← Organic compounds

9 Mechanisms & curly arrows

A curly arrow shows the movement of a PAIR of electrons, from a lone pair or bond (the source) to where the new bond forms (the head). Watch: Mechanisms & curly arrows Opens a YouTube search: pick a video that clicks for you ℹ

Not asked for in the last five papers

No paper from 2021 to 2025 asked for a mechanism or a curly arrow, and the marking schedules never use the words mechanism, nucleophile, electrophile or carbocation. This section is here because it explains why the reactions you do get asked about happen. Read it for understanding; revise the sections above it for marks.
Reading the diagrams: what each symbol means
Every arrow below starts at a real electron source (a lone pair or a bond) and ends where those electrons go.
Nucleophilic substitution (SN2), a nucleophile (electron-pair donor) attacks a carbon bearing a leaving group. Primary haloalkanes react by SN2 in one concerted step:
SN2: OH⁻ + CH₃Br → CH₃OH + Br⁻
Arrow 1: lone pair on HO⁻ → carbon. Arrow 2: C–Br bonding pair → Br. One step, backside attack, inversion of configuration.
Electrophilic addition, the C=C π electrons act as the nucleophile and attack an electrophile (e.g. the δ+ H of HBr), forming a carbocation, which is then attacked by the nucleophile (Br⁻):
Electrophilic addition: ethene + HBr
Step 1: π electrons attack H, H–Br breaks (Br⁻ leaves) → carbocation. Step 2: Br⁻ attacks the carbocation.
Elimination, a base removes a β-hydrogen; those electrons form a new C=C as the leaving group departs, making an alkene:
Elimination: bromoethane + OH⁻ (ethanolic) → ethene
Ethanolic KOH/NaOH favours elimination; aqueous favours substitution. Same reagent, different solvent, different product.
✓

Mnemonic: leaving group ability

“Weak bases are great leavers”, the weaker the base, the better the leaving group. Down the halogens: I⁻ > Br⁻ > Cl⁻ > F⁻ (“Iブ Can Fly”, iodide flies away fastest). Why? I⁻ is the biggest, most stable anion, so it holds the negative charge comfortably. OH⁻ is a terrible leaving group (strong base), which is why alcohols need acid to react (protonate the OH first so water leaves instead). ✓

Mnemonic: aqueous vs ethanolic (substitution vs elimination)

“Water substitutes; ethanol eliminates.” NaOH in water → nucleophilic substitution (alcohol). KOH in ethanol + heat → elimination (alkene). Identical reagent, different solvent, completely different product: a favourite exam trick.

Curly-arrow rules

  • Arrow tail starts at the electron source: a lone pair or the middle of a bond.
  • Arrow head points to where the new bond forms (an atom) or to an atom taking the electrons.
  • A bond breaking has an arrow leaving it; a bond forming has an arrow pointing to it.
  • Charges must balance across the mechanism. Track them at every step.

Practise just this