Topic 6 of 34
Name the change and track where the electrons begin
Reaction classes describe the net change; mechanisms explain the individual electron movements.
A-Level 9476 (2026-2027)
Name the change and track where the electrons begin
Reaction classes describe the net change; mechanisms explain the individual electron movements.
| Term | Meaning |
|---|---|
| Electrophile / Lewis acid | An electron-pair acceptor, often positive or partially positive; examples H+ and a polarised Br2 molecule. |
| Nucleophile / Lewis base | An electron-pair donor, often with a lone pair or negative charge; examples OH-, CN-, NH3 and H2O. |
| Carbocation | An organic ion with a positively charged carbon centre; a simple alkyl carbocation is approximately planar. |
| Free radical | A species with an unpaired electron, represented by a single dot. |
| Homolytic fission | Each atom receives one electron from a broken covalent bond, producing radicals. |
| Heterolytic fission | Both bonding electrons go to one atom, producing ions from a neutral bond. |
A full-headed curly arrow shows the movement of an electron pair; its tail begins at a bond or lone pair and its head points to the atom or bond receiving the pair. A single-barbed fishhook shows movement of one electron. Arrows do not show atoms moving. A positive charge is not an electron source.
| Type | What changes |
|---|---|
| Addition | Groups add across a multiple bond, producing one main organic product from the combining reactants. |
| Substitution | An atom or group is replaced by another. |
| Elimination | A small molecule is removed while a multiple bond forms. |
| Condensation | Molecules join while eliminating a small molecule such as H2O or HCl. |
| Hydrolysis | A bond is cleaved by reaction with water or its acid/base components; products depend on the medium. |
| Oxidation | Organic carbon typically gains bonds to oxygen/electronegative atoms or loses bonds to hydrogen. |
| Reduction | The reverse change, often gaining hydrogen or losing oxygen. [O] and [H] may represent oxidising/reducing equivalents in equations. |
For an alcohol or halogenoalkane, primary/secondary/tertiary describes how many carbon groups are attached to the carbon carrying OH or halogen. For amines, it counts carbon groups attached to nitrogen: RNH2, R2NH and R3N. R4N+ is a quaternary ammonium ion. A quaternary carbon is bonded to four other carbons; it is not a neutral nitrogen with four bonds and an extra lone pair.
| Effect | How it changes a reaction |
|---|---|
| Delocalisation | Electron density is spread over adjacent overlapping orbitals; this can stabilise an ion or make a lone pair less available for donation. Resonance drawings are not different molecules rapidly swapping positions. |
| Electron donation or withdrawal | Groups change electron density through bonds and, where possible, conjugation. Alkyl groups donate towards a carbocation; electronegative substituents withdraw inductively. |
| Steric hindrance | Bulky groups physically obstruct an approaching reagent or an effective geometry. This is distinct from electron-pair repulsion or bond strength. |