--- title: "Termination Steps in Organic Chemistry" description: "Termination steps end a radical chain reaction when two free radicals combine or disproportionate, stopping propagation in Organic Chemistry." canonical: "https://fiveable.me/organic-chem/key-terms/termination-steps" type: "key-term" subject: "Organic Chemistry" unit: "Unit 6" --- # Termination Steps in Organic Chemistry ## Definition Termination steps are the final radical reactions in Organic Chemistry that end a chain reaction by removing free radicals, usually when two radicals form stable products. ## What It Is Termination steps are the last stage of a radical chain reaction in Organic Chemistry, when the reactive chain shuts down because free radicals are used up. Instead of making more radicals, the reaction ends with stable products. The basic idea is simple: two species with unpaired electrons meet and react. Since radicals are highly reactive, this meeting usually removes both unpaired electrons at once. That is why termination stops the chain rather than extending it. There are a few common ways this happens. In radical-radical coupling, two radicals bond together to make one larger molecule. In radical-radical combination, the result is also one stable product, and in many classes these terms are used very similarly. In disproportionation, one radical pulls a hydrogen atom from the other, giving two different products. One product ends up more reduced and the other more oxidized. You usually see termination after initiation and propagation have already built up radicals in the system. Initiation creates the first radicals, propagation keeps the chain going, and termination cleans up the radicals so the reaction can no longer continue. Because termination depends on two radicals colliding, it often happens less often than propagation, but it becomes more likely as the radical concentration rises. A good way to picture it is to compare it to a relay race. Propagation passes the radical along to the next molecule, while termination is the handoff that never gets passed again. Once the radicals are gone, the chain reaction winds down and the mixture moves toward stable products. In practice, termination can leave behind a mix of products, including the desired organic product and side products from radical coupling or disproportionation. That is one reason radical reactions can be useful but not perfectly selective. ## Why It Matters Termination steps tell you why radical reactions do not run forever. In Organic Chemistry, the chain only keeps going while radicals keep getting regenerated, so the termination step marks the point where the mechanism loses its reactive intermediates. This matters any time you analyze a radical mechanism, because you need to separate the chain-starting steps from the chain-stopping steps. If you can point out where radicals are formed, how they propagate, and how they finally disappear, you can explain the whole reaction pathway instead of memorizing products by sight. Termination also helps explain product mixtures. A reaction that seems straightforward on paper may give extra byproducts because radicals can couple with each other or undergo disproportionation before the chain fully shuts down. That is especially useful in problems involving halogenation, polymerization, or other radical processes where side reactions matter. It also connects to reaction control. Conditions that increase radical concentration or give radicals more chances to collide can change how much termination happens, which changes yield and selectivity. So termination is not just the ending of a mechanism, it is part of why radical chemistry can be efficient, messy, or both depending on the setup. ## Connections ### Free Radicals Termination steps only happen because free radicals exist in the first place. These species have unpaired electrons, which makes them unstable and very reactive. When two radicals meet, their unpaired electrons can be paired up, which is what ends the chain reaction. ### Propagation Steps Propagation is the part of the radical chain reaction that keeps the mechanism going, while termination stops it. In propagation, one radical reacts and another radical is created, so the chain continues. Termination breaks that cycle by removing radicals instead of replacing them. ### [Disproportionation](/organic-chem/key-terms/disproportionation) Disproportionation is one specific termination pathway. Instead of making one coupled product, one radical transfers a hydrogen atom to another radical, producing two different molecules. That gives you one product with a double bond and another that is more saturated. ### [Homolytic Bond Cleavage](/organic-chem/key-terms/homolytic-bond-cleavage) Homolytic bond cleavage is how radicals are often formed in the first place. One electron from the bond goes to each atom, creating radical species that can enter the chain reaction. Without homolytic cleavage, you would not have the radicals that later undergo termination. ## On the AP Exam A mechanism question may give you a radical chain reaction and ask you to label the step where the reaction ends. You should look for two radical species combining, coupling, or undergoing disproportionation to form stable products. That is your termination step. In a reaction sequence, this can also show up as the final arrow in a mechanism map or as a side-product question. If radicals disappear without being regenerated, the step is terminating the chain. If the reaction creates another radical, it is propagation instead. On a problem set, you may be asked to compare possible radical pathways and identify which one stops the chain fastest or explains extra products. The skill is tracing where the unpaired electrons go, then deciding whether the chain survives or shuts down. ## Termination Steps vs Propagation Steps Propagation and termination both involve radicals, but they do opposite jobs. Propagation keeps the chain reaction going by creating a new radical, while termination ends the chain by removing radicals from the system. If the number of radicals stays the same or grows, you are probably looking at propagation. If radicals disappear, it is termination. ## Key Takeaways - Termination steps are the final radical reactions that stop a chain reaction in Organic Chemistry. - They happen when two radicals react to form stable products, so the unpaired electrons are no longer part of the mechanism. - Coupling and combination usually make one larger product, while disproportionation gives two different products. - Termination comes after initiation and propagation, and it explains why radical reactions eventually slow down and stop. - When you trace a radical mechanism, ask whether a step creates another radical or removes radicals, because that tells you whether it is propagation or termination. ## FAQs ### What is termination steps in Organic Chemistry? Termination steps are the last reactions in a radical chain mechanism. Two free radicals react and form stable products, which stops the chain from continuing. In Organic Chemistry, this is how a radical reaction loses the reactive intermediates it needs to keep going. ### How is termination different from propagation steps? Propagation keeps the chain alive by producing a new radical after each step. Termination does the opposite, because two radicals combine or disproportionate and no new radical is made. If the mechanism no longer regenerates a radical, you have reached termination. ### What are the main types of termination steps? The common termination paths are radical-radical coupling, radical-radical combination, and disproportionation. Coupling or combination gives one stable product from two radicals. Disproportionation gives two products, usually one with a double bond and one with a saturated bond. ### Why do termination steps matter in radical reactions? They explain why radical reactions do not continue forever and why side products can form. If radicals collide with each other instead of with the main substrate, the chain shuts down early or makes unexpected products. That affects yield, selectivity, and the product mixture you see in a mechanism problem. ## Related Study Guides - [6.6 Radical Reactions](/organic-chem/unit-6/radical-reactions/study-guide/H12vj2gxuKPWwqR7) ## 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/termination-steps#resource","name":"Termination Steps in Organic Chemistry","url":"https://fiveable.me/organic-chem/key-terms/termination-steps","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/organic-chem/key-terms/termination-steps#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:23:42.322Z","isPartOf":{"@type":"Collection","name":"Organic Chemistry Key Terms","url":"https://fiveable.me/organic-chem/key-terms"},"author":{"@type":"Organization","@id":"https://fiveable.me/editorial#content-team","name":"Fiveable Content Team","url":"https://fiveable.me/editorial","parentOrganization":{"@id":"https://fiveable.me/#organization"},"member":[{"@type":"Person","name":"Amanda DoAmaral","sameAs":["https://www.linkedin.com/in/adoamaral"]},{"@type":"Person","name":"Becky Bahr","sameAs":["https://www.linkedin.com/in/becky-bahr-b52743132"]}]},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/organic-chem/key-terms/termination-steps#term","name":"Termination Steps","description":"Termination steps are the final radical reactions in Organic Chemistry that end a chain reaction by removing free radicals, usually when two radicals form stable products.","url":"https://fiveable.me/organic-chem/key-terms/termination-steps","inDefinedTermSet":{"@type":"DefinedTermSet","name":"Organic Chemistry Key Terms","url":"https://fiveable.me/organic-chem/key-terms"}},{"@type":"FAQPage","mainEntity":[{"@type":"Question","name":"What is termination steps in Organic Chemistry?","acceptedAnswer":{"@type":"Answer","text":"Termination steps are the last reactions in a radical chain mechanism. 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