Chemistry A2 · Analytical techniques. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
A carbon-13 NMR spectrum has one peak for each different environment of carbon atom in the molecule. Carbon atoms are in the same environment if they are joined to exactly the same atoms and groups, so that the molecule's symmetry makes them equivalent. Equivalent carbons give one shared peak.
The position of a peak is its chemical shift, δ, in ppm. It shows what kind of carbon it is: carbons bonded to oxygen, or part of C=C or C=O, have larger shifts.
Each peak is a single line. There is no splitting to work out, and peak heights do not show the number of carbon atoms.
Students count carbon atoms and not environments. Find the plane of symmetry first: in a benzene ring with one group attached, the six ring carbons give only four peaks.
Answer: Pentan-3-one
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