WJEC A Level Chemistry
WJEC A Level Chemistry Unit 4: Organic Chemistry and Analysis Exam Questions
Free WJEC A Level Chemistry exam questions for Unit 4: Organic Chemistry and Analysis, with a mark scheme for every pack. Covers stereoisomerism, benzene, phenols, carbonyls, carboxylic acids and derivatives, amines, azo dyes, amino acids, synthesis and spectroscopy.
Unit 4: Organic Chemistry and Analysis exam questions
8 question packs of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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A-Level Chemistry Flashcards for 2027 Onwards
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About WJEC A Level Chemistry Unit 4: Organic Chemistry and Analysis
Unit 4 is an A2 unit of the WJEC A Level Chemistry course covering organic chemistry and analysis. It includes optical isomerism, the structure and reactions of benzene, alcohols and phenols, aldehydes and ketones, carboxylic acids and their derivatives, amines and diazonium salts, amino acids, peptides and proteins, and organic synthesis and analysis. It builds on the hydrocarbon, halogenoalkane and alcohol chemistry of Unit 2. Examiners set mechanism questions on electrophilic substitution and nucleophilic addition, ask for chemical tests with clear observations, and include structure determination questions combining NMR, IR, mass spectra and chemical evidence.
What you need to know
- 4.1 and 4.2 Stereoisomerism and aromaticity Optical isomers have a chiral carbon and rotate plane-polarised light in opposite directions; racemic mixtures are optically inactive. Benzene's delocalised structure explains its stability and its electrophilic substitution reactions (nitration, halogenation, Friedel-Crafts).
- 4.3 Alcohols and phenols Phenol is a weak acid that reacts with sodium hydroxide but not carbonates, gives a white precipitate with bromine water and a purple colour with neutral iron(III) chloride.
- 4.4 Aldehydes and ketones Both react with 2,4-DNPH to give orange precipitates. Aldehydes give a silver mirror with Tollens' and red precipitate with Fehling's. NaBH4 reduces carbonyls; HCN adds by nucleophilic addition; iodoform test identifies CH3CO groups.
- 4.5 Carboxylic acids and derivatives Acidity, esterification, ester hydrolysis, acyl chlorides and their reactions, amides, and the hydrolysis of nitriles to carboxylic acids.
- 4.6 and 4.7 Amines and amino acids Basicity of amines; making phenylamine; diazotisation below 10 C and coupling to form azo dyes. Amino acids form zwitterions, peptides and proteins, and can be separated by chromatography.
- 4.8 Synthesis and analysis Plan multi-step syntheses including changing carbon chain length. Use 1H NMR (shift, integration, splitting), 13C NMR, IR and mass spectra together to deduce structures.
Common mistakes in Unit 4: Organic Chemistry and Analysis
- Drawing the benzene electrophilic substitution intermediate with a complete ring of delocalisation.
- Allowing diazotisation to warm above 10 C, which decomposes the diazonium salt to phenol and nitrogen.
- Saying ketones react with Tollens' reagent. Only aldehydes give a silver mirror.
- Forgetting to protonate the alkoxide intermediate in nucleophilic addition of HCN.
- Misreading splitting patterns: a triplet means two hydrogens on adjacent carbons, not three.
Try three Unit 4: Organic Chemistry and Analysis exam-style questions
1. Draw the mechanism for the nitration of benzene, including the formation of the electrophile. (4 marks)
Show the mark scheme answer
HNO3 + 2H2SO4 gives NO2+ + H3O+ + 2HSO4-; arrow from delocalised ring to NO2+; intermediate with broken delocalisation and positive charge; arrow from C to H bond back into the ring giving nitrobenzene and H+.
2. A compound C3H6O gives an orange precipitate with 2,4-DNPH and a silver mirror with Tollens' reagent. Its 1H NMR spectrum has three peaks with areas 3 : 2 : 1. Identify the compound and explain. (3 marks)
Show the mark scheme answer
Orange precipitate shows a C=O group; silver mirror shows an aldehyde; three environments in ratio 3 : 2 : 1 fit CH3, CH2 and CHO, so the compound is propanal, CH3CH2CHO.
3. Explain why amino acids have relatively high melting temperatures. (2 marks)
Show the mark scheme answer
They exist as zwitterions, H3N+CHRCOO-; strong ionic attractions between zwitterions need a lot of energy to overcome.
Unit 4: Organic Chemistry and Analysis FAQs
What does WJEC A Level Chemistry Unit 4 cover?
It covers stereoisomerism, aromaticity, alcohols and phenols, aldehydes and ketones, carboxylic acids and derivatives, amines and azo dyes, amino acids and proteins, organic synthesis and analysis.
How do I revise Unit 4?
Combine a reaction map of all organic conversions with regular practice of mechanisms and spectroscopy problems, checking your answers against every piece of data given.
How do I approach structure deduction questions in Unit 4?
Use the molecular formula and mass spectrum first, then chemical tests and IR for functional groups, then NMR for environments and splitting, and finally check the proposed structure fits all the evidence.
