Question:

1. Determine the structure of the carboxylic acid whose proton NMR spectrum appears below.
2. Draw the NMR spectrum you would expect from the corresponding aldehyde whose oxidation gives this carboxylic acid.
3. Point out two distinctive differences in the spectra of the aldehyde and the acid.

Proton NMR gives information about the number of hydrogen atoms present in a molecule, and this NMR usually gives sharp signals because of the most sensitive nucleus. We can determine how many hydrogen atoms are there from the chemical shift value.

Structure of carboxylic acid

NMR spectrum gives information about the chemical environment of the organic molecules. Two types of NMR one is proton NMR, and the other is carbon-13 NMR.By identifying the singlets, doublets, or multiplets, we can determine the molecule's structure.

(b) The multiplet between chemical shifts 2 and 3 is drawn as pentet; the splitting is equally by an aldehyde proton and group. The coupling constants were unequal, by which the actual splitting pattern will be a complex multiplet.

The NMR spectrum of aldehyde

The aldehyde has a chemical shift ranging between 9 and 10 due to carbonyl group anisotropy. Acid’s chemical shift is 9.5 and above.

(c) The difference between the spectra of aldehyde and acid is that, in the case of aldehyde proton, the chemical shift is between 9 and 10, not as far downfield as that of the acid proton.

In acid proton, it always appears as a singlet, whereas the aldehyde proton is split into a triplet by CH₂ group. The CH₂ is split up by an extra proton, giving multiplet having complex splitting.