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CAIE A-Level Chemistry A2 37.3 Carbon 13 Nmr Spectroscopy Questions

Practise counting carbon environments, assigning chemical shifts and testing possible molecular structures.

Syllabus
2028–2030
Course
Chemistry 9701
Level
A2

Exam points

  • treat each chemically distinct carbon environment as one 13C NMR peak and account for molecular symmetry
  • use chemical-shift ranges to distinguish alkyl, aryl, carboxyl and other carbonyl environments
  • combine peak count and shifts with formulae or chemical tests to accept or reject structural isomers

Covered syllabus points

37.3.1—Carbon-13 NMR interpretation37.3.2—Carbon-13 NMR peak count

Question 1

[Maximum number: 2]

Benzene can be used to make benzoic acid in the two-step process shown in Fig. 7.1.

Fig. 7.1

Fig. 7.1

Methylbenzene and benzoic acid each have five different peaks in the carbon (13C)\left({ }^{13} \mathrm{C}\right) NMR spectrum.

Table 7.1

Table 7.1

Use Table 7.1 to complete the two sentences to suggest descriptions of these two spectra.
The carbon (13C)\left({ }^{13} \mathrm{C}\right) NMR spectrum of methylbenzene:
- has peak(s) in the chemical shift range of and
- has peak(s) in the chemical shift range of .

The carbon (13C)\left({ }^{13} \mathrm{C}\right) NMR spectrum of benzoic acid:
- has peak(s) in the chemical shift range of and
- has peak(s) in the chemical shift range of

。

Question 2

[Maximum number: 1]

Asparagine is an amino acid that contains a chiral carbon atom and displays stereoisomerism.

Separate samples of asparagine are dissolved in CDCl3\mathrm{CDCl}_{3} and analysed using carbon-13 and proton (1H)\left({ }^{1} \mathrm{H}\right) NMR spectroscopy.

Fig. 6.1

Fig. 6.1

Predict the number of peaks seen in the carbon-13 and proton ( 1H{ }^{1} \mathrm{H} ) NMR spectra of asparagine.

Table for Question 2 — CAIE A-Level Chemistry A2

Question 3

[Maximum number: 3]

Question (a)

(a)

Deduce the number of peaks that would be present in the carbon-13 NMR spectrum of benzophenone.
number of peaks

[ 1 ]

Question (b)

(b)

Identify two different environments of carbon atom that would result in different chemical shift ranges in this carbon-13 NMR spectrum of benzophenone.

Table for Question (b) — CAIE A-Level Chemistry A2
[ 2 ]
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