Explain what happens at a molecular level during the absorption of IR radiation by carbon dioxide, CO₂.

(i)     Identify the bonds responsible for the peaks A, B and C in the IR spectrum of X.

A:

B:

C:

(ii)     In the mass spectrum of X, deduce which ions the m/z values at 74, 45 and 29 correspond to.

m/z = 74:

m/z = 45:

m/z = 29:

(iii)     Identify the peak at 11.73 ppm in the $^{\text{1}}{\text{H}}\,{\text{NMR}}$ spectrum.

(iv)     Deduce the structure of X.

change in bond length / bond stretching / asymmetric stretch;

change in bond angle / bending (of molecule);

Allow [1 max] for only stating vibrations.

induces molecular polarity/dipole moment / OWTTE;

(i)     A: O–H

B: C=O

C: C–O

Award [2] for three correct, [1] for two correct.

(ii)     m/z = 74: ${{\text{C}}_2}{{\text{H}}_5}{\text{COO}}{{\text{H}}^ + }$ / ${{\text{C}}_3}{{\text{H}}_6}{\text{O}}_2^ +$;

m/z = 45: ${\text{COO}}{{\text{H}}^ + }$;

m/z = 29: ${{\text{C}}_2}{\text{H}}_5^ +$;

Penalize missing + charge once only.

Do not award mark for m/z = 29: CHO⁺.

(iii)     –COOH

Accept –OH.

(iv)     ${\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{COOH}}$ / ${\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{O}}_2}{\text{H}}$;

More detailed structural formula may be given.

In (b) the main misconception stated by candidates was that non-polar compounds do not absorb infrared radiation. Most candidates scored a mark for vibrations, but many misunderstood the difference between symmetric and asymmetric stretching.

Part (c)(i) was well answered by the great majority of candidates; giving C–H bond instead of O–H for A was a popular incorrect answer. In (ii) the most common mistake was missing the + sign. Most candidates answered (iii) and (iv) correctly.