What is the expected multiplicity of the signal of the circled hydrogen atoms in a 1H NMR spectrum?

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To analyze the 1H NMR multiplicity, we apply the n+1 rule, where the number of lines in a signal is one more than the number of equivalent neighboring protons (n) that couple to the observed nucleus.

First, consider what the circled hydrogen atoms are adjacent to. If each circled hydrogen experiences coupling only with three equivalent protons on an adjacent carbon (for example, a CH group next to a CH3 group where the three hydrogens are equivalent), then the observed signal splits into 4 lines, i.e., a quartet.

Now evaluate potential alternatives conceptually:
- If the circled hydrogens had two equivalent neighboring protons, the splitting would be a triplet (n+1 = 3 lines).
- If there were four equivalent neighboring protons, the splitting would be a quintet (n+1 = 5 lines).
- If there were no neighboring protons (isolated singlet), the signal would be a singlet (n = 0, n+1 = 1 line).

Since the common scenario for a CH coupling to a methyl group yields three neighboring protons (n = 3), the resulting multiplicity is four lines, i.e., a quartet.

In summary, the quartet arises specifically from coupling to three equivalent neighboring protons, producing an n+1 pattern characteristic of this coupling situation.

MUF0042 Chemistry Unit 2 - Semester 1, 2025 8.20 Quiz: NMR Spectroscopy

What is the expected multiplicity of the signal of the circled hydrogen atoms in a 1H NMR spectrum?

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