CIE A Level Chemistry Analytical Techniques Exam Questions

About CIE A Level Chemistry Analytical Techniques

Analytical techniques in Cambridge International 9701 covers the methods chemists use to identify organic compounds. At AS you interpret infrared spectra to identify functional groups and use mass spectra to find the molecular ion, the number of carbon atoms from the M+1 peak, the presence of chlorine or bromine from M+2 peaks, and fragmentation patterns. The A Level extension covers thin-layer and gas chromatography, including Rf values and retention times, carbon-13 NMR, and proton NMR with chemical shift, integration, splitting using the n+1 rule and the use of D2O to identify OH and NH protons. Examiners often combine several techniques to deduce a structure.

What you need to know

• Infrared spectroscopy Bonds absorb IR at characteristic frequencies. Learn O-H in alcohols (broad, 3200 to 3600 cm-1), O-H in acids (very broad, 2500 to 3000 cm-1), C=O (1670 to 1740 cm-1) and N-H, using the data booklet.
• Mass spectrometry The molecular ion peak M+ gives Mr. Number of carbon atoms n = 100 x (abundance of M+1)/(1.1 x abundance of M+). M and M+2 in a 3:1 ratio show one Cl, and 1:1 show one Br.
• Fragmentation Fragment peaks such as m/z 15 (CH3+), 29 (C2H5+ or CHO+) and 43 (C3H7+ or CH3CO+) help identify structure. Only positive ions are detected.
• Chromatography (A Level) TLC separates by adsorption onto a stationary phase; Rf = distance moved by spot/distance moved by solvent front. GC separates by partition between a liquid stationary phase and a carrier gas, and peak area shows relative amount.
• Carbon-13 NMR (A Level) The number of peaks equals the number of different carbon environments. Chemical shift identifies the type of carbon, for example C=O at about 160 to 220 ppm.
• Proton NMR (A Level) Chemical shift shows the proton environment, peak area (integration) shows relative numbers of protons, and splitting follows the n+1 rule from neighbouring H. TMS is the reference and CDCl3 is used as solvent.
• D2O exchange (A Level) Adding D2O replaces OH and NH protons with D, so their peaks disappear from the proton NMR spectrum, confirming them.

Common mistakes in Analytical Techniques

• Assuming the tallest peak is the molecular ion; M+ is the peak with the highest m/z (apart from M+1 and M+2).
• Applying the n+1 rule using protons on the same carbon; n counts H atoms on adjacent carbons only.
• Forgetting that OH and NH protons usually appear as singlets and do not split neighbours.
• Measuring Rf to the back of the spot or from the bottom of the plate rather than from the baseline.
• Identifying a C=O compound as an acid without checking for the very broad O-H absorption.

Try three Analytical Techniques exam-style questions