed to the hydogens closer to the carboxyl group. The triplet at ~7.45ppm being meta to it, and the doublet at ~7.79ppm being ortho to it. These are seen further downfield because of the slight deshielding present from the carboxyl group. I checked benzoic acid, o-toluic acid, methyl salicylate, methyl benzoate, benzene, toluene, phenol, any compound on the extra credit list that had a similar aromatic, and many more and the best I could find was 1-phenylethanone which has an HNMR spectra.
 * [Would you expect triplets for these hydrogens? JCB]**
 * [You have to provide more concrete evidence to truly assign each hydrogen in the ring - try to compare with NMRs of similar molecules for hints JCB]**

http://www.chemspider.com/Chemical-Structure.7132.html I do find it peculiar that there are triplet of triplets and doublet of doublets. Based on the N+1 rule, and assuming their neighboring hydrogens on the ring are equivilent, the hydrogens would show as solo doublet and triplets like the spectra shows. However, if there non-equivilent, they would be doublet of doublets instead of triplets. There might be some function or characteristic of aromatics i'm missing that might normally make this different. However for this spectra, the hydrogen assignments look to be correct. Based on the hydroxyl effect of strengthing the field (pg. 571 in Wade), it's only logical that the hydrogens ortho and para to the hydroxyl group are the doublet and triplet seen near 6.9ppm. From there, the most parsimonious explanation is that the ortho hydrogen is the doublet, and so on for the triplet. Under the same fashion, the remaining hydrogens can be dictated as doublet and triplet. It's not a concrete analysis, but most of the similar molecules don't have HNMR spectra. So it's more of a process of elimination.
 * [Looks good - in aromatic systems you get meta coupling as well (about 2Hz) so that causes more splitting JCB]**