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Magazine Chapter 20: Q8P a. (page 1053)
Question:
- Determine the structure of the carboxylic acid whose proton NMR spectrum appears below.
- Draw the NMR spectrum you would expect from the corresponding aldehyde whose oxidation gives this carboxylic acid.
- Point out two distinctive differences in the spectra of the aldehyde and the acid.
Short Answer
Answer
- The structure of carboxylic acid is given below.
(b). The NMR spectrum is given below.
(c) 1) The aldehyde proton has a chemical shift between 9 and 10, not as far downfield as the acid proton.
2) The proton of aldehyde splits into a triplet by the group, unlike the COOH group, which always appears as a singlet.
Step by step solution
Proton NMR spectrum
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
- A 3H triplet at chemical shift 1.15 and a 2H at chemical shift 2.4 tell the pattern is ethyl. The only peak at chemical shift 11.9 is of the COOH group. The structure of carboxylic acid, whose proton NMR spectrum is given below:
Structure of carboxylic acid
NMR spectrum
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.
Structure of carboxylic acid
(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
NMR spectrum of aldehyde and acid
The aldehyde has a chemical shift ranging between 9 and 10 due to carbonyl group anisotropy. Acid鈥檚 chemical shift is 9.5 and above.
Difference between the spectra of the aldehyde and the acid
(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.
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