AQA A-Level Chemistry 3.3 Organic chemistry: a deep dive on functional groups, mechanisms and analysis

What module 3.3 actually demands

Organic chemistry is the largest mechanism-driven part of AQA A-Level Chemistry. Module 3.3 runs from the language of naming and drawing molecules, through each homologous series and its characteristic reactions, into isomerism, polymers and biological molecules, and finishes with the analytical methods used to determine structure. The examiners test three linked skills: precise recall of reagents, conditions and observations; the ability to draw and explain mechanisms with correct curly arrows; and the application of spectra and chromatograms to deduce an unknown structure.

This guide walks through the whole module in a sensible learning order, then sets out the exam patterns AQA repeats. Each cluster has a matching dot-point page with practice questions; this overview ties them together.

The language of organic chemistry

Everything starts with nomenclature, formulae and isomerism. You name compounds by IUPAC rules from the longest carbon chain, with a suffix for the principal functional group and numbered prefixes for substituents. You must move fluently between molecular, empirical, displayed, structural and skeletal formulae, and recognise a homologous series as a family sharing a functional group and general formula, each member differing by $CH_2$.

Isomerism appears throughout. Structural isomers share a molecular formula but differ in connectivity. E-Z stereoisomerism arises from restricted rotation about a $C=C$ double bond, assigned using CIP priority. Optical isomerism arises from a chiral carbon bonded to four different groups, giving non-superimposable mirror-image enantiomers that rotate plane-polarised light in opposite directions; a 50:50 racemic mixture is optically inactive. Learn the language of mechanisms too: curly arrows show the movement of electron pairs, nucleophiles donate them, electrophiles accept them, and free radicals carry an unpaired electron shown by a single-headed arrow.

The homologous series and their reactions

Build the series in order, because later reactions reuse earlier ones.

Alkanes are saturated hydrocarbons from crude oil, separated by fractional distillation and broken down by cracking. They burn (complete and incomplete combustion, with pollutants and catalytic converters) and react with halogens by free-radical substitution in initiation, propagation and termination steps under UV light.

Halogenoalkanes undergo nucleophilic substitution (with hydroxide to alcohols, cyanide to nitriles, ammonia to amines) and elimination to alkenes. Reactivity follows bond enthalpy: $C-I$ is weakest so iodoalkanes react fastest.

Alkenes are unsaturated and react by electrophilic addition with bromine, hydrogen halides and steam, explained through carbocation stability and Markovnikov addition. They also polymerise.

Alcohols are made by fermentation or hydration of alkenes, classified as primary, secondary or tertiary, oxidised by acidified dichromate (orange to green) to aldehydes, carboxylic acids or ketones, and dehydrated to alkenes.

Aldehydes and ketones share the polar carbonyl group. Aldehydes are oxidised (Tollens and Fehlings distinguish them from ketones); both are reduced by $NaBH_4$ and undergo nucleophilic addition of HCN to hydroxynitriles, giving a racemate when a chiral centre forms.

Carboxylic acids and derivatives are weak acids that fizz with carbonates, form esters with alcohols (hydrolysed by acid or base), and are acylated through reactive acyl chlorides and anhydrides reacting with water, alcohols, ammonia and amines.

Aromatic chemistry centres on the delocalised model of benzene, supported by bond-length and enthalpy-of-hydrogenation evidence, and its electrophilic substitution reactions (nitration with $NO_2^+$ and Friedel-Crafts acylation with $RCO^+$).

Amines are bases and nucleophiles, prepared from halogenoalkanes, nitriles or nitro compounds, with base strength running aromatic amine, then ammonia, then aliphatic amine.

Polymers and biological molecules

Addition polymers come from alkenes with no loss of atoms and an inert backbone. Condensation polymers (polyesters and polyamides) form with loss of water and can be hydrolysed back to monomers, making them more biodegradable; deduce monomers by breaking each link and adding water.

Biological molecules apply these ideas. Amino acids carry both an amine and a carboxylic acid group, exist as zwitterions, and condense into proteins joined by peptide (amide) bonds that hydrolyse back to amino acids. Enzymes are protein catalysts with stereospecific active sites. DNA is a polymer of nucleotides whose strands are held by hydrogen bonds in complementary base pairs (A-T, C-G), and the drug cisplatin binds DNA to block replication in cancer cells.

The analytical toolkit

Structure determination ties the module together. Simple test-tube reactions identify functional groups. Mass spectrometry gives the molecular mass from the M peak and structural clues from fragments. Infrared spectroscopy identifies bonds from characteristic wavenumbers, especially the broad $O-H$ and the strong $C=O$ near $1700 \text{ cm}^{-1}$, with a unique fingerprint region.

NMR is the heavyweight technique. Carbon-13 NMR counts carbon environments; high-resolution proton NMR adds chemical shift (type of proton), integration (ratio of hydrogens) and spin-spin splitting interpreted with the n+1 rule, referenced to TMS at $\delta = 0$ in a deuterated solvent. Chromatography (TLC and column with Rf values, gas chromatography with retention time, and GC-MS) separates and identifies the components of a mixture.

How module 3.3 is examined

A typical AQA profile for organic chemistry:

• Mechanisms. Drawing one of the five named mechanisms with correct curly arrows, electrophile or nucleophile, and conditions.
• Reagents and conditions. Stating the exact reagent, catalyst, solvent and whether to distil or reflux for a named conversion.
• Synthesis. Planning a multi-step route between functional groups, often via a nitrile to add a carbon.
• Analysis. Deducing an unknown structure from a combination of mass spectrum, IR and NMR data, or interpreting a chromatogram.
• Application and evaluation. The biofuel debate, the choice of anhydride over acyl chloride, polymer disposal, and the action of cisplatin.

Check your knowledge

A mix of recall and application questions covering the whole of module 3.3. Attempt them under timed conditions, then check against the solutions.

1. Name the five mechanisms studied in AQA organic chemistry and give one reaction for each. (5 marks)
2. Describe how you would distinguish between an aldehyde and a ketone in the laboratory. (3 marks)
3. Give the reagents and conditions to convert a primary alcohol into a carboxylic acid, and explain how you would avoid stopping at the aldehyde. (3 marks)
4. State two pieces of evidence that benzene has a delocalised ring of electrons. (2 marks)
5. Explain why a primary aliphatic amine is a stronger base than phenylamine. (3 marks)
6. Explain how you would deduce the monomers of a condensation polymer from its repeat unit. (2 marks)
7. Ethanol shows three peaks in its proton NMR. State the splitting pattern of the $CH_3$ and $CH_2$ signals and explain why. (3 marks)
8. Calculate the Rf value of a spot that travels 4.5 cm when the solvent front travels 9.0 cm. (1 mark)