--- title: "Free Radical Reaction | Organic Chemistry" description: "Free radical reaction is a chain mechanism using radicals and homolytic bond cleavage, often seen in Organic Chemistry allylic bromination with NBS." canonical: "https://fiveable.me/organic-chem/key-terms/free-radical-reaction" type: "key-term" subject: "Organic Chemistry" unit: "Unit 10" --- # Free Radical Reaction | Organic Chemistry ## Definition A free radical reaction is a reaction in Organic Chemistry that uses radicals with unpaired electrons to start, continue, and end a chain mechanism. It shows up most clearly in allylic bromination and other light- or peroxide-driven reactions. ## What It Is A free radical reaction in Organic Chemistry is a reaction that runs through radicals, which are species with an unpaired electron. Because that unpaired electron is so reactive, the reaction often moves as a chain instead of stopping after one step. The chain usually starts with initiation. A covalent bond breaks by homolytic cleavage, meaning each atom takes one electron from the bond. That creates the first radical, which can come from a radical initiator such as benzoyl peroxide or from light striking a weak bond. Next comes propagation. One radical reacts with a stable molecule and makes a new bond, but it also creates a new radical in the process. That new radical keeps the chain going. In allylic bromination, for example, a bromine radical can remove an allylic hydrogen, and the resulting carbon radical can then react with bromine to form the product and regenerate another radical. This is why free radical reactions can keep going even when only a small amount of initiator is present. The reaction does not need a new radical for every product molecule, because each propagation step passes the radical along. The chain ends during termination, when two radicals meet and combine into a stable product. That stops the radical process, so termination is basically the reaction shutting itself off. In real lab conditions, you often get a mix of propagation and termination events at the same time. Organic Chemistry students usually meet this term through allylic bromination with NBS. The point of that reaction is not to add across the double bond, but to replace an allylic hydrogen while leaving the alkene intact. Understanding the radical chain explains why the reagent choices, light, heat, and low bromine concentration all matter. ## Why It Matters Free radical reaction is the mechanism behind some of the most distinctive alkene transformations in Organic Chemistry, especially allylic bromination. If you know the chain steps, you can predict why the double bond survives while the allylic position gets substituted. It also gives you a different way to think about reactivity. Instead of asking only which atom is most positive or most negative, you look at bond strength, radical stability, and how easily a hydrogen can be abstracted. That shift matters in synthesis problems, where the location of a reaction can change the whole product. This term also connects a lot of smaller ideas in the chapter. Benzoyl peroxide, NBS, homolytic cleavage, and hydrogen abstraction all make more sense once you can place them inside the radical chain. Once you see the sequence, reaction conditions stop looking random and start looking purposeful. When you recognize a radical mechanism, you can also avoid a common mistake: assuming every reaction involving bromine is an addition reaction. In this topic, bromine can be part of a chain process that substitutes at the allylic position instead of adding across the alkene. ## Connections ### Radical Initiator A radical initiator is what gets the chain started by making the first radicals. In allylic bromination, heat, light, or a peroxide initiator can trigger homolytic cleavage so the reaction can begin. Without initiation, the substrate usually just sits there because radicals are too hard to form on their own. ### [Hydrogen Abstraction](/organic-chem/key-terms/hydrogen-abstraction) Hydrogen abstraction is the propagation step where a radical removes a hydrogen atom from another molecule. In allylic bromination, this step creates the allylic radical that later becomes the product. The ease of abstraction depends a lot on bond strength and radical stability, which is why allylic C-H bonds react more readily than many others. ### [N-bromosuccinimide](/organic-chem/key-terms/n-bromosuccinimide) N-bromosuccinimide, or NBS, is the reagent that makes allylic bromination selective. It keeps the concentration of bromine low, which favors substitution at the allylic position instead of full addition to the alkene. That choice helps the radical chain do the specific job the chapter is focused on. ### [Homolytic Cleavage](/organic-chem/key-terms/homolytic-cleavage) Homolytic cleavage is the bond-breaking step that gives each atom one electron and forms radicals. This is the opposite of heterolytic cleavage, where both electrons go to one atom. In free radical reactions, homolysis is the starting point because it creates the unpaired electrons that drive the chain. ## On the AP Exam A quiz question on this term usually asks you to identify the radical steps in a reaction scheme or explain why a product formed at the allylic position instead of across the double bond. You may need to label initiation, propagation, and termination, or choose the condition that favors radical chemistry, such as light or a peroxide initiator. In mechanism problems, the move is to track the unpaired electron from step to step. If a step forms a new radical, that is propagation. If two radicals combine and the chain stops, that is termination. If you can follow the radical and explain why a hydrogen is abstracted from an allylic carbon, you are doing the exact kind of reasoning this topic asks for. ## Free Radical Reaction vs Halonium Ion A free radical reaction is a chain mechanism built around species with unpaired electrons, while a halonium ion is a bridged cation intermediate in electrophilic addition to alkenes. If bromine and an alkene appear together, check the conditions. Light, peroxide, or NBS points to a radical pathway, while Br2 without radical conditions usually points to a halonium-ion mechanism. ## Key Takeaways - A free radical reaction in Organic Chemistry runs through radicals, not through ordinary ions or carbocations. - The chain has three parts: initiation makes the first radical, propagation keeps the chain moving, and termination stops it. - Homolytic cleavage is how the first radical is usually formed, especially with light or a peroxide initiator. - Allylic bromination is the classic example, because the reaction swaps in bromine at the allylic position while keeping the alkene double bond intact. - If you can trace where the unpaired electron goes, you can usually explain the whole mechanism. ## FAQs ### What is free radical reaction in Organic Chemistry? It is a reaction mechanism that uses radicals, species with unpaired electrons, to start and maintain a chain reaction. In Organic Chemistry, the classic example is allylic bromination, where the radical chain places bromine at the allylic position instead of adding across the double bond. ### How does a free radical reaction start? It starts with initiation, when a covalent bond breaks homolytically and makes the first radical. Heat, UV light, or a radical initiator like benzoyl peroxide can provide the energy needed for that first step. ### Why is NBS used in free radical reactions? NBS keeps the bromine concentration low, which favors allylic substitution over addition to the alkene. That makes it useful for allylic bromination, where you want to preserve the double bond and install bromine next to it. ### What is the difference between free radical reaction and halonium ion reaction? A free radical reaction moves through unpaired-electron intermediates and chain steps, while a halonium ion reaction goes through a bridged cation. The conditions usually give it away: radicals need light, heat, or peroxides, but halonium-ion addition usually does not. ## Related Study Guides - [10.3 Preparing Alkyl Halides from Alkenes: Allylic Bromination](/organic-chem/unit-10/preparing-alkyl-halides-from-alkenes-allylic-bromination/study-guide/9uBIzuEgsimnIEwt) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/organic-chem/key-terms/free-radical-reaction#resource","name":"Free Radical Reaction | Organic Chemistry","url":"https://fiveable.me/organic-chem/key-terms/free-radical-reaction","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/organic-chem/key-terms/free-radical-reaction#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:23:40.731Z","isPartOf":{"@type":"Collection","name":"Organic Chemistry Key Terms","url":"https://fiveable.me/organic-chem/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/organic-chem/key-terms/free-radical-reaction#term","name":"Free Radical Reaction","description":"A free radical reaction is a reaction in Organic Chemistry that uses radicals with unpaired electrons to start, continue, and end a chain mechanism. 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