Chapter summary
Organic Chemistry, at a glance
Read structures, reason through electron movement, and connect functional groups with the conditions that transform them.
A-Level 9476 (2026-2027)
Quick revision
Revisit the essentials, then return to an explanation when you need it.
| Term | Definition to write |
|---|---|
| Electrophile | An electron-pair acceptor. |
| Nucleophile | An electron-pair donor. |
| Homolytic fission | The breaking of a covalent bond in which each atom takes one of the two bonding electrons, forming two free radicals. |
| Heterolytic fission | The breaking of a covalent bond in which one atom takes both bonding electrons, forming a cation and an anion. |
| Free radical | A species with an unpaired electron. |
| Structural (constitutional) isomers | Compounds with the same molecular formula but different structural formulae. |
| Stereoisomers | Compounds with the same molecular formula and the same structural formula, but a different arrangement of atoms in space. |
| Chiral centre | A carbon atom bonded to four different atoms or groups. |
| Enantiomers | A pair of stereoisomers that are non-superimposable mirror images of each other. |
| Site or condition | Mechanism or change |
|---|---|
| Alkane + halogen, UV | Radical substitution: initiation, propagation, termination. |
| Alkene pi bond + electrophile | Electrophilic addition through a carbocation: slow step forms C+, fast step adds the nucleophile. The more stable carbocation sets the orientation. |
| Activated electrophile + aromatic ring | Electrophilic substitution; temporary loss then restoration of aromaticity. |
| Nucleophile + saturated C-X | SN1 if carbocation formation is favoured; SN2 if concerted backside attack is accessible. |
| Nucleophile + aldehyde/ketone C=O | Nucleophilic addition; move pi electrons onto O before protonation. |
| Hydrolysable ester/amide or acyl chloride | Use the stated medium to choose acid, carboxylate, amine or ammonium products. |
Check every reagent with its essential condition: aqueous versus ethanolic NaOH; UV versus Lewis-acid halogenation; cold alkaline versus hot acidified KMnO4; distillation versus reflux; acidic versus alkaline hydrolysis. Nitration specifically uses 50 °C for benzene and 30 °C for methylbenzene.
For structure deductions, combine observations: 2,4-DNPH detects aldehyde/ketone carbonyls; Tollens and Fehling help distinguish aldehydes; iodoform identifies the relevant methyl-carbonyl or oxidisable methyl-carbinol unit. Bromine-water decolourisation is not unique to an alkene. Track carbon count through CN substitution and every subsequent hydrolysis or reduction.
For mechanisms, arrows begin at electrons, intermediates must have valid bonds and charges, and catalysts must be regenerated. For stereochemistry, distinguish connectivity, restricted C=C rotation, tetrahedral chirality, SN2 inversion and SN1 loss of configuration. For polymers, draw bonds through repeat-unit brackets and identify the bond that hydrolysis actually cleaves.
Quick recall
Which product forms mainly when HBr adds to propene, and why?
Revisit an explanation
- A formula should tell you exactly which atoms are connected
- Recognise the functional group before choosing a reaction
- Hybrid orbitals form the sigma framework; unhybridised p orbitals form pi bonding
- Isomers can differ in connectivity or in arrangement around a fixed framework
- Chirality is non-superimposability on a mirror image
- Name the change and track where the electrons begin
- Look for accessible electron-rich and electron-poor sites
- Radical substitution is a chain reaction
- An alkene donates its pi electrons to an electrophile
- Aromatic substitution restores the delocalised ring
- SN2 forms and breaks bonds in one concerted step
- SN1 forms a carbocation before the nucleophile attacks
- Cyanide attacks the carbonyl carbon, then oxygen is protonated
- An alkene offers a reactive pi bond that an alkane lacks
- Hot oxidation reveals what was attached to each double-bond carbon
- The ring and side-chain respond to different conditions
- Combustion products have different environmental effects
- An aqueous nucleophile substitutes; hot ethanolic base favours elimination
- A covalently attached halogen must first be released before an ionic halide test
- The carbon bearing OH determines the oxidation product
- Use a structural pattern for iodoform, and delocalisation for phenol
- First detect a carbonyl, then distinguish its class and methyl-carbonyl unit
- Carboxylic acids are stabilised by their delocalised conjugate bases
- Choose between acid-base reaction and changing the acyl group
- An acyl chloride reacts readily with water, alcohols, phenols and amines
- Acid hydrolysis is reversible; alkaline hydrolysis traps the carboxylate
- Track every carbon when preparing an amine
- Basicity depends on how available the nitrogen lone pair is
- An amide nitrogen is electronically coupled to its carbonyl
- An amino acid can donate and accept protons
- The repeat unit must preserve the correct bonds and functional groups
- Proteins are condensation polymers of alpha-amino acids
- A hydrolysable link helps degradation, but conditions still determine the rate
- Build a route by changing one functional group at a time