AQA A Level Chemistry 3.3.6 Organic Analysis Exam Questions

About AQA A Level Chemistry 3.3.6 Organic Analysis

Organic analysis is about identifying unknown organic compounds. On the AQA A Level course you use test-tube reactions to identify alkenes, alcohols, aldehydes and carboxylic acids (Required practical 6), use high resolution mass spectrometry to find molecular formulae from precise relative molecular masses, and interpret infrared spectra using characteristic absorptions and the fingerprint region. You also link infrared absorption by bonds in CO2, methane and water vapour to the greenhouse effect. These skills come back in 3.3.15 NMR and 3.3.16 Chromatography, and in combined spectroscopy problems. Examiners expect exact observations and specific wavenumber ranges taken from the data booklet.

What you need to know

• Test-tube tests Alkene: bromine water turns from orange to colourless. Primary or secondary alcohol and aldehyde: acidified K2Cr2O7 turns from orange to green. Aldehyde: Tollens' gives a silver mirror and Fehling's gives a brick-red precipitate. Carboxylic acid: sodium carbonate gives effervescence of CO2.
• High resolution mass spectrometry Measures Mr to four or more decimal places. Using precise atomic masses (H 1.0078, C 12.0000, O 15.9949, N 14.0031), compounds with the same integer Mr can be told apart.
• Infrared spectroscopy Bonds absorb infrared radiation at characteristic wavenumbers. Key ranges: O-H in alcohols 3230 to 3550 cm-1 (broad), O-H in acids 2500 to 3000 cm-1 (very broad), C=O 1680 to 1750 cm-1, C-H 2850 to 3300 cm-1.
• Fingerprint region The region below 1500 cm-1 is unique to each compound. Matching it to a database spectrum identifies a compound or shows the presence of impurities.
• Global warming CO2, methane and water vapour absorb infrared radiation emitted from the Earth's surface, which contributes to global warming.

Common mistakes in 3.3.6 Organic Analysis

• Identifying functional groups from the fingerprint region. Use the characteristic absorptions above 1500 cm-1 for functional groups.
• Mixing up the O-H absorption in alcohols and in carboxylic acids. The acid band is very broad and lower, at 2500 to 3000 cm-1.
• Writing 'silver precipitate' or 'goes cloudy' for Tollens' reagent. The expected observation is a silver mirror.
• Writing 'gas produced' for the carbonate test. Say effervescence and, if asked, that the gas turns limewater cloudy.
• Rounding precise masses in high resolution mass spectrometry calculations. Keep four decimal places throughout.

Try three 3.3.6 Organic Analysis exam-style questions