H-NMR for Benzaldehyde obtained from ChemSpider. [Note spectrum is inverted - can be corrected on JSpecView JCB]
Benzaldehyde_HNMR_complete.jpg
Figure 1: Complete HNMR for benzaldehyde.




























Figure 1 shows the complete H-NMR spectrum for benzaldehyde, with a sharp peak at about 10 ppm and a collection of peaks between 7 to 8 ppm.

Benzaldehyde_HNMR_7-8.jpg
Figure 2: Partial zoom of benzaldehyde HNMR showing the 7-8 ppm region.

Figure 2 shows a partial zoom of the benzaldehyde H-NMR, focusing on the 7-8 region. The theoretical H-NMR of benzaldehyde would show three groups of peaks in this region:

  1. A doublet of doublets should be seen around 7.5 corresponding to hydrogens meta to the aldehyde. With further distance from the aldehyde, these hydrogens should be more shielded than the ortho hydrogens and therefore have peaks toward 7 ppm. The doublet of doublets coupling arises from coupling with the ortho hydrogens as well as the para hydrogen.
  2. A doublet, probably toward the 8 ppm region corresponding to the hydrogens ortho to the aldehyde group. Since they are the closest to the aldehyde, these hydrogens should be the most deshielded and show peaks toward the end of the benzene region. They are also coupled by their neighboring hydrogens to produce a doublet.
  3. Finally, a triplet should be seen near 7 ppm corresponding to the hydrogen para to the aldehyde. At this position, the hydrogen should feel the least effect of the aldehyde and resonate at 7 ppm. The triplet arises from coupling with the neighboring meta hydrogens (1 on each side).

The actual NMR, shown in figure 2 is not as clean as the theoretical spectra, but can still be interpreted to an extent. As seen in figure 2, there is a doublet at 7.8 ppm (outlined in red), probably corresponding to the ortho hydrogens. As predicted, this region is the most deshielded and has a doublet pairing of peaks. Therefore, this is most likely the o-hydrogens. The blue-outlined region in figure 2 shows 6 peaks and is likely a composite of the meta and para hydrogen peaks. A symmetrical pair of doublets can be seen in this region, but the entire peak structure is too compressed to be certain of which peaks correspond to which hydrogens.[You can tell - only one peak should be a triplet of triplets - but you'll have to zoom in more to see it JCB]
Benzaldehyde_HNMR_10.jpg
Figure 3: Partial zoom of benzaldehyde HNMR showing the 9-10 ppm region.

The peak shown in figure 3 matches the theoretical peak for an aldehyde hydrogen at 9-10 ppm. This peak corresponds to the carbonyl hydrogen in benzaldehyde.

Benzaldehyde_HNMR_structure.jpg
Figure 4: Comparison of Benzaldehyde structure and HNMR.

As Figure 4 shows, the experimental HNMR is consistent with the structure of benzaldhyde.



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