Interpret first-order spin–spin splitting and multiplicity

Neighbours split one environment

In CH₃CH₂Br, CH₃ is split by two equivalent CH₂ protons into a triplet; CH₂ is split by three CH₃ protons into a quartet.

Use n+1 only for first-order coupling to n equivalent neighbouring protons under the supplied simple conditions.

Use splitting only after identifying which protons form one signal. Then count equivalent neighbouring protons in the simple first-order environment stated by the question.

Multiplicity and coupling

Spin–spin coupling splits a signal according to neighbouring nuclear spin states.

Singlet, doublet, triplet, quartet and septet describe line count; they do not state integration.

The n + 1 rule predicts line number, while Pascal's triangle predicts the relative line intensities for ideal first-order multiplets. Neither rule gives the signal's integral or chemical shift.

Read an ethyl pattern

A 3H triplet plus 2H quartet with matching coupling supports CH₃CH₂. Pascal intensities are 1:2:1 for a triplet and 1:3:3:1 for a quartet.

Equivalent protons do not split one another in this first-order treatment; labile-proton coupling may be absent through exchange.

Coupled partners share the same spacing, called the coupling constant. Matching spacing in a triplet and quartet is useful evidence that the two signals belong to the same ethyl fragment.

The simple rule can fail when neighbouring sets are non-equivalent or strongly coupled. Stay within the first-order cases supplied rather than forcing every complex multiplet into one n + 1 label.

Assign bromoethane

CH₃ has n = 2 neighbours, so n+1 = 3; CH₂ has n = 3, so four lines.

Combine with integrals 3H and 2H rather than assigning from multiplicity alone.

For bromoethane, show the reasoning in both directions: CH₃ sees two CH₂ neighbours and becomes a triplet; CH₂ sees three CH₃ neighbours and becomes a quartet.

Recognise isopropyl

Supplied data: a 1H septet and a 6H doublet with the same coupling constant.

Use n + 1 to count the neighbours each multiplicity implies, then check that the two signals can be coupled to each other.

The isopropyl pattern is convincing only as a pair, with matching coupling.

Transfer to propan-2-ol

Structure: propan-2-ol, (CH₃)₂CHOH.

Ignore coupling to OH for the carbon-bound signals, then say separately how the OH signal may appear.

Predict the carbon-bound signals first, then state separately that rapid O–H exchange may remove observable coupling. This keeps the structural rule and experimental limitation distinct.

Do not split a group by itself

The three equivalent protons within one CH₃ environment do not split one another.

Count equivalent neighbours on adjacent atoms in the stated first-order system.

Equivalent protons within one signal do not split one another in this treatment. A methyl group is a quartet only when it has three suitable neighbouring protons, not because it contains three protons itself.

Combine n+1 with shift and area

After the check questions, interpret the ether splitting pattern without notes. Return later for a different isopropyl-ketone question.

Try this next: use TMS and δ.

A complete annotation gives integral, multiplicity, neighbour count and relative line intensities, then checks that the paired signals describe a chemically possible fragment.