37.3.2—Carbon-13 NMR peak count

The number of carbon-13 NMR signals equals the number of chemically distinct carbon environments in the molecule. Symmetry and rapid equivalence reduce the count.

Label carbons, compare their attached groups and surroundings, then merge positions related by a symmetry operation. Do not count equivalent carbons twice.

Butan-2-one has four carbons but three environments: the terminal methyl groups are not equivalent because one is next to C=O and the other is not.

Signal count is a structural count, not a carbon atom count and not a direct measure of signal height.