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Radical Reactions: Peroxides & Halogenation

Understand radical chains, HBr peroxide regioselectivity, and selectivity in radical halogenation.

Step-by-step lessonsCurated examplesInteractive practicePrintable worksheets

1. Understand the concept

Radical reactions involve species with unpaired electrons. Typical chain reactions have initiation, propagation, and termination steps.

Learning goal: Understand radical chains, HBr peroxide regioselectivity, and selectivity in radical halogenation.

2. Use a reliable prediction strategy

  1. Recognize radical initiators such as peroxides and light where appropriate.
  2. For alkene + HBr with peroxides, use the radical chain that normally leads to anti-Markovnikov HBr addition.
  3. For ordinary ionic HBr addition without peroxides, predict Markovnikov regioselectivity.
  4. For alkane halogenation, evaluate possible hydrogen abstraction sites; a mixture of products is common.

3. Follow the reaction roadmap

Three verified reaction transformations with substrates, reagents and labeled products for Radical Reactions
Illustrative pathways; reagent choice and selectivity matter. Structures are shown in condensed notation.

4. Work through examples

Worked example 1

Propene + HBr + peroxide

CH₃CH=CH₂ + HBr/ROOR → CH₃CH₂CH₂Br

The radical chain favors 1-bromopropane (anti-Markovnikov). This peroxide effect is characteristic of HBr, not a universal rule for HX.

Worked example 2

Propene + HBr, no peroxide

CH₃CH=CH₂ + HBr → CH₃CHBrCH₃

Ordinary electrophilic addition gives 2-bromopropane under typical conditions.

5. Common mistakes

  • Extending the peroxide effect equally to HCl and HI; chain energetics make this unreliable.
  • Calling every bromination radical; Br₂ added to an alkene without light/peroxide usually proceeds via bromonium-ion electrophilic addition.
  • Treating radical alkane chlorination as a perfectly selective single-product synthesis.

6. Check understanding

Compare HBr addition to propene with and without peroxides.

Without peroxides 2-bromopropane is expected; with ROOR, 1-bromopropane is favored by radical addition.

Why might radical monochlorination of butane form more than one structural product?

Butane has primary and secondary C–H positions, giving 1-chlorobutane and 2-chlorobutane (and stereoisomeric products where relevant).

7. Practice the reactions

Use the quiz below to apply the rules to new substrates and reagents. A correct answer is explained, not merely marked.

INTERACTIVE PRACTICE

Try the reaction challenge

Answer eight curated questions. Review the explanation after each answer.

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