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3.3 Electron sharing reactions (Radicals)

Syllabus
First assessment 2025
Topic
3.3
Level
HL

Free Radicals

A radical is a highly reactive species containing an unpaired electron. Show the unpaired electron with a dot next to the atom that carries it.

the arrow has one curved shaft and exactly one terminal barb; the symbol therefore represents movement of one electron; no double-barbed arrow or unrelated molecular content is present.
two single-barbed arrows split the Cl-Cl bonding pair equally; one arrow terminates at each chlorine atom; the products are exactly two neutral Cl radicals with one radical dot each.

Homolytic fission creates radicals because each covalent-bond fragment retains one bonding electron.

The dot in Cl· or CH₃· represents one unpaired electron, not a positive or negative charge. Radicals react readily because pairing that electron can form a bond; track the dot through every equation so electron and atom accounting remain explicit.

Recognizing Radical Notation

1 mark

Which radical is most likely to form during the breakdown of one covalent bond of dichlorofluoromethane, CHCl2 F\mathrm{CHCl}_{2} \mathrm{~F}, in the upper atmosphere?

Homolytic Fission and Initiation

Cl2(g)→2Cl⋅(g)underUVlightorheatCl2(g) → 2Cl·(g) under UV light or heat

two single-barbed arrows split the Cl-Cl bonding pair equally; one arrow terminates at each chlorine atom; the products are exactly two neutral Cl radicals with one radical dot each.
Cl2 homolysis is shown with two single-electron fish-hook arrows; each reactant chlorine has three lone pairs; each product chlorine has three lone pairs and one unpaired electron; no formal charge is introduced.

Homolytic cleavage gives one electron to each fragment. Use single-electron arrows to show radical movement in the chain mechanism.

Draw two single-barbed arrows from the breaking X–X bond, one toward each atom, to account for both electrons. The UV or heat step creates radicals and is initiation; a step that consumes one radical and forms another belongs to propagation.

Writing the Initiation Step

1 mark

Write an equation for the initiation reaction.

Free-Radical Substitution

Initiation creates radicals; propagation abstracts H from an alkane and then regenerates the halogen radical; termination combines radicals. A mixture can form because substitution may occur at different positions.

Cl2 homolysis is shown with two single-electron fish-hook arrows; each reactant chlorine has three lone pairs; each product chlorine has three lone pairs and one unpaired electron; no formal charge is introduced.
the equation is Cl radical plus CH4 to HCl plus CH3 radical; one fish-hook arrow begins at the chlorine radical electron and ends at H; the second fish-hook arrow begins at the breaking C-H bond and ends at carbon; four reactant hydrogens become one HCl hydrogen plus three methyl-radical hydrogens.
the equation is CH3 radical plus Cl2 to CH3Cl plus Cl radical; the carbon radical electron moves to the nearer chlorine; one electron from the Cl-Cl bond moves to the departing chlorine; the chlorine radical is regenerated and all atoms are conserved.

C2H6+Cl⋅→C2H5⋅+HCl;C2H5⋅+Cl2→C2H5Cl+Cl⋅C2H6 + Cl· → C2H5· + HCl; C2H5· + Cl2 → C2H5Cl + Cl·

For methane chlorination, initiation forms 2Cl· from Cl₂ under UV. Propagation uses Cl· + CH₄ → HCl + CH₃· and CH₃· + Cl₂ → CH₃Cl + Cl·; termination combines two radicals. Further substitution creates a mixture, so the mechanism does not guarantee only CH₃Cl.

Writing Chain-Substitution Equations

4 marks

Explain the reaction mechanism by writing equations for each step.

One initiation step:
Two propagation steps:

One termination step:

Radical Chain Summary

Retrieve the route: locate the unpaired electron, split the bond homolytically, initiate with UV or heat, propagate by single-electron steps and terminate by radical combination.

Every propagation step must regenerate a radical, and every radical symbol and single-electron movement must be shown where required.