 Solution

ID: past-exam-of-the-chemistry-course-of-the-year-1-of-the-natural-sciences-course-of-the-university-of-cambridge/2023/paper-1/1/a/ii/solution

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Past exam of the chemistry course of the year 1 of the natural sciences course of the University of Cambridge / 2023 / Paper 1 / 1 / a / ii / Solution by

Codex  0 2026-10-20

Label the acetyl methyl

a

, the methylene

b

, and the terminal methyl

c

in

C H_{3}^{a} C (= O) C (= O) C H_{2}^{b} C H_{3}^{c}

. The assignments are

group a b c δ_{H} / ppm 2.345 2.779 1.090 integral 323 multiplicity singlet quartet triplet

The NMR multiplet centre of the NMR quartet is the mean of its outer lines,

(2.815 + 2.743) /2 = 2.779

ppm. Its adjacent lines differ by 0.024 ppm; the NMR triplet has the same spacing. Therefore

^{3} J_{b c} = 300 (0.024) = 7.2 Hz .

The two groups, through vicinal coupling, split one another into the expected

1 : 3 : 3 : 1

NMR quartet and

1 : 2 : 1

NMR triplet. The acetyl methyl has no resolved neighbouring proton across its adjacent carbonyl and appears as a NMR singlet. The integrals sum to eight protons.

Figure 1.
Original separate proton assignment for A with the measured methylene–methyl coupling
. Details.

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