Lesson 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)
I can use the specified reaction terminology.Syllabus
Syllabus 9476, 11.3(a). Use the specified reaction terminology.
I can use the specified structural-reactivity concepts.Syllabus
Syllabus 9476, 11.3(b). Use the specified structural-reactivity concepts.
I can apply reaction and reactivity concepts to mechanisms.Syllabus
Syllabus 9476, 11.3(i). Apply reaction and reactivity concepts to mechanisms.
I can track electron flow in polar mechanisms.Syllabus
Syllabus 9476, 11.3(j). Track electron flow in polar mechanisms.
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 curly arrow with a full head shows an electron pair moving. Its tail starts at a bond or a lone pair. Its head points to the atom or bond that receives the pair. A half-headed fishhook arrow shows one electron moving. Arrows show electrons moving, not atoms. A positive charge is never the start of an arrow, because it is not a source of electrons.
| 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 and tertiary tell you how many carbon groups are joined to the carbon that carries the OH or halogen. For amines, count the carbon groups joined to the nitrogen: RNH2, R2NH and R3N. R4N+ is a quaternary ammonium ion. A quaternary carbon is a carbon bonded to four other carbons.
| 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. |
Check your understanding
In the reaction of OH- with bromomethane, CH3Br, which curly arrow correctly shows the new C-O bond forming?
Show the answer
From a lone pair on the oxygen of OH- to the delta+ carbon
A curly arrow shows a pair of electrons moving. It starts where the pair is now (a lone pair on O) and ends where the new bond forms (the delta+ carbon).
Check your understanding
Match each fact to the effect that explains it. (a) Phenylamine is a weaker base than ethylamine. (b) (CH3)3C+ forms more easily than CH3CH2+. (c) ClCH2COOH is a stronger acid than CH3COOH. (d) (CH3)3CBr reacts very slowly by SN2.
Show the answer
(a) delocalisation (b) electron donation (c) electron withdrawal (d) steric hindrance
(a) The N lone pair overlaps with the ring's pi electrons, so it is less available for H+. (b) Three alkyl groups donate electron density and spread the positive charge. (c) Electronegative Cl pulls density away and stabilises COO-. (d) Three methyl groups block the back of the carbon from the nucleophile.