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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/c/i/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 / c / i / Solution by

Codex  0 2026-10-20

Use

a

for the acetyl methyl,

b

for the O-bound methylene group pair,

c

for the internal vinyl proton, and

d, e

for the terminal vinyl protons respectively trans and cis to

c

. The assignments are

protons a b c d e δ_{H} / ppm 2.081 4.570 5.919 5.314 5.235 integral 32111 resolved pattern singlet doublet of triplets doublet of doublets of triplets doublet of apparent quartets doublet of apparent quartets

The two terminal protons are diastereotopic: one is cis and the other trans to

c

, so they have different shifts and different vicinal couplings. Each terminal proton also couples through geminal coupling to the other and through long-range scalar coupling to the two

b

protons. When those small couplings coincide, an underlying doublet of doublets of NMR triplets collapses to the observed NMR doublet of apparent NMR quartets. The O-bound protons are enantiotopic and have chemical equivalence in NMR in this achiral environment; they are split by

c

and by the two terminal protons. The integrated terminal envelope contains two protons, not two equivalent protons at one shift.

Figure 1.
Original stereochemically labelled proton structure for C with all resolved vicinal, geminal and long-range couplings
. Details.

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