Radical Substitution
Radical Substitution - Related Videos
01:31
Radical Substitution: Allylic Chlorination
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2023
Typically, when alkenes react with halogens at low temperatures, an addition reaction occurs. However, upon increasing the temperature or under reaction conditions that form radicals, providing a low but steady concentration of halogen radicals, allylic substitution reaction is favored. This is because allylic hydrogens are very reactive as the formed intermediate is resonance stabilized. For example, when propene is treated with chlorine in the gas phase at 400 °C, it undergoes allylic...
01:27
Radical Substitution: Allylic Bromination
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2025
In organic synthesis, the formation of products can be altered by changing the reaction conditions. For example, a dibromo addition product is formed when propene is treated with bromine at room temperature. In contrast, propene undergoes allylic substitution in non-polar solvents at high temperatures to give 3-bromopropene. In order to avoid the addition reaction, the bromine concentration must be kept as low as possible throughout the reaction. This can be achieved using N-bromosuccinimide...
01:27
Radical Substitution: Halogenation of Alkanes and Alkyl Substituents
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2023
In the presence of heat or light, alkanes react with molecular halogens to form alkyl halides by a substitution reaction called radical halogenation. This reaction has three steps: initiation, propagation, and termination, as seen in the radical chlorination of methane to produce methyl chloride. In the initiation step of the reaction, the chlorine molecule undergoes homolytic cleavage in the presence of light or heat, forming two highly reactive chlorine radicals. Propagation occurs in two...
14:22
Free Radicals in Chemical Biology: from Chemical Behavior to Biomarker Development
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Cited by 9 •
2013
Radical-based biomimetic chemistry has been applied to building-up libraries necessary for biomarker development.
01:26
Radical Substitution: Hydrogenolysis of Alkyl Halides with Tributyltin Hydride
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2023
Radical substitution reactions can be used to remove functional groups from molecules. The hydrogenolysis of alkyl halides is one such reaction, where the weak Sn–H bond in tributyltin hydride reacts with alkyl halides to form alkanes. Here, the reagent Bu3SnH yields tributyltin halide as a byproduct. The bonds formed in this reaction are stronger than the bonds broken, making it energetically favorable. The reaction follows a radical chain mechanism similar to radical halogenation reactions,...
FAQs
Ultraviolet light or heat supplies the energy needed for a halogen molecule to undergo homolytic fission. In this process, the bond breaks so that each atom retains one electron, producing two halogen radicals. These highly reactive species begin the chain sequence by attacking molecules of the alkane, rather than serving merely as passive reaction conditions.
Propagation proceeds through two linked reactions. First, a halogen radical abstracts a hydrogen atom from an alkane, forming a hydrogen halide and an alkyl radical. The alkyl radical then reacts with another halogen molecule, producing an alkyl halide and a new halogen radical. Because the radical is regenerated, the sequence can continue through repeated cycles.
Termination removes radicals from the reaction sequence when two radicals combine. This stops the chain at that event because the reactive radical species are no longer available to continue propagation. Termination therefore competes with the steps that generate alkyl halide, and its occurrence helps explain why radical chain reactions do not proceed indefinitely.
An alkane may contain more than one carbon site where hydrogen abstraction can occur. Formation of alkyl radicals at different sites can therefore lead to different alkyl halides, known as isomers. This competing substitution lowers selectivity when a single product is desired, making site preference an important consideration in applying the reaction to synthesis.
A typical setup combines an alkane with a halogen and exposes the mixture to ultraviolet light or heat. The energy initiates homolytic fission of the halogen molecule, after which hydrogen abstraction and halogen transfer proceed through radical intermediates. The chosen alkane structure determines which carbon sites are available for substitution and whether multiple products may form.
Radical substitution provides a route for converting relatively unreactive carbon-hydrogen bonds in alkanes into carbon-halogen-containing products, specifically alkyl halides. This transformation is useful when an alkane framework must be functionalized. However, the possibility of substitution at several carbon sites can complicate product control, so its synthetic value must be considered alongside its selectivity limitations.
Product analysis should consider both the formation of the intended alkyl halide and the possible presence of isomers. Different carbon sites can undergo hydrogen abstraction, so the reaction may generate a mixture rather than one compound. The observed products provide information about how substitution was distributed across the alkane and reveal the practical challenge of controlling reaction selectivity.