4-hydroxybenzaldehyde

http://www.chemspider.com/Chemical-Structure.123.html



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Figure 1: 4-hydroxybenzaldehyde Chemical Structure
The chemical structure of this molecule is displayed, making the parts of it clearly identifiable. The hydroxyl (OH) group is evident, along with the aromatic benzene. The aldehyde (CHO) portion is seen on the left of molecule.


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Figure 2: Complete HNMR of 4-hydroxybenzaldehyde
An extremely sharp peak is evident around 4.8 ppm, along with smaller peaks around 6.9 ppm, 7.8 ppm, and 9.6 ppm.


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Figure 3: Enlargement of Peak around 4.8 ppm
This peak was the abnormally large peak found on the complete HNMR. It relates to the hydroxyl group of the molecule. OH groups are usually found are 4.5 ppm, and this particular peak is probably slightly higher due to the fact that it is attached to an aromatic ring.
[If the spectrum was taken in D2O as solvent the 4.8 ppm peak is HDO from the exchange with the phenol - you can check the solvent by right clicking on the spectrum and looking at properties JCB]

Correction: Since the solvent was D2O, the peak at 4.8 ppm is due to the HDO formed because of the interaction with the hydroxyl group attached to the benzene ring. This then explains why the peak around 4.8 ppm was abnormally large, because the solvent must have been in excess, warranting having a large peak as a result.


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Figure 4: Enlargement of Peak around 9.6 ppm
There is one peak found right around 9.6 ppm. It is the defining peak for the aldehyde (CHO) group in the molecule since aldehydes can be between 9 and 10 ppm. This is slightly on the higher side probably due to the connection to the aromatic ring.


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Figure 5: Enlargement of Peaks around 6.9 ppm and 7.8 ppm
Having been able to relatively clearly identify the other groups of the molecule, the only portion left to address is the aromatic ring itself. Generally, aromatic rings around found between 7 and 8 ppm. This matches the peaks left in the graph because they are relatively in that range. There are 2 doublets featured and in order to find which C-H they match to, it is necessary to consider which end of the molecule, the OH or CHO, would cause the number to be higher. Being attached to an aldehyde would cause the HNMR value to be slightly higher, although still in the range for aromatic rings. Thusly, the peaks around 7.8 ppm belong to C2 and C6, leaving C3 and C5 with the peaks around 6.9 ppm.
[You have to provide evidence for your assignments of the ring hydrogens - hint: look at the NMRs of similar molecules JCB]

Comparing with the HNMR for 3-(4-hydroxyphenyl)-2-oxo-propanoic acid (http://www.chemspider.com/Chemical-Structure.954.html), which also contains a hydroxyl group that is at a para position with another functional group, two doublets are also found. One is around 6.9 ppm and another around 7.13 ppm. The doublet at 6.9 ppm is consistent with that for 4-hydroxybenzaldehyde, providing evidence of the 6.9 ppm doublet being a result of the OH (ortho). Furthermore, the textbook also talks about upfield movement on an HNMR spectrum resulting from hydroxyl groups, along with alkoxy and amino groups. Downfield movement can be a result of carbonyl groups, along with nitro and cyano groups.
[Good analysis JCB]