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Analytical Techniques: Question 5

Syllabus 37.4

Multiple choice A2 1 mark

In the proton (1H^1\text{H}) NMR spectrum of a compound, one signal is a quartet integrating for 2H, and a second signal is a triplet integrating for 3H. No other signals are present, and there is no evidence of an exchangeable O–H or N–H proton.

Which structural fragment is consistent with this pattern?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Applying the n + 1 rule

The n + 1 rule states that a proton’s signal is split into (n+1)(n+1) lines by nn equivalent protons on the adjacent carbon. Testing an isolated ethyl group, −CH2−CH3-\text{CH}_2-\text{CH}_3 (flanked, on the far side, by an atom or group with no protons of its own. Such as an oxygen, a carbonyl carbon, or a halogen):

  • The CH2\text{CH}_2 protons are adjacent to the CH3\text{CH}_3 group (3 equivalent H), so n+1=4n+1 = 4: a quartet, integrating for 2H.
  • The CH3\text{CH}_3 protons are adjacent to the CH2\text{CH}_2 group (2 equivalent H), so n+1=3n+1 = 3: a triplet, integrating for 3H.

This is exactly the pattern described. A 2H quartet paired with a 3H triplet, and no other signals, because the CH2\text{CH}_2 and CH3\text{CH}_3 are coupled only to each other.

Why the other options are wrong

  • B: an isopropyl group, −CH(CH3)2-\text{CH}(\text{CH}_3)_2, has two equivalent CH3\text{CH}_3 groups (6H total) split into a doublet by the single adjacent CH\text{CH} proton, and the CH\text{CH} proton (1H) split into a multiplet (a septet, from 6 equivalent neighbouring protons), a doublet + multiplet, not a quartet + triplet.
  • C: a propyl group, −CH2−CH2−CH3-\text{CH}_2-\text{CH}_2-\text{CH}_3, has three distinct proton environments (terminal CH3\text{CH}_3, middle CH2\text{CH}_2, and the CH2\text{CH}_2 attached to the rest of the molecule), giving three signals rather than two.
  • D: an isolated CH3\text{CH}_3 group with no neighbouring protons on any adjacent carbon gives a single singlet, with no quartet involved at all.

Final answer

A, an isolated ethyl group, −CH2−CH3-\text{CH}_2-\text{CH}_3.