Reaction Mechanisms Summary

Reaction mechanisms describe the step-by-step bond-breaking and bond-forming processes by which reactants are converted into products. At A-Level, you need to be able to draw, describe, and recognise several key mechanism types. This lesson consolidates all the organic mechanisms required for the Edexcel specification.

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Curly Arrow Conventions

Before reviewing the mechanisms, it is essential to understand curly arrow notation:

• A full curly arrow (↷) represents the movement of a pair of electrons
• A half curly arrow (⇀) represents the movement of a single electron (used only in free radical mechanisms)
• Curly arrows always start from the electron source (a lone pair, a bond, or a negative charge) and point to the electron sink (an atom or bond accepting the electrons)
• Each curly arrow must start and finish precisely — examiners mark this strictly

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1. Free Radical Substitution

Where it occurs: Alkanes reacting with halogens (e.g., CH₄ + Cl₂) in the presence of UV light

Three stages:

Initiation

UV light provides the energy to break the Cl–Cl bond homolytically (evenly), producing two chlorine free radicals:

Cl₂ → 2Cl•

Homolytic fission means each atom gets one electron from the bond.