A compound has the molecular formula C5H10O and the 1H-NMR spectrum contains four peaks. The compound could be

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Question restatement: A compound with formula C5H10O and a 1H-NMR spectrum showing four peaks could be which of the given structures? Options: a) 3-methylbutan-2-one, b) pentan-2-one, c) pentanal, d) pentan-3-one.

Option a: 3-methylbutan-2-one. View this structure as CH3-CO-CH(CH3)-CH3. In this molecule there are four distinct types of hydrogen environments:
- the acetyl methyl group (the CH3 directly attached to the carbonyl), which should appear as a singlet or a small multiplet around ~2.1–2.3 ppm,
- the single methine proton (the CH) on C3, which is a unique environment due to being adjacent to both a carbonyl-bearing carbon and a methyl group,
- the methyl group attached to that same carbon (the CH(CH3) portion), which is a distinct environment from the acetyl methyl,
- the terminal methyl group at the end of the chain, which is different from the others.
Thus, four distinct proton environments are present, corresponding to four peaks in the 1H-NMR.

Option b: pentan-2-one. Structure: CH3-CO-CH2-CH2-CH3. This has several proton environments, but count carefully: the acetyl CH3 (3H) is one environment; the CH2 adjacent to the carbonyl (2H) is another; the middle CH2 (2H) is a third; and the terminal CH3 (3H) is a fourth. Depending on coupling, these could still collapse into fewer observed signals if some environments are equivalent by symmetry; however, pentan-2-one lacks symmetry that would merge the two terminal methyls, so you would typically expect more distinct signals than four, often close to four or five depending on splitting and coincidence. In strict counting without degeneracy, this option is less clearly consistent with a simple four-peak spectrum.

Option c: pentanal. Structure: CH3-CH2-CH2-CH2-CHO. This aldehyde-containing chain has several non-equivalent proton environments: the aldehyde proton (CHO, ~9–10 ppm) is a distinct peak, the aldehyde-adjacent CH2 (2H), the next CH2s (two distinct environments along the chain), and the terminal methyl (3H). That already yields at least five different proton environments, so a four-peak spectrum would be unlikely.

Option d: pentan-3-one. Structure: CH3-CH2-CO-CH2-CH3. Here there are several non-equivalent environments: the two terminal methyls are equivalent in this case, giving one signal for both ends, the CH2 groups adjacent to the carbonyl are distinct, and the methyl group attached to the far end is another. This arrangement generally yields more than four distinct proton environments or fewer only if coincidences occur, but the typical expectation would not pin to a clean four-peak pattern as clearly as the candidate in (a).

In summary, evaluating the count and differentiation of proton environments against the four-peak requirement points to a structure where the four unique environments align cleanly with four distinct signals in the 1H-NMR, which is most consistently satisfied by option a, given its arrangement CH3-CO-CH(CH3)-CH3 and the four clearly non-equivalent sets of protons it contains.

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

A compound has the molecular formula C5H10O and the 1H-NMR spectrum contains four peaks. The compound could be

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