Determine proton equivalence and signal count

Count environments, not atoms

Ethane's six protons are equivalent by symmetry and rapid rotation, so they give one ¹H environment.

CH₃CH₂Cl has two environments: CH₃ and CH₂, despite five proton nuclei.

Before counting signals, draw the complete structure and mark any symmetry operation or rapid rotation that interchanges protons. Counting carbon atoms or written groups is only a starting clue.

Chemical equivalence

Equivalent protons have the same chemical environment and chemical shift at the stated treatment.

A symmetry operation or rapid motion can interchange equivalent protons; merely sharing a carbon is not a universal proof.

The replacement test is useful: replace each candidate proton in turn by an imagined label. If the labelled structures are related by an allowed symmetry operation, the original protons are chemically equivalent at this level.

Use substitution and symmetry tests

Replace one candidate proton with a label and compare the resulting structures; symmetry-related replacements support equivalence at this level.

In propane the two terminal CH₃ groups are equivalent, while the central CH₂ is distinct: two signals with 6H and 2H integrals.

Equivalence is determined for the molecular situation stated in the question. Rapid rotation can average positions on the NMR timescale, while a stereogenic or otherwise asymmetric environment can make apparently similar protons non-equivalent.

Count propanone signals

The two CH₃ groups in CH₃COCH₃ are interchanged by symmetry.

All six protons form one environment, so propanone gives one 6H singlet in this simple treatment.

For propanone, a symmetry operation exchanges the two methyl groups and rotation exchanges the three protons within each methyl. All six therefore contribute to one environment.

Count ethyl ethanoate

Ethyl ethanoate is CH₃CO₂CH₂CH₃: a methyl group on the carbonyl carbon, then the ester oxygen, then an ethyl group.

Build an environment table with three columns: structural location, number of equivalent protons and reason for equivalence or difference. This prevents an integral from being mistaken for the number of signals.

Test each CH₃ against the other: is there a symmetry operation or rapid motion that interchanges them, or are their neighbours different?

Transfer to para-disubstituted benzene

para-Dichlorobenzene is a benzene ring with Cl at positions 1 and 4 and H at positions 2, 3, 5 and 6.

State the symmetry assumption you use and count sets of equivalent protons rather than individual H atoms.

One proton does not mean one signal

A signal belongs to an environment shared by equivalent protons.

Conversely, protons on one carbon may become non-equivalent in a chiral/asymmetric context, though use only supplied stereochemical information.

Protons joined to the same carbon are not automatically equivalent, and protons on different carbons are not automatically different. The molecular symmetry and stereochemical context decide.

Count before interpreting area

After the check questions, count the environments in methyl tert-butyl ether without notes. Return later for a different disubstituted-benzene question.

Try this next: use integration.

In an exam, show the grouping on the structure before stating the signal count. A bare number earns less confidence than a count supported by labelled environments and symmetry.