Reaction Mechanisms — Curly Arrows and Electron Movement

A reaction mechanism is a step-by-step account of how a reaction proceeds at the molecular level: which bonds break, which bonds form, and how electrons move. The curly arrow is the most important symbol in organic chemistry — it shows exactly where electrons are moving from and to. Mastering curly arrows is essential for every mechanism question in A-Level Chemistry. This lesson covers OCR A-Level Chemistry A (H432) specification 4.1.1 (f)–(h).

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1. Why Mechanisms Matter

Without a mechanism, a reaction is just reactants → products. With a mechanism, we can:

• Explain why certain products form and others do not (e.g., Markovnikov's rule).
• Predict the product of a reaction we haven't seen before.
• Rationalise the influence of solvent, temperature, and concentration.
• Understand reactive intermediates and transition states.

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2. Bond Breaking — Homolytic vs Heterolytic Fission

A covalent bond is a pair of shared electrons. When a bond breaks, those two electrons must go somewhere. There are two possibilities.

2.1 Homolytic Fission

In homolytic fission, each atom takes one electron from the broken bond. This forms two radicals — species with an unpaired electron.

Key Definition — Homolytic fission: Breaking of a covalent bond in which one electron goes to each of the two atoms originally bonded.

Shown in curly arrow notation using half-headed (single-barb) arrows:

A — B  →  A•  +  •B

Each half-arrow moves a single electron. Homolytic fission typically happens when:

• The bond is non-polar (e.g., Cl–Cl, Br–Br).
• There is enough energy to break the bond (heat or UV light).
• The resulting radicals are reasonably stable.

A classic example: Cl₂ absorbs UV light and undergoes homolytic fission to give two chlorine radicals, Cl•.

2.2 Heterolytic Fission

In heterolytic fission, both electrons from the broken bond go to one of the atoms, forming a pair of ions.

Key Definition — Heterolytic fission: Breaking of a covalent bond in which both electrons go to one of the two atoms originally bonded, forming a cation and an anion.

Shown in curly arrow notation using full-headed (double-barb) arrows:

A — B  →  A⁺  +  :B⁻

The curly arrow starts at the bond and ends at the atom that takes both electrons.

Heterolytic fission is favoured when the bond is polar, because the more electronegative atom is already pulling the electron pair. For example, C–Br in bromomethane breaks heterolytically to give CH₃⁺ and Br⁻ (the mechanism of nucleophilic substitution).

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3. Curly Arrow Rules — The Golden Rules

A curly arrow represents the movement of a pair of electrons (full-headed arrow) or a single electron (half-headed arrow) in a mechanism.

Follow these rules strictly:

1. The arrow starts at the source of the electrons (a lone pair on an atom, or a bond).
2. The arrow ends at the destination of the electrons (an atom, or the centre of a new bond).
3. For a pair of electrons, use a full-headed arrow.
4. For a single electron, use a half-headed arrow.
5. Arrows never start from a positive charge or from a hydrogen atom (except in very specific proton-transfer steps where an O–H bond pair moves).
6. Every arrow must account for both where the electrons came from and where they go.
7. Charges must balance on both sides of each arrow-pushing step.

3.1 Example — Heterolytic Bond Breaking

Bromomethane reacting with hydroxide:

HO⁻  →  CH3 — Br   (arrow from HO⁻ lone pair to C)
              ↓
       CH3 — Br   (arrow from C–Br bond to Br)

After both arrows move:

• A new C–O bond has formed.
• The old C–Br bond has broken, with both electrons going to Br.
• Br leaves as Br⁻.

3.2 Example — Homolytic Bond Breaking (UV)

  Cl — Cl  →  Cl•  +  •Cl

Two half-arrows, each starting at the centre of the Cl–Cl bond and ending at one of the chlorine atoms.

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4. Three Key Species: Nucleophiles, Electrophiles, and Radicals

4.1 Nucleophiles

Key Definition — Nucleophile: An electron pair donor (a species attracted to a region of positive or partial positive charge).