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

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

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 interpretation 37.3.2—Carbon-13 NMR peak count

[Maximum number: 3]

(a)

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

[ 1 ]

(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 ]

environment of carbon atom	chemical shift range (δ)
carbonyl / RCOR	190–220
arene / benzene	110–160

[Maximum number: 1]

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

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

Fig. 6.1

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

Table for Question 2 — CAIE A-Level Chemistry A2

& carbon-13 NMR & proton NMR

number of peaks in $\mathrm{CDCl}_{3}$ & 4 & 5

[Maximum number: 5]

(a)

A student analyses an aromatic compound, $\mathbf{X}, \mathrm{C}_{8} \mathrm{H}_{8} \mathrm{O}_{3}$ , using NMR spectroscopy.

$Fig. 6.4 shows the carbon-13 NMR spectrum of a sample of \(\mathbf{X$

Fig. 6.4 shows the carbon-13 NMR spectrum of a sample of \(\mathbf{X

Separate samples of X were analysed using proton $\left({ }^{1} \mathrm{H}\right)$ NMR spectroscopy.
Table 6.1 gives information obtained from this analysis.

Table 6.1

[ 5 ]

(i)

Identify the number of different carbon environments present in X.

[ 1 ]

(ii)

Aromatic compound X gives a yellow precipitate when it reacts with alkaline $\mathrm{I}_{2}$ (aq).

Use the information in (c) to suggest a structure for X.
Explain your reasoning.

\mathrm{X}, \mathrm{C}_{8} \mathrm{H}_{8} \mathrm{O}_{3}

Table 6.2

[ 4 ]

Figure for Answer to Question (ii) — CAIE A-Level Chemistry A2

Molecule must not be symmetrical (as eight different C environments)

Viable structures of X

- methyl ketone / $\mathrm{CH}_{3} \mathrm{CO}$ group because (yellow) ppt / $\ce{CHI3}$ formed (with alkaline $\left.\mathrm{I}_{2}(\mathrm{aq})\right)$

any two linked statements:

- two - $\ce{OH}$ groups

because ${ }^{1} \mathrm{H}$ NMR signals lost in $\mathrm{D}_{2} \mathrm{O}$

- ketone / carbonyl / $\mathrm{CH}_{3} \underline{\mathrm{CO}}$

as $\left({ }^{13} \mathrm{C}\right.$ NMR) signal with $\Delta=205 / 190-220 \mathrm{ppm}$

- methyl / carbon next to $\ce{C=O}$ because

$\left({ }^{13} \mathrm{C} \mathrm{NMR}\right)$ signal with $\Delta=30-65 \mathrm{ppm}$

All question bank results loaded

Covered syllabus points

Question 1

Question (a)

Question (b)

Question 2

Question 3

Question (a)

Question (i)

Question (ii)