CAIE A-Level Chemistry 37.4.4 Need for Deuterated Solvents

Practise selecting a deuterated solvent such as CDCl₃ and explaining why ordinary protonated solvent is unsuitable.

Exam points

suggest CDCl3 when a suitable deuterated solvent is required for a proton NMR spectrum
explain that deuterium is not detected as 1H, so the solvent does not produce a large interfering proton signal
separate this solvent requirement from D2O exchange used deliberately to identify labile OH or NH protons

[Maximum number: 1]

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

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

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

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

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

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37.4.4—Need for deuterated solvents, e.g. CDCl ₃, question 1