Discussion
In the proton decoupled 13C NMR spectrum of 7-norbornanone, the number of signals obtained is
Answer: B
The spectrum shows 3 signals.
7-Norbornanone is bicyclo[2.2.1]heptan-7-one: the carbonyl sits on the one-carbon bridge. The molecule has C2v symmetry, with one mirror plane through C7 and both bridgeheads, which swaps the two ethano bridges, and a second through C7 and the midpoints of the C2-C3 and C5-C6 bonds, which swaps the two bridgeheads. That leaves three sets of equivalent carbons:
- C7, the carbonyl carbon;
- C1 and C4, the two bridgehead CH carbons;
- C2, C3, C5 and C6, the four CH2 carbons.
7 counts every carbon as different and ignores symmetry. 4 or 5 would need the two ethano bridges, or the two ends of each bridge, to differ, which the mirror planes rule out.
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