10.2 Preparing Alkyl Halides from Alkanes: Radical Halogenation
3 min read•may 7, 2024
transforms into through a series of steps. This process involves the substitution of hydrogen atoms with halogen atoms, creating a mix of products due to varying reactivity of different hydrogen types and potential .
The reactivity of hydrogens follows a specific order, with tertiary being the most reactive. Factors like bond strength, , and halogen type influence the outcome. Understanding these aspects helps predict and control the products of radical halogenation reactions.
Radical Halogenation of Alkanes
Process of radical halogenation
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- Converts alkanes to alkyl halides by substituting a hydrogen atom with a halogen atom (Cl, Br)
- step:
- of the diatomic halogen molecule () by heat or light generates two halogen radicals ()
- steps:
- : Halogen radical abstracts a hydrogen from the alkane, forming an alkyl radical and HX
- : Alkyl radical reacts with another molecule, forming the alkyl halide product and regenerating the halogen radical
- steps:
- : Two radicals combine to form a stable molecule
- Alkyl radicals can combine with each other or with halogen radicals (ethane, 2-methylpropane)
- : Two alkyl radicals react, with one abstracting a hydrogen from the other, forming an alkane and an alkene (propane and propene)
- : Two radicals combine to form a stable molecule
Mixtures in radical halogenation products
- Alkanes contain different types of hydrogens (primary, secondary, tertiary) with varying reactivity towards radical halogenation
- Alkyl radical formed during the reaction can undergo rearrangements
- or lead to more stable radicals (1-methylpropyl radical to 2-methylpropyl radical)
- Multiple propagation steps can occur before termination allowing for the formation of various (1-chlorobutane, 2-chlorobutane)
- Termination steps involving different radicals can lead to a variety of byproducts
- Combination of alkyl radicals forms higher molecular weight alkanes (hexane from propyl radicals)
- Disproportionation of alkyl radicals forms alkanes and alkenes (butane and 2-butene from butyl radicals)
Reactivity of hydrogens in chlorination
- Reactivity order: Tertiary (3°) > Secondary (2°) > Primary (1°) based on the stability of the resulting alkyl radical
- Tertiary radicals are the most stable due to stabilization by with three neighboring alkyl groups
- Secondary radicals are more stable than primary radicals with stabilization by hyperconjugation with two neighboring alkyl groups
- Primary radicals are the least stable, stabilized by hyperconjugation with only one neighboring alkyl group
- More stable radicals have lower required for formation leading to higher reaction rates and preferential formation of more substituted alkyl halides (2-chloro-2-methylbutane over 1-chloro-2-methylbutane)
- The relative stability of radicals affects the of the reaction, favoring the formation of more stable radical intermediates
Factors affecting radical halogenation
- : The strength of the C-H bond being broken influences the ease of hydrogen abstraction
- Radical stability: More stable radicals form more readily, affecting the product distribution
- Selectivity: The preference for forming certain products based on radical stability and reaction conditions
- : Different halogens (Cl, Br) have varying reactivities, impacting the overall reaction rate and product distribution
Key Terms to Review (31)
Activation Energy: Activation energy is the minimum amount of energy required to initiate a chemical reaction. It represents the energy barrier that reactants must overcome in order to form products. This concept is central to understanding the mechanisms and kinetics of organic reactions.
Activation energy, ΔG‡: Activation energy (ΔG‡) is the minimum amount of energy required to initiate a chemical reaction, specifically the energy needed to reach the transition state from the reactants. It's a crucial factor in determining the rate at which a reaction will occur in organic chemistry.
Alkanes: Alkanes are a class of saturated hydrocarbons composed entirely of single-bonded carbon and hydrogen atoms. They are the simplest organic compounds and serve as the foundation for many other organic molecules and reactions.
Alkyl Halides: Alkyl halides are organic compounds that consist of an alkyl group (a hydrocarbon chain) bonded to a halogen atom (fluorine, chlorine, bromine, or iodine). They are widely used in organic synthesis and have various applications in chemistry and biology.
Bond Dissociation Energy: Bond dissociation energy is the amount of energy required to break a specific chemical bond between two atoms, separating them into individual, free atoms. This term is crucial in understanding the stability and reactivity of molecules, as well as the energetics of chemical reactions.
Bond dissociation energy, D: Bond dissociation energy is the amount of energy required to break a bond between two atoms in a molecule into two separate, radical species. It is measured in kilojoules per mole (kJ/mol) and varies depending on the type of bond and the molecules involved.
Chain reaction: In organic chemistry, a chain reaction is a sequence of reactions where a reactive intermediate, often a free radical, initiates and propagates multiple steps that lead to the formation of products, with the process potentially continuing until terminated. This self-sustaining mechanism amplifies the effect of a single reactant through successive stages.
Chain Reaction: A chain reaction is a sequence of reactions where the products of one reaction trigger additional reactions, leading to a self-sustaining and amplifying process. This concept is central to understanding radical additions to alkenes and the preparation of alkyl halides from alkanes.
Combination: Combination is the process of two or more reactants coming together to form a new product. This term is particularly relevant in the context of radical reactions, radical additions to alkenes, radical halogenation of alkanes, and chain-growth polymerization.
Disproportionation: Disproportionation is a chemical reaction in which a single reactant is simultaneously oxidized and reduced, resulting in the formation of two or more different products. This process is an important concept in various areas of organic chemistry, including radical reactions, radical additions to alkenes, radical halogenation of alkanes, and chain-growth polymerization.
Free Radical Substitution: Free radical substitution is a type of organic reaction where a free radical intermediate is involved in the replacement of one atom or group in a molecule with another. This process is particularly relevant in the context of preparing alkyl halides from alkanes through radical halogenation.
Free Radicals: Free radicals are highly reactive chemical species that possess one or more unpaired electrons in their outer shell. These unstable molecules are capable of initiating chain reactions and can have significant impacts on various chemical processes, including those encountered in organic chemistry.
Halogen Abstraction: Halogen abstraction is a type of radical substitution reaction in organic chemistry where a halogen atom (such as chlorine, bromine, or iodine) is selectively removed from an alkane or other organic compound by a radical species. This process is a key step in the radical halogenation of alkanes, leading to the formation of alkyl halides.
Halogen Reactivity: Halogen reactivity refers to the chemical properties and behaviors of the halogen group of elements, which includes fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). These elements share similar characteristics and tend to form highly reactive compounds due to their high electronegativity and ability to accept or share electrons.
Homolytic Cleavage: Homolytic cleavage refers to the breaking of a covalent bond in a molecule in a way that results in the formation of two neutral radical species, each retaining one of the shared electrons from the original bond. This process is a key feature in radical reactions and is central to understanding the industrial preparation and use of alkenes, as well as the preparation of alkyl halides from both alkanes and alkenes.
Hydrogen Abstraction: Hydrogen abstraction is a fundamental reaction in organic chemistry where a hydrogen atom is removed from a molecule, often by a reactive species such as a radical or a base. This process is central to various topics in organic chemistry, including radical reactions, biological additions of radicals to alkenes, and the preparation of alkyl halides from alkanes and alkenes.
Hydrogen Shifts: Hydrogen shifts refer to the rearrangement of hydrogen atoms within a molecule during chemical reactions, particularly in the context of radical halogenation reactions involving alkanes. This process is an important consideration when preparing alkyl halides from alkanes.
Hyperconjugation: Hyperconjugation is a type of conjugation in organic chemistry where the sigma bonds of alkyl groups (such as methyl or ethyl) interact with adjacent pi bonds, leading to increased stability of the molecule. This stabilizing effect is particularly important in understanding the stability of carbocations and the orientation of electrophilic additions.
Initiation: Initiation is the first and critical step in various processes, marking the beginning of a sequence of events or reactions. This term is particularly relevant in the contexts of radical additions to alkenes, radical halogenation of alkanes, transcription of DNA, and translation of RNA during protein biosynthesis.
Isomeric Alkyl Halides: Isomeric alkyl halides refer to organic compounds that have the same molecular formula but different structural arrangements of the atoms. This concept is particularly relevant in the context of preparing alkyl halides from alkanes through radical halogenation, as the reaction can produce multiple isomeric products.
Primary Hydrogens: Primary hydrogens refer to the hydrogen atoms attached to the carbon atoms that are bonded to only one other carbon atom in an organic compound. These hydrogens are considered the least substituted or least alkyl-substituted positions on the carbon chain.
Propagation: Propagation refers to the process of continuing or extending a reaction or phenomenon, particularly in the context of radical reactions and chain-growth polymerization. It describes the steps that sustain and propagate a reaction once it has been initiated.
Propagation step: In the context of preparing alkenes through elimination reactions in organic chemistry, the propagation step is a phase within a chain reaction where intermediates react with stable molecules to produce new intermediates and propagate the reaction sequence. This step repeats itself, sustaining the chain reaction until terminated.
Radical Halogenation: Radical halogenation is a type of substitution reaction where a hydrogen atom in an alkane is replaced by a halogen atom, typically chlorine or bromine, through a free radical mechanism.
Radical Stability: Radical stability refers to the relative stability of a radical species, which is an uncharged molecule or fragment that contains an unpaired electron. The stability of a radical influences its reactivity and the likelihood of it participating in various chemical reactions.
Rearrangements: Rearrangements are a class of organic reactions where the atoms in a molecule are reorganized, leading to the formation of a new compound with a different structure. This term is particularly relevant in the context of polar reaction mechanisms and the preparation of alkyl halides from alkanes through radical halogenation.
Secondary Hydrogens: Secondary hydrogens refer to hydrogen atoms that are bonded to a carbon atom that is connected to two other carbon atoms. These hydrogen atoms are considered secondary because they are not the primary or terminal hydrogens on a carbon chain.
Selectivity: Selectivity refers to the ability of a chemical reaction to preferentially form one product over another, even when multiple potential products are possible. It is a critical concept in organic chemistry that describes the specificity and directionality of a reaction.
Skeletal Rearrangements: Skeletal rearrangements refer to the process where the carbon skeleton of an organic compound is reorganized, often resulting in the formation of a new structural isomer. This phenomenon is particularly relevant in the context of preparing alkyl halides from alkanes through radical halogenation.
Termination: Termination is the final step in a radical reaction mechanism where reactive radicals are converted into stable products, effectively stopping the chain reaction. This process is crucial in both synthetic organic chemistry and biological systems, as it ensures that the chain reactions do not continue indefinitely, leading to uncontrolled product formation or cellular damage.
Tertiary Hydrogens: Tertiary hydrogens refer to the hydrogen atoms bonded to a carbon atom that is connected to three other carbon atoms in an organic molecule. These hydrogens are considered tertiary because the carbon atom they are attached to has three other carbon atoms bonded to it, making it a tertiary carbon.