Homolytic Fission (DP IB Chemistry): Revision Note

Richard Boole

Written by: Richard Boole

Reviewed by: Philippa Platt

Updated on

Homolytic fission

  • In a reaction mechanism, curly arrows show the movement of electrons

  • A single-headed curly arrow shows the movement of a single electron

    • These arrows can be called single barbed arrows, fish-hook arrows or half-curly arrows

What is homolytic fission?

  • Homolytic fission is breaking a covalent bond in such a way that each atom takes an electron from the bond to form two radicals

    • Remember: A radical is a chemical species that contains an unpaired electron

  • The homolytic fission of halogens is the initiation step (first step) in a sequence of steps that form a chain reaction

Homolytic fission of a chlorine-chlorine bond

Chemical reaction equation showing chlorine molecule (Cl2) splitting into two chlorine radicals (2Cl•) with arrows indicating bond breaking.
The covalent bond breaks evenly and each chlorine atom receives one electron resulting in the formation of two chlorine radicals, Cl•
  • The mechanism of homolytic fission can also be represented using Lewis formulas

Mechanism of homolytic fission using Lewis formulas

Chemical reaction showing a chlorine molecule splitting into two chlorine radicals; curved arrows indicate the bond breaking process.
The mechanism of homolytic fission using Lewis formulas shows the specific movement of electrons
  • When drawing mechanisms, ensure:

    • That the curly arrows start at an electron-rich region

      • In this case, the curly arrows should start from the middle of the covalent bond

    • That the curly arrows finish at their correct destination

      • In this case, each curly arrow should finish at a chlorine atom

Types of homolytic fission

  • Since bond breaking is an endothermic process, energy is required for homolytic fission to occur

  • The amount of energy required depends on the strength of the covalent bond being broken

  • Thermolytic fission: For weaker bonds, simply heating the compound could provide sufficient energy

Xbold colonX rightwards arrow with heat on top X• + X• 

  • Photolytic fission: For stronger bonds such as halogen bonds, exposing the compound to high-energy UV light provides the required energy

Xbold colonX begin mathsize 20px style rightwards arrow with U V space light on top end style X• + X• 

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Richard Boole

Author: Richard Boole

Expertise: Chemistry Content Creator

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.

Philippa Platt

Reviewer: Philippa Platt

Expertise: Chemistry Content Creator

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener

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