AQA A-Level Chemistry: Organic Chemistry Foundations — Complete Revision Guide (7405)

Organic foundations is the topic that unlocks every Paper 2 organic question on AQA 7405. Once you can name a structure, classify its isomers, draw a mechanism with the right curly arrows, and predict the product of a substitution or addition step, the rest of organic chemistry stops looking like a list of unrelated reactions and starts looking like a small set of mechanistic patterns repeated across new functional groups.

The mechanisms you learn here — free-radical substitution, electrophilic addition, nucleophilic substitution (SN1 and SN2), elimination (E1 and E2), oxidation, dehydration and esterification — are the same arrow-pushing patterns you will see in carbonyl, aromatic, amine and biological-molecule chemistry. Getting the foundations fluent now is the single highest-leverage decision you can make for your organic mark.

Synoptic Preview: How Organic Foundations Connect to the Rest of 7405

Organic foundations sits at the centre of AQA 7405. The covalent bonding and electronegativity ideas from bonding determine which bonds polarise and why a C-Br bond is the target of nucleophilic attack rather than a C-H bond. The kinetics framework from kinetics and equilibrium tells you why SN1 is favoured by tertiary substrates (rate-determining carbocation formation) while SN2 is favoured by primary substrates (one concerted bimolecular step).

The analytical techniques in analytical chemistry — infrared spectroscopy and proton NMR — are the tools you use to identify the functional groups introduced here. An O-H stretch at around 3200-3550 cm-1, a C=O at around 1680-1750 cm-1, a CHO triplet at around 9.5 ppm — every fingerprint depends on knowing the functional group.

Finally, the entire organic advanced module — carbonyls, carboxylic acids, esters, aromatics, amines, amino acids and condensation polymers — extends the mechanisms you meet here. Without fluent foundations, advanced organic collapses into memorisation. With them, it becomes pattern recognition.

Guide Overview: Ten Sub-Topics

This guide covers, in order:

1. IUPAC nomenclature and functional groups
2. Isomerism: structural and stereoisomers
3. Optical isomerism and stereochemistry
4. Alkanes and free-radical substitution
5. Alkenes and electrophilic addition
6. Addition polymers and polymer properties
7. Halogenoalkanes and nucleophilic substitution (SN1, SN2, E1, E2)
8. Alcohols: oxidation, dehydration and esterification
9. Organic mechanisms master class
10. Required practicals: organic foundations (RP4, RP5, RP6)

The 10-lesson organic foundations course walks through each topic with worked examples, AI tutor feedback and exam-style practice.

What the AQA 7405 Specification Covers

AQA 7405 is examined through Paper 1 (Physical and Inorganic, 2 h, 105 marks), Paper 2 (Organic and Physical, 2 h, 105 marks) and Paper 3 (any content plus practicals, 2 h, 90 marks). Organic foundations spans specification sections §3.3.1-§3.3.7 and is examined heavily on Papers 2 and 3.

Sub-topic	Spec area	Typical paper weight
Nomenclature and functional groups	§3.3.1	2-4 marks
Isomerism (structural and E/Z)	§3.3.1, §3.3.4	3-5 marks
Optical isomerism	§3.3.7 (introduced)	2-4 marks
Alkanes and free-radical substitution	§3.3.2	4-6 marks
Alkenes and electrophilic addition	§3.3.4	5-8 marks
Addition polymers	§3.3.4	2-4 marks
Halogenoalkanes (SN1, SN2, elimination)	§3.3.3	6-10 marks
Alcohols (oxidation, dehydration, esterification)	§3.3.5	5-8 marks
Mechanisms (curly arrows, RDS)	§3.3.2-§3.3.5	embedded
Required practicals RP4-RP6	§3.3.5, §3.3.6	Paper 3

These weights are estimates modelled on recent 7405 patterns. Reliable: at least one extended mechanism question — typically six to ten marks for a full arrow-pushing scheme — appears on Paper 2 every year.

IUPAC Nomenclature and Functional Groups

The IUPAC system assigns a unique name to every organic compound by identifying the longest carbon chain (the parent), naming substituents as prefixes, and indicating positions by locants chosen to give the lowest set of numbers.

The principal A-Level functional groups: alkane (C-C), alkene (C=C), halogenoalkane (C-X), alcohol (C-OH), aldehyde (CHO), ketone (C=O internal), carboxylic acid (COOH), ester (COOR), amine (NH2), amide (CONH2) and nitrile (CN). Each has a characteristic suffix or prefix (-ane, -ene, -ol, -al, -one, -oic acid, -oate, -amine, -amide, -nitrile) and a priority order when more than one is present.

Worked example. Name CH3CH(OH)CH2CH3. The longest chain is four carbons (butane). The OH group is on carbon 2, giving butan-2-ol.

A common pitfall is to number from the wrong end so locants are unnecessarily high. Another is to omit hyphens or write "but-2-anol" instead of butan-2-ol. A third is to miss the longest continuous backbone on a branched chain — count again if a substituent looks unexpectedly long.

See the nomenclature lesson.

Isomerism: Structural and Stereoisomers

Isomers are compounds with the same molecular formula but different structures, divided into structural isomers (different connectivity) and stereoisomers (same connectivity, different spatial arrangement).

Structural isomerism has three sub-types: chain (different carbon skeleton, e.g. butane vs methylpropane); position (different position of a functional group, e.g. butan-1-ol vs butan-2-ol); functional group (different functional groups entirely, e.g. propan-1-ol vs methoxyethane, both C3H8O).

Stereoisomerism at A-Level covers E/Z isomerism around a C=C double bond and optical isomerism (treated separately below). E/Z arises because rotation about C=C is restricted; substituents can be on the same side (Z) or opposite sides (E). The Cahn-Ingold-Prelog priority rules assign priorities to the two substituents on each double-bond carbon: higher atomic number wins. Z is when both higher-priority groups are on the same side; E is when they are on opposite sides.

Worked example. For but-2-ene (CH3CH=CHCH3), the two carbons of the C=C each carry a CH3 and an H. CH3 outranks H. If both CH3 groups are on the same side, the isomer is Z (cis). If on opposite sides, it is E (trans).

A common pitfall is to think only molecules with two identical substituent pairs can show E/Z isomerism — actually any C=C with two different groups on each carbon qualifies. See the isomerism lesson.

Optical Isomerism and Stereochemistry

Optical isomers (or enantiomers) are non-superimposable mirror images. They arise when a carbon carries four different substituents — a chiral centre. The two enantiomers rotate plane-polarised light in equal and opposite directions: one clockwise (+), one anticlockwise (-). A 50:50 mixture is a racemate and shows no net rotation.

The discovery of optical isomerism is associated with Pasteur, who in 1848 separated the two crystal forms of sodium ammonium tartrate by hand and showed each rotated polarised light in opposite directions. The modern priority system used to label enantiomers as R or S was developed by Cahn, Ingold and Prelog.

Worked example. 2-bromobutane (CH3CHBrCH2CH3) has a chiral centre at C2 — bonded to H, Br, CH3 and CH2CH3 — and exists as two enantiomers.

Optical isomerism matters biologically because enzymes are themselves chiral and bind only one enantiomer of many substrates. Thalidomide is the standard cautionary example: marketed in the late 1950s and early 1960s as a sedative, it was later linked to severe birth defects, and one enantiomer was subsequently shown to carry the harmful effect while the other had the intended therapeutic effect. The two enantiomers interconvert in the body, so administering the "safe" form alone would not have prevented the tragedy — but the case made pharmaceutical chirality a permanent priority.

A common pitfall is marking a carbon as chiral when two substituents are identical. See the optical isomerism lesson.

Alkanes and Free Radical Substitution

Alkanes are saturated hydrocarbons CnH2n+2. They contain only single C-C and C-H bonds — both strong and non-polar, which is why alkanes are generally unreactive. The A-Level reactions are combustion (complete to CO2 and H2O; incomplete to CO or C plus H2O) and free-radical substitution with halogens under UV light.

Free-radical substitution proceeds in three stages: initiation, propagation and termination.

Initiation: UV light homolytically cleaves the halogen — Cl2 → 2 Cl• (one electron of the bonding pair to each atom).

Propagation: Cl• + CH4 → •CH3 + HCl, then •CH3 + Cl2 → CH3Cl + Cl•. The chlorine radical is regenerated, so each photolysis can drive many substitution events.

Termination: any two radicals combine — 2 Cl• → Cl2, •CH3 + Cl• → CH3Cl, 2 •CH3 → C2H6 (a possible side product).

A common pitfall is drawing curly arrows in a radical mechanism — radical mechanisms use fish-hooks (half-arrows with single barbs) because each arrow moves only one electron. Another is omitting the second propagation step that regenerates the chain carrier; both steps are needed for full marks. See the alkanes lesson.

Alkenes and Electrophilic Addition

Alkenes contain at least one C=C and have formula CnH2n. The C=C is a region of high electron density — sigma plus pi — and is attacked by electrophiles. The characteristic alkene reaction is electrophilic addition.

The mechanism is two steps. Step 1: the pi electrons attack the electrophile, forming a new C-E bond and leaving a positive charge on the other carbon — a carbocation intermediate. Step 2: a nucleophile attacks the carbocation, forming the second new bond.

Markovnikov's rule: when an unsymmetrical electrophile (e.g. HBr) adds to an unsymmetrical alkene (e.g. propene), the H goes to the carbon that already has more hydrogens. The reason: the more substituted carbocation is more stable (tertiary > secondary > primary), and the major product comes from the more stable intermediate.

Worked example. Propene (CH3CH=CH2) plus HBr. H+ adds to CH2, generating the secondary carbocation CH3CH+CH3. Bromide then attacks, giving 2-bromopropane as the major product. The minor product, 1-bromopropane, comes from the less stable primary carbocation.

A common pitfall is forgetting the carbocation (drawing a one-step concerted addition) or predicting the wrong regiochemistry. See the alkenes lesson.

Addition Polymers and Polymer Properties

When alkenes undergo repeated addition, the C=C bonds open and link into a long chain — an addition polymer: n CH2=CHX → [-CH2-CHX-]n. The monomer is the alkene; the repeat unit is in brackets; the polymer is named "poly(monomer)".

Common examples: poly(ethene) from ethene; poly(propene) from propene; poly(chloroethene) (PVC) from chloroethene; poly(phenylethene) (polystyrene) from phenylethene; poly(tetrafluoroethene) from tetrafluoroethene.

Addition polymers are inert and non-biodegradable because the C-C and C-H backbone bonds are strong and non-polar — there are no functional groups for enzymes or hydrolysis to attack. Disposal routes include landfill (slow degradation), incineration (energy recovery, possible toxic emissions from chlorinated polymers), and mechanical or chemical recycling.

The discovery of Ziegler-Natta catalysts in the 1950s enabled controlled polymerisation of propene and other alkenes at low pressure, producing stereoregular polymers (isotactic and syndiotactic forms) with much higher crystallinity than the random (atactic) form. Ziegler and Natta received the Nobel Prize in 1963 for this work.

A common pitfall is to draw the monomer with a C=C still in the repeat unit — the double bond becomes a single bond as it opens during polymerisation. See the polymers lesson.

Halogenoalkanes and Nucleophilic Substitution (SN1, SN2, E1, E2)

Halogenoalkanes contain a C-X bond (X = F, Cl, Br, I). The bond is polar (Cδ+, Xδ-), making the carbon electrophilic — a target for nucleophiles like OH-, CN-, NH3 and H2O.

There are two substitution mechanisms.

SN2 is concerted: the nucleophile attacks the back of the C-X bond as the halide leaves, passing through a five-coordinate transition state with inversion of stereochemistry. Rate = k[RX][Nu-]; bimolecular. Favoured by primary halogenoalkanes (least steric hindrance).

SN1 is two-step: C-X breaks heterolytically to form a planar carbocation (slow, rate-determining); the nucleophile then attacks (fast). Rate = k[RX]; unimolecular. The planar intermediate gives racemisation if the carbon was chiral. Favoured by tertiary halogenoalkanes (most stable carbocation). Secondary halogenoalkanes show a mixture depending on conditions.

Elimination competes with substitution. The base removes a hydrogen from the carbon adjacent to C-X, the C-C bond becomes C=C, and the halide leaves. E2 is concerted (bimolecular); E1 goes via a carbocation (unimolecular). Concentrated alcoholic KOH favours elimination; aqueous KOH favours substitution.

A common pitfall is drawing arrows incorrectly: in SN2 the nucleophile arrow goes into C while the C-X pair leaves onto X; in SN1 the C-X arrow leaves first to form the cation. See the halogenoalkanes lesson.

Alcohols: Oxidation, Dehydration and Esterification

Alcohols contain C-OH and are classified as primary (OH on a CH2 at chain end), secondary (OH on a CH in the middle), or tertiary (OH on a C with no H attached). The classification matters because oxidation behaviour differs.

Oxidation with acidified potassium dichromate (K2Cr2O7 / dilute H2SO4) converts primary alcohols stepwise: first to aldehydes (RCHO) if the aldehyde is distilled off as it forms, then to carboxylic acids (RCOOH) under reflux. Secondary alcohols oxidise to ketones (R2C=O) and stop there. Tertiary alcohols do not oxidise under these conditions because there is no C-H bond on the alcohol carbon to remove. The dichromate colour change orange → green is the visible test.

Dehydration removes a water molecule across C-OH and an adjacent C-H, producing an alkene. Concentrated sulfuric or phosphoric acid catalyses the reaction at elevated temperature. The mechanism proceeds via protonation of OH, loss of water to give a carbocation, and loss of a proton to form C=C. The most stable (most substituted) alkene is usually the major product.

Esterification with a carboxylic acid (catalysed by concentrated H2SO4) produces an ester and water — a reversible condensation: ROH + R'COOH ⇌ R'COOR + H2O. Esters have characteristic fruity smells and are used as flavourings and solvents.

A common pitfall is forgetting that distillation stops oxidation at the aldehyde — under reflux a primary alcohol goes all the way to carboxylic acid. See the alcohols lesson.

Organic Mechanisms Master Class

The organic foundations module ends with a consolidation lesson on every mechanism in the section. A mechanism is a step-by-step account of bond breaking and bond forming, with curly arrows for electron pairs (or fish-hook half-arrows for single electrons in radical mechanisms).

Arrow conventions: a curly arrow always starts from a bond or a lone pair and ends at the atom that receives the new bond or charge. The arrow shows where the electrons go, not where the atoms move. An arrow that starts at a positive carbon is wrong — that carbon has no electrons to donate.

The five core mechanisms across organic foundations:

1. Free-radical substitution (alkanes + halogens, UV): fish-hooks; initiation/propagation/termination.
2. Electrophilic addition (alkenes + electrophiles): pi electrons attack electrophile; carbocation; nucleophile attacks.
3. Nucleophilic substitution SN1 and SN2 (halogenoalkanes + nucleophiles): SN1 via carbocation; SN2 concerted with inversion.
4. Elimination E1 and E2 (halogenoalkanes + bulky base, or alcohols + concentrated acid): formation of C=C with loss of HX or H2O.
5. Esterification / hydrolysis (alcohol + carboxylic acid ⇌ ester + water): addition-elimination at the carbonyl carbon.

The Hammond postulate is a useful heuristic: the transition state of an exothermic step resembles the reactants; the transition state of an endothermic step resembles the products. This is why the carbocation-forming step of SN1 is rate-determining and geometrically resembles the carbocation.

A common pitfall is to draw arrows that start from atoms rather than from electrons (bonds or lone pairs). See the mechanisms master class.

Required Practicals: Organic Foundations (RP4, RP5, RP6)

AQA 7405 names twelve required practicals; three sit in organic foundations.

RP4: Distillation of a product from a reaction. Typically the oxidation of a primary alcohol to an aldehyde. Apparatus: pear-shaped flask with anti-bumping granules, still-head, Liebig condenser (water in at the bottom), receiver. The aldehyde distils off as it forms and is collected before further oxidation. Expected colour change: orange (Cr2O72-) to green (Cr3+).

RP5: Tests for organic functional groups. A suite of standard tests: bromine water decolourises with alkenes; warm acidified dichromate goes orange-to-green with primary or secondary alcohols (stays orange with tertiary); Tollens' reagent (silver mirror) and Fehling's (brick-red Cu2O) distinguish aldehydes (positive) from ketones (negative); sodium hydrogencarbonate fizzes with carboxylic acids (CO2). Link each test to its functional group with the expected observation.

RP6: Preparation of an organic solid (purification of an organic liquid). Typical preparation: aspirin from 2-hydroxybenzoic acid, or recrystallisation of a named solid, or purification of a liquid by separating funnel, drying agent and distillation. Skills assessed: choice of recrystallisation solvent (hot dissolves, cold does not), melting-point as a purity check, and percentage yield.

A common pitfall is to give vague observations — write "orange to green" rather than "colour change", and "white solid" rather than "precipitate forms". See the required practicals lesson.

Common Mark-Loss Patterns

• Drawing curly arrows starting from atoms instead of electrons.
• Using curly arrows in radical mechanisms instead of fish-hooks.
• Predicting anti-Markovnikov regiochemistry without justification.
• Missing the propagation step that regenerates the chain carrier.
• Marking a carbon as chiral when two substituents are identical.
• Confusing E and Z when applying Cahn-Ingold-Prelog priorities.
• Drawing a polymer repeat unit with a C=C left in.
• Forgetting that distillation is required to stop primary-alcohol oxidation at the aldehyde.
• Choosing the wrong test for the functional group present.
• Giving vague observations ("changes colour" instead of "orange to green").

How to Revise This Topic

• Drill nomenclature daily — twenty structures a day for a fortnight until naming and drawing are automatic in both directions.
• Build an isomer-counting card for C4H10X, C5H12O and C4H8 — know how many constitutional and stereoisomers exist for each.
• Practise five mechanisms a day, rotating SN1, SN2, E1, E2 and electrophilic addition until curly arrows are muscle memory.
• Memorise the test-reagent table — bromine water, dichromate, Tollens', Fehling's, sodium hydrogencarbonate — by group and observation.
• Use the LearningBro practice quizzes for timed practice and the AI tutor to debug mechanisms in real time.

Linking to Other Topics

Organic foundations is the prerequisite for nearly every later topic. The mechanisms here are reused throughout organic advanced — addition-elimination at carbonyls, electrophilic substitution in aromatics, and the chemistry of amines, amino acids and condensation polymers. Bonding and polarity come from bonding; the kinetics underpinning SN1 vs SN2 is kinetics and equilibrium in action; the functional-group testing in RP5 is exactly what you read off infrared and NMR spectra in analytical chemistry. Time invested in foundations returns many-fold across Papers 2 and 3.

Final Word

Organic chemistry rewards pattern recognition over memorisation. There are only five core mechanisms to know fluently — radical substitution, electrophilic addition, SN1/SN2, E1/E2, and addition-elimination at carbonyls — and once those are automatic, every named reaction in the spec becomes a substitution into a pattern you already know. Drill nomenclature, drill mechanisms, drill isomer counting, drill the test-reagent tables. The 10-lesson LearningBro organic foundations course walks through every sub-topic with worked examples and AI tutor feedback. Get the foundations fluent and Paper 2 stops being the paper students fear.