DP Chemistry (last assessment 2024)

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Question 20N.3.hl.TZ0.e

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Date November 2020 Marks available [Maximum mark: 2] Reference code 20N.3.hl.TZ0.e
Level hl Paper 3 Time zone TZ0
Command term Explain Question number e Adapted from N/A
e.
[Maximum mark: 2]
20N.3.hl.TZ0.e

Aspirin is formed by reacting salicylic acid with ethanoic anhydride. The structure of aspirin is given in section 37 of the data booklet.

Explain how IR spectroscopy can be used to distinguish aspirin from salicylic acid.

[2]

Markscheme

salicylic acid contains absorption in the range 3200−3600 «cm−1» ✔

due to phenol/hydroxyl/OH group not present in aspirin ✔


Award [2] for “additional OH «stretch» in IR for salicylic acid at higher wavenumber than corresponding OH  «stretch» in aspirin” OR “aspirin has two absorption bands/one stronger absorption band in 1700–1750 «cm–1»while salicylic acid has one/weaker absorption band in that region”.

Award [1 max] for “fingerprint regions will be different for both”.

Examiners report

This question which asked for an explanation of how IR spectroscopy can be used to distinguish aspirin from salicyclic acid was generally very well answered. The majority stated that salicyclic acid contains an absorption in the IR spectrum in the 3200-3600 cm-1 range due to the phenolic OH group, which is not present in aspirin. A few stated that aspirin has a methyl group and hence the CH stretch will appear in the 2850-3090 cm-1 region of the IR spectrum in aspirin (using Section 26 of the Data Booklet) which will not appear in the corresponding IR spectrum for salicyclic acid. This is somewhat incorrect as in salicyclic acid the benzene ring will also have CH bonds and the CH stretch for the benzene ring will occur in a similar region of the IR spectrum (as indicated in Section 26 of the Data Booklet) and hence cannot be used to distinguish fully between the two structures per se if using the Data Booklet range. Of course, in practice the alkyl CH stretch would be at a slightly lower wavenumber (e.g. 2850-2950 cm-1) in the IR spectrum compared to the aromatic CH stretch (3030 cm-1), but virtually no candidate gave this type of precise detail.

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