Discussion
The number of signals expected for the following compound in 1H-NMR is :
Answer: C
The compound shows 06 signals.
The lactone is 4-methyldihydrofuran-2(3H)-one, with a stereocentre at C4 (the carbon carrying the methyl). Count the proton environments:
- the CH3 group (a doublet);
- the C4 methine H;
- the two H atoms of C3, next to the carbonyl;
- the two H atoms of C5, next to the ring oxygen.
Because C4 is a stereocentre, the two protons of each neighbouring CH2 group are diastereotopic: one is cis to the methyl and one trans, so they are chemically different and give separate signals. That gives 1 + 1 + 2 + 2 = 6 signals.
03 counts only the kinds of carbon bearing H, and 05 treats one CH2 as a single environment; both overlook diastereotopicity. 08 counts each of the eight protons separately, but the three methyl protons are equivalent and give one signal.
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