Name: alkane halogenation: stability and reaction rates
Uploaded: 2023-04-30T11:21:58.000Z
Duration: 1 min 34 s
Channel: JoVE

L’halogénation radicalaire des alcanes suit un ordre de réactivité différent pour différents halogènes.
Cette différence peut être comprise à partir de la thermodynamique de la réaction.
Pour l’halogénation aux alcanes, le terme d’entropie est négligeable car le nombre de réactifs et de molécules de produit est égal. Par conséquent, ΔG devient équivalent à ΔH.
Considérons la fluoration radicale du méthane. L’enthalpie de la réaction est estimée à partir des énergies de dissociation des liaisons rompues et des liaisons formées.
Le ΔH global de la réaction est négatif et important, ce qui rend la réaction thermodynamiquement favorable mais hautement explosive et peu pratique.
À l’inverse, pour l’iodation, la valeur positive de ΔH la rend thermodynamiquement défavorable et impossible à atteindre. 
Par conséquent, seules la chloration et la bromation avec des valeurs négatives de ΔH sont thermodynamiquement favorables et pratiquement réalisables.
Une comparaison des deux réactions révèle que la bromation est plus lente que la chloration.
Un examen plus approfondi des différentes étapes de propagation montre que la première étape de propagation de la bromation est plus endothermique, ce qui affecte la vitesse de réaction globale.
Comme la réaction nette est toujours exothermique, la bromation se produit, mais lentement.

halogénation radicale : thermodynamique

Radical Halogenation: Thermodynamics

La favorabilité thermodynamique d'une réaction est déterminée par la variation de l'énergie libre de Gibbs (ΔG). ΔG a deux composantes : l'enthalpie (ΔH) et l'entropie (ΔS). La composante d'entropie est négligeable pour l'halogénation des alcanes car le nombre de réactifs et de molécules de produits sont égaux. Dans ce cas, le ΔG est régi uniquement par la composante enthalpique. Le facteur le plus crucial qui détermine ΔH est La puissance des liaisons. ΔH peut être déterminé en comparant l’énergie entre les liaisons rompues et les liaisons formées.

Sur la base de la thermodynamique de la réaction, l'halogénation radicalaire des alcanes a un ordre de réactivité différent pour la fluoration, la bromation et l'iodation. Le ΔH pour l'iodation radicalaire est positif (+55 kJ/mol), ce qui suggère que la valeur ΔG est également positive pour cette réaction. Par conséquent, l'iodation est thermodynamiquement défavorable et la réaction n'a pas lieu. D'autre part, le ΔH global pour la fluoration radicalaire du méthane est grand et négatif (-431 kJ/mol), ce qui rend la réaction thermodynamiquement favorable mais hautement exothermique et n'ayant aucune utilité pratique. La valeur ΔH pour la chloration et la bromation est respectivement de -104 kJ/mol et -33 kJ/mol, ce qui rend ces réactions thermodynamiquement favorables et pratiquement réalisables. La comparaison des vitesses de réaction entre la chloration et la bromation montre que la bromation est plus lente que la chloration. L'étape déterminante pour cette réaction est la première étape de propagation ou l'étape d'extraction d'hydrogène. La première étape de propagation de la réaction de chloration est exothermique et l'énergie d'activation est faible, tandis que pour la bromation, cette étape est endothermique et l'énergie d'activation est grande, ce qui explique pourquoi la bromation est plus lente que la chloration.

Halogénation radicale : thermodynamique

La faveur thermodynamique d’une réaction est déterminée par la variation de l’énergie libre de Gibbs (ΔG). ΔG a deux composantes : l’enthalpie (ΔH) ...

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alkane halogenation: stability and reaction rates

Alkane halogenation is shaped by thermodynamic stability and reaction rate. In radical halogenation, the key question is whether the reaction is favorable and how fast it happens. Gibbs free energy, or ΔG, helps answer the first question.

ΔG has two parts: enthalpy, or ΔH, and entropy, or ΔS. For alkane halogenation, the entropy term is negligible because the number of reactant and product molecules is the same. That means ΔG is controlled mainly by enthalpy.

Bond strength is the most important factor in ΔH. Scientists estimate ΔH by comparing the energy needed to break bonds with the energy released when new bonds form. This comparison explains why fluorination, chlorination, bromination, and iodination do not all behave the same way.

Radical iodination has a positive ΔH of +55 kJ/mol. That makes ΔG positive as well, so the reaction is thermodynamically unfavorable and does not take place. Radical fluorination of methane has a very large negative ΔH of -431 kJ/mol, so it is favorable but highly exothermic and not practical.

Chlorination and bromination are both thermodynamically favorable. Their ΔH values are -104 kJ/mol for chlorination and -33 kJ/mol for bromination. This makes both reactions feasible, but bromination happens more slowly than chlorination.

The rate-determining step is the first propagation step, also called hydrogen abstraction. For chlorination, this step is exothermic and has a small activation energy. For bromination, the same step is endothermic and has a large activation energy, which explains the slower rate.

Alkane Halogenation: Stability and Reaction Rates

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Radical geometry depends on the electronic structure around the radical center. An unpaired electron, like a lone pair, affects the shape of the molecule. For a trivalent radical, the structure falls between the geometries of carbocations and carbanions. A carbocation with sp2 hybridization is trigonal planar. A carbanion with sp3 hybridization is trigonal pyramidal. The difference comes from the number of nonbonding electrons. The carbocation has none, while the carbanion has two. A carbon...

electron paramagnetic resonance (epr) spectroscopy: organic radicals

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EPR Hyperfine Splitting in Organic Radicals

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Homolysis and Radical Bonds

Homolysis is a bond-breaking process that splits a shared electron pair evenly. When a bond is broken with enough energy, the two electrons can also move to one atom in heterolysis, which forms ions. Homolysis gives one electron to each atom and creates radicals, which are highly reactive particles with an unpaired electron. HCl shows the difference between these bond cleavages. In solution, HCl can break into H+ and Cl− ions, and chlorine takes both bonding electrons, leaving a naked proton.

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Radical Abstraction in Chain Reactions

Radical abstraction helps drive radical chain reactions. In this process, a relatively unstable radical removes an atom from a compound and forms a new radical that is more stable. The atom taken in this step can be hydrogen or a halogen. A radical initiator can first form radicals by homolysis, which is the breaking of a spin-paired molecule into two radicals. That is different from abstraction. In abstraction, a radical reacts with a spin-paired molecule and produces one new radical plus one...

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Radical Addition and Spin Conservation

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Radicals from Elimination Reactions

Radicals can form through elimination reactions. This route is the opposite of the addition route. It is driven by the instability of the radical. Dibenzoyl peroxide is one example of this process. It first undergoes homolysis, which is an even split of a bond, to give a pair of unstable radicals. Because the radical is unstable, it then undergoes elimination on its own. During elimination, a C-C bond breaks. The cleavage occurs between the alpha and beta positions relative to the radical...

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Radical Reaction Pathways and Stability

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Electrophilic Radicals and Alkene Reactions

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Radical Cyclization in One Molecule

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Autoxidation in Oils and Ethers

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Selective MnO2 Oxidation of Alcohols

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How Alkanes Form Alkyl Halides by Halogenation

Alkanes can react with halogens to form alkyl halides when heat or light is present. This reaction is called radical halogenation. It is a substitution reaction, which means a hydrogen atom on the alkane is replaced by a halogen atom. Radical chlorination of methane shows the process clearly. The reaction begins with initiation, when a chlorine molecule splits evenly under light or heat. This homolytic cleavage forms two very reactive chlorine radicals. The next stage is propagation. In the...

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Alkane halogenation is shaped by thermodynamic stability and reaction rate. In radical halogenation, the key question is whether the reaction is favorable and how fast it happens. Gibbs free energy, or ΔG, helps answer the first question. ΔG has two parts: enthalpy, or ΔH, and entropy, or ΔS. For alkane halogenation, the entropy term is negligible because the number of reactant and product molecules is the same. That means ΔG is controlled mainly by enthalpy. Bond strength is the most...

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Radical Halogenation: Chiral Products

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Allylic Chlorination by Radical Chain Reaction

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radical substitution: allylic bromination

Allylic Bromination with NBS

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Alkyl Halide Reduction with Tributyltin Hydride

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radical anti-markovnikov addition to alkenes: overview

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Peroxide Effect in HBr Addition to Alkenes

Hydrogen bromide can add to alkenes through a peroxide-driven radical pathway. In this reaction, hydroperoxides or peroxides change the usual addition pattern and lead to alkyl bromides with anti-Markovnikov regiochemistry, which means bromine ends up on the less substituted carbon. This regioselectivity can be explained by radical stability. When the bromine radical adds to the alkene at the less substituted carbon, the reaction forms a more stable tertiary radical intermediate. The same...

radical anti-markovnikov addition to alkenes: mechanism

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Hydrogen Bromide Addition to Alkenes by Radicals

Hydrogen bromide can add to alkenes through a radical chain reaction when hydroperoxides or peroxides are present. This pathway gives anti-Markovnikov addition, which means the bromine ends up on the less substituted side of the double bond. The reaction begins with chain initiation. First, mild heating breaks the weak peroxide bond by homolytic cleavage, which means each atom keeps one electron and two alkoxy radicals form. Next, an alkoxy radical abstracts a hydrogen atom and forms a bromine...

radical anti-markovnikov addition to alkenes: thermodynamics

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HBr Selectivity in Alkene Addition Thermodynamics

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vicinal diols via reductive coupling of aldehydes or ketones: pinacol coupling overview

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Pinacol Coupling and Solvent Effects

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alkenes via reductive coupling of aldehydes or ketones: mcmurry reaction

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Titanium-Coupled Alkenes from Carbonyls

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&alpha;-hydroxy ketones via reductive coupling of esters: acyloin condensation overview

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Acyloin Condensation: Ester Coupling Pathway

Acyloin condensation is a reaction that joins two ester molecules to form an alpha-hydroxy ketone. The product is also called an acyloin. In this reaction, sodium metal is used in an aprotic solvent, which supports the reductive coupling step. The reaction is related to the pinacol and McMurry reactions, which also involve reductive coupling of ketones or aldehydes. Here, the ester first becomes a ketyl. These ketyl species undergo radical dimerization and form an unstable tetrahedral...

key terms and concepts

Explore how bond strength, ΔH, and ΔG affect radical halogenation of alkanes, and why chlorination, bromination, fluorination, and iodination differ.

Alkane Halogenation: Stability and Reaction Rates | JoVE K12

organic chemistry

JoVE Core Chimie Organique explique les concepts de base de la stéréochimie, des réactions et des mécanismes à l'aide de leçons vidéo animées concises et faciles à comprendre. De plus, les vidéos de scientifiques à l'action présentent l'application de ces concepts grâce à des expériences de recherche classiques et originales réalisées dans des laboratoires modernes.

Alkane Halogenation: Stability and Reaction Rates

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