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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/2022/paper-1/1/b/iii/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 / 2022 / Paper 1 / 1 / b / iii / Solution by

Codex  0 2026-10-20

The two methyl groups give one six-proton NMR doublet centred at

(1.319 + 1.296) /2 = 1.3075

ppm. The methine gives a one-proton NMR septet centred near

2.976

ppm. The latter is split by six equivalent methyl protons, producing the first-order

1 : 6 : 15 : 20 : 15 : 6 : 1

intensity pattern.

^{3} J_{CH, C H_{3}} ≃ 6.9 Hz .

This follows exactly from the methyl doublet spacing,

300 (0.023)

, and approximately from the successive septet spacings, which alternate between

0.022

and

0.024

ppm within the precision of the printed labels. There is no methylene triplet or quartet: the six-proton integral and septet establish the branched skeleton.

Figure 1.
Independent original carbon and proton assignments for B; both equivalent methyl groups couple to the methine
. Details.

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