Name: hbr selectivity in alkene addition thermodynamics
Uploaded: 2025-05-22T09:05:43.000Z
Duration: 1 min 32 s
Channel: JoVE

Rappelons que l’addition anti-Markovnikov n’est observée qu’avec HBr. L’addition radicale avec HCl et HI est thermodynamiquement défavorable.
Mais pourquoi?
Examinons la thermodynamique de chaque étape de propagation impliquée dans le mécanisme radicalaire.
Pour qu’une réaction soit spontanée, la variation de l’énergie libre de Gibbs, ∆G, doit être négative.
Pour la première étape de propagation, le signe de ∆G est dicté par la compétition entre l’enthalpie et l’entropie.
Dans le cas de HBr et HCl, le terme d’enthalpie domine à basse température, et ∆G devient négatif, ce qui implique une réaction thermodynamiquement favorable.
Cependant, dans le cas de HI, les termes d’enthalpie et d’entropie sont positifs, ce qui implique que ∆G est positif, quelle que soit la température, indiquant une réaction thermodynamiquement défavorable.
Pour la deuxième étape de propagation, le terme d’entropie est proche de zéro car le nombre de réactifs et de molécules de produit est égal. Ainsi, le terme d’enthalpie dicte le signe de ∆G.
Comme il est évident, ∆G est positif pour HCl, ce qui rend la réaction thermodynamiquement défavorable.
Dans l’ensemble, les deux étapes de propagation ne sont thermodynamiquement favorables que dans le cas de HBr.

ajout radical anti-markovnikov aux alcènes : thermodynamique

Radical Anti-Markovnikov Addition to Alkenes: Thermodynamics

L'addition anti-Markovnikov d'halogénures d'hydrogène à un alcène n'est thermodynamiquement réalisable qu'avec HBr. La réaction d'addition radicale avec d'autres halogénures d'hydrogène comme HCl et HI est thermodynamiquement défavorable.

<img alt="Figure1" src="/files/ftp_upload/12904/12904_PT_Figure_1.svg" style="text-align: center; height: 125px;">

Facteurs thermodynamiques

La température influence la spontanéité d'une réaction, qui peut être évaluée en fonction de la variation de l'énergie libre de Gibbs, ΔG. Si la variation de l’énergie libre de Gibbs, ΔG, est négative, la réaction se produit spontanément. Comme indiqué ci-dessous, ΔG peut être évalué directement à partir des valeurs des deux termes, ΔH et −TΔS.

<img alt="Figure2" src="/files/ftp_upload/12904/12904_PT_Figure_2.svg" style="text-align: center; height: 14px;">

La faisabilité thermodynamique de l'addition d'anti-Markovnikov avec HBr peut être prédite à partir de chaque étape de propagation impliquée dans le mécanisme radicalaire.

Dans la première étape de propagation de la réaction radicalaire, le signe ∆G est déterminé par la compétition entre les termes d'enthalpie et d'entropie. À des températures plus basses, l’addition radicalaire avec HBr et HCl domine le terme d’enthalpie. Par conséquent, ΔG est négatif, indiquant que la réaction est thermodynamiquement favorable. Cependant, à haute température, la réaction est endothermique et thermodynamiquement défavorable. En revanche, avec HI, quelle que soit la température, les termes d’enthalpie et d’entropie sont positifs. En conséquence, ∆G devient positif, ce qui signifie que la réaction est thermodynamiquement défavorable.

Dans la deuxième étape de propagation, le terme d'entropie est proche de zéro car le nombre de réactifs et de molécules de produits sont égaux, ce qui signifie que le signe ∆G dépend du terme d'enthalpie. Puisque le terme d’enthalpie domine pour HBr et HI, la réaction est exothermique et thermodynamiquement favorable. Cependant, avec HCl, ∆G est positif, ce qui signifie que la réaction est endothermique et thermodynamiquement défavorable.

En résumé, les deux étapes de propagation de la réaction ne sont thermodynamiquement favorables qu'avec HBr.

Ajout radical anti-Markovnikov aux alcènes : thermodynamique

L’ajout anti-Markovnikov d’halogénures d’hydrogène à un alcène n’est réalisable thermodynamiquement qu’avec HBr. La réaction d’addition radicale avec d’autres ...

radical-anti-markovnikov-addition-to-alkenes-thermodynamics

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Organic Chemistry

Chimie organique

Unit 1

Unité 1

Radical Chemistry

Chimie radicalaire

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

hbr selectivity in alkene addition thermodynamics

Anti-Markovnikov addition of hydrogen halides to alkenes depends on thermodynamics. In this radical reaction, only HBr makes the overall process feasible. With HCl and HI, the same addition is thermodynamically unfavorable.

Temperature affects whether the reaction is spontaneous. Spontaneity is measured with Gibbs free energy, or ΔG. When ΔG is negative, the reaction can occur on its own. ΔG is determined by two terms: ΔH, which is the enthalpy change, and −TΔS, which is the entropy term.

The first propagation step depends on the balance between enthalpy and entropy. At lower temperatures, radical addition with HBr and HCl is driven by the enthalpy term, so ΔG is negative and the step is favorable. At higher temperatures, the reaction becomes endothermic and thermodynamically unfavorable. With HI, both the enthalpy and entropy terms are positive at any temperature, so ΔG stays positive.

The second propagation step is different because the number of reactant and product molecules is equal. That makes the entropy term close to zero, so ΔG depends mainly on enthalpy. For HBr and HI, the enthalpy term makes the step exothermic and favorable. For HCl, ΔG is positive, so the step is endothermic and unfavorable. Taken together, both propagation steps are thermodynamically favorable only with HBr.

HBr Selectivity in Alkene Addition Thermodynamics

hbr-selectivity-in-alkene-addition-thermodynamics

radicals: electronic structure and geometry

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Radical Geometry: Planarity and Inversion

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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radical formation: overview

How Radicals Are Generated

Radicals are generated when bonds break in specific ways or when one radical leads to another. In homolytic bond cleavage, a bond splits evenly and forms radicals. In heterolytic bond cleavage, the bond breaks to form ions instead. A fishhook arrow shows the movement of a single electron during homolytic cleavage. Radicals can come from spin-paired molecules by homolysis or by electron transfer. Homolysis forms two radicals. Electron transfer adds an electron to the molecule, and this process...

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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.

radical formation: abstraction

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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radical formation: elimination

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...

radical reactivity: overview

Radical Reaction Pathways and Stability

Radical reactions follow a small set of pathways that help highly reactive species become more stable. A radical can react with another radical, with a spin-paired molecule, or through unimolecular decomposition, which means one radical species breaks down on its own. In radical-radical coupling, two radicals join to form a spin-paired molecule. In a radical reaction with a spin-paired molecule, a new radical and a new spin-paired molecule are formed. In radical decomposition, one radical...

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Electrophilic Radicals and Alkene Reactions

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Nucleophilic Radicals and Alkene Traps

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radical reactivity: intramolecular vs intermolecular

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Radical Cyclization in One Molecule

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radical autoxidation

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

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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...

radical halogenation: stereochemistry

Radical Halogenation: Chiral Products

Radical halogenation can change the chirality of a molecule, which is the three-dimensional arrangement of its atoms. The products depend on whether the reaction creates a new chiral center, reacts at an existing chiral center, or adds a second chiral center. When radical halogenation forms a new chiral center, the product can be a racemic mixture. For example, halogenation of butane gives 2-chlorobutane and 1-chlorobutane. The 2-chlorobutane product has a chiral center, but it is formed as a...

radical substitution: allylic chlorination

radical-substitution-allylic-chlorination

Allylic Chlorination by Radical Chain Reaction

Allylic chlorination is a radical chain reaction in alkenes. It happens when halogen reactions move away from simple addition and toward substitution at the allylic position, which is the carbon next to a double bond. At low temperatures, alkenes usually react with halogens by addition. When the temperature increases, or when the reaction conditions create radicals, allylic substitution becomes favored. This works because allylic hydrogens are very reactive and the intermediate formed is...

radical substitution: allylic bromination

Allylic Bromination with NBS

Allylic bromination with NBS changes propene into allyl bromide under radical conditions. In this reaction, the goal is to favor substitution at the allylic position instead of bromine addition across the double bond. Using N-bromosuccinimide, or NBS, helps keep the bromine concentration low so the addition product is less likely to form. The reaction conditions matter. Propene gives a dibromo addition product with bromine at room temperature. In non-polar solvents at high temperature, propene...

radical substitution: hydrogenolysis of alkyl halides with tributyltin hydride

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Alkyl Halide Reduction with Tributyltin Hydride

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radical anti-markovnikov addition to alkenes: overview

radical-anti-markovnikov-addition-to-alkenes-overview

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

radical-anti-markovnikov-addition-to-alkenes-mechanism

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

Anti-Markovnikov addition of hydrogen halides to alkenes depends on thermodynamics. In this radical reaction, only HBr makes the overall process feasible. With HCl and HI, the same addition is thermodynamically unfavorable. Temperature affects whether the reaction is spontaneous. Spontaneity is measured with Gibbs free energy, or ΔG. When ΔG is negative, the reaction can occur on its own. ΔG is determined by two terms: ΔH, which is the enthalpy change, and −TΔS, which is the entropy term. The...

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

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Pinacol Coupling and Solvent Effects

Pinacol coupling is a radical dimerization reaction that joins aldehydes or ketones to form vicinal diols. Wilhelm Rudolph Fittig first discovered this reaction in 1859. It is a reductive coupling, which means the molecules are joined after they gain electrons. The reaction begins with a single electron transfer from metals such as sodium or magnesium to an aldehyde or ketone. This forms a ketyl, a radical anion. In a ketyl, the carbon atom can carry radical character while the oxygen atom...

alkenes via reductive coupling of aldehydes or ketones: mcmurry reaction

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Titanium-Coupled Alkenes from Carbonyls

The McMurry reaction turns aldehydes or ketones into alkenes with titanium metal. It first forms a diol-like intermediate through a coupling step, and then removes oxygen to make the double bond. In some cases, this process can be compared to pinacol coupling, which also joins carbonyl compounds. Titanium acts as a source of electrons in the first step. It transfers a single electron to the carbonyl group and forms a ketyl intermediate. That intermediate can lead to a vicinal diol, which is a...

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

hydroxy-ketones-via-reductive-coupling-esters-acyloin-condensation

Acyloin Condensation: Ester Coupling Pathway

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key terms and concepts

See why anti-Markovnikov addition to alkenes is thermodynamically favorable only with HBr, and how ΔG, enthalpy, entropy, and temperature affect each step.

HBr Selectivity in Alkene Addition Thermodynamics | JoVE K12

organic chemistry

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HBr Selectivity in Alkene Addition Thermodynamics

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