CAIE A-Level Chemistry 37.4 Proton NMR Spectroscopy

Practise combining proton environments, integration, chemical shift, splitting and D₂O exchange evidence.

Exam points

use peak count for proton environments, integration for relative proton numbers and δ for group assignment
apply n+1 splitting to neighbouring non-equivalent hydrogens while recognising singlets and multiplets
use loss of OH or NH peaks after D2O exchange and the TMS δ 0 reference as supporting evidence

Covered syllabus points

37.4.1—Proton NMR interpretation 37.4.2—The chemical shifts and splitting patterns 37.4.3—Of tetramethylsilane, TMS, as the standard 37.4.4—Need for deuterated solvents, e.g. CDCl ₃,37.4.5—O-H and N-H proton exchange with D2O

[Maximum number: 5]

Amino acids are molecules that contain $-\mathrm{NH}_{2}$ and -COOH functional groups.
Glycine, $\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{COOH}$ , is the simplest stable amino acid.

(a)

Compound V is another amino acid.

The proton $\left({ }^{1} \mathrm{H}\right)$ NMR spectrum of V shows hydrogen atoms in five different environments, a , b , c, d and e, as shown in Fig. 6.2.

Fig. 6.2

Table 6.1

[ 5 ]

(i)

Complete Table 6.2 for the proton $\left({ }^{1} \mathrm{H}\right)$ NMR spectrum of V taken in $\mathrm{CDCl}_{3}$ .

Table 6.2

[ 4 ]

proton | a | b | c | d | e

$\delta$ | 9.0-13.0 | 6.0-9.0 | 2.3-3.0 | 3.2-4.0 | 1.0-5.0

splitting | singlet | multiplet | triplet | triplet | singlet

any three [1] any five [2] any seven [3] all nine [4]

(ii)

Complete Table 6.3 by placing a tick ( $\checkmark$ ) to indicate any protons whose peaks are still present in the proton $\left({ }^{1} \mathrm{H}\right)$ NMR spectrum of V taken in $\mathrm{D}_{2} \mathrm{O}$ .

Table 6.3

[ 1 ]

proton & a & b & c & d & e

present in $\mathrm{D}_{2} \mathrm{O}$ & & & & &

[Maximum number: 6]

(a)

State the number of peaks that would be seen in the proton ( ${ }^{1} \mathrm{H}$ ) NMR spectrum of methyl butanoate, $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{CH}_{3}$ . Name all the splitting patterns seen in this spectrum.
number of peaks
splitting patterns

[ 2 ]

(b)

D and E are both esters with the molecular formula $\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O}_{2}$ . Their proton $\left({ }^{1} \mathrm{H}\right) \mathrm{NMR}$ spectra are shown in Fig. 9.2 and Fig. 9.3.

Fig. 9.2

Fig. 9.3

Table 9.2

[ 4 ]

(i)

Deduce the structures of the two esters D and E and draw their displayed formulae in the boxes below.

\begin{gathered} \text { D } \\ \mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O}_{2} \end{gathered}

Figure for Question (i) — CAIE A-Level Chemistry A2

[ 2 ]

$=\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}[1] \mathrm{E}=\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCO}_{2} \mathrm{CH}_{3}$

(ii)

The spectrum of D includes a quartet at $\delta$ 4.1.

Identify the protons responsible for this quartet on your structure in (i) by labelling these protons with the letter F.

Explain why this peak is split into a quartet.

[ 1 ]

$\mathrm{O}-\mathrm{CH}_{2}$ labelled F

AND three protons on neighbouring carbon / adjacent $\mathrm{CH}_{3}$

(iii)

The spectrum of E has a doublet at $\delta$ 1.1.

Identify the protons responsible for this doublet on your structure in (i) by labelling these protons with the letter G.

Explain why this peak has a chemical shift of 1.1.

[ 1 ]

both $\mathbf{C H}_{\mathbf{3}}$ in isopropyl group labelled G

AND alkane / alkyl (protons)

[Maximum number: 5]

Lactic acid, $\mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{COOH}$ , is the only monomer needed to form the polymer polylactic acid, PLA.

(a)

The proton $\left({ }^{1} \mathrm{H}\right) \mathrm{NMR}$ spectrum of $\mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{COOH}$ in $\mathrm{CDCl}_{3}$ is shown in Fig. 8.1. The proton NMR chemical shift ranges are shown in Table 8.1.

Lactic acid

Table 8.1

[ 5 ]

(i)

Use Fig. 8.1 and Table 8.1 to complete Table 8.2.

Table 8.2

[ 3 ]

(ii)

Name the substance responsible for the peak at $\delta=0.0$ .

[ 1 ]

(iii)

Explain why $\mathrm{CDCl}_{3}$ is a better solvent than $\mathrm{CHCl}_{3}$ for use in proton NMR .

[ 1 ]

$\mathrm{CDCl}_{3}$ will not give an absorption / peak OR

$\mathrm{CHCl}_{3}$ will give an absorption / peak [1]

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Covered syllabus points

37.4 Proton NMR spectroscopy question 1

Question (a)

Question (i)

Question (ii)

37.4 Proton NMR spectroscopy question 2

Question (a)

Question (b)

Question (i)

Question (ii)

Question (iii)

37.4 Proton NMR spectroscopy question 3

Question (a)

Question (i)

Question (ii)

Question (iii)