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Task:
Can you explain why there are 8 peaks in the NMR of 2-methyl-4-heptanol while there are only 7 peaks in the NMR of 2-methyl-4-heptanone?
Solution:
The number of peaks in the NMR spectrum depends on the number of nonequivalent hydrogen atoms in the structure of the compound.
2-methyl-4-heptanol contains 9 sets of hydrogen atoms (3 methyl groups (CH3), 3 methylene groups (CH2), 2 methine groups (CH) and one hydroxyl group(OH). However, neighboring methyl groups (at the second carbon atom) are equivalent and have one common signal in the spectrum. Therefore, in the end, we get 8 nonequivalent hydrogen atoms and 8 signals in the NMR spectrum. It should be noted that hydroxyl peaks will disappear in the presence of water. Only if the proton exchange rate is very slow, you will observe them.
2-methyl-4-heptanone contains 7 sets of hydrogen atoms (3 methyl groups (CH3), 3 methylene groups (CH2) and one methine group(CH). However, neighboring methyl groups (at the second carbon atom) are equivalent and have one common signal in the spectrum. Therefore, in the end, we get 6 nonequivalent hydrogen atoms and 6 signals in the NMR spectrum.
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