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

Es sei daran erinnert, dass die Anti-Markovnikow-Addition nur bei HBr beobachtet wird. Die radikalische Zugabe mit HCl und HI ist thermodynamisch ungünstig.
Aber warum?
Untersuchen wir die Thermodynamik für jeden Ausbreitungsschritt, der am Radikalmechanismus beteiligt ist.
Damit eine Reaktion spontan ist, muss die Änderung der freien Gibbs-Energie ∆G negativ sein.
Für den ersten Ausbreitungsschritt wird das Vorzeichen von ∆G durch die Konkurrenz zwischen dem Enthalpie- und dem Entropieterm bestimmt.
Im Fall von HBr und HCl dominiert bei niedrigen Temperaturen der Enthalpieterm, und ∆G wird negativ, was eine thermodynamisch günstige Reaktion impliziert.
Im Fall von HI sind jedoch sowohl die Enthalpie- als auch die Entropieterme positiv, was bedeutet, dass ∆G unabhängig von der Temperatur positiv ist, was auf eine thermodynamisch ungünstige Reaktion hinweist.
Für den zweiten Ausbreitungsschritt ist der Entropieterm nahe Null, da die Anzahl der Reaktanten und Produktmoleküle gleich ist. Der Enthalpieterm diktiert also das Vorzeichen ∆G.
Es zeigt sich, dass ∆G positiv für HCl ist, was die Reaktion thermodynamisch ungünstig macht.
Insgesamt sind die beiden Anzuchtstufen nur im Fall von HBr thermodynamisch günstig.

radikale anti-markovnikov-addition an alkene: thermodynamik

Radical Anti-Markovnikov Addition to Alkenes: Thermodynamics

Die Anti-Markovnikov-Addition von Halogenwassersubstanzenn an ein Alken ist thermodynamisch nur mit HBr möglich. Die radikalische Additionsreaktion mit anderen Halogenwassersubstanzenn wie HCl und HI ist thermodynamisch ungünstig.

<img alt="Figure1" src="/files/ftp_upload/12904/12904_PT_Figure_1.svg" style="text-align: center; height: 125px;">

Thermodynamische Faktoren

Die Temperatur beeinflusst die Spontaneität einer Reaktion, die anhand der Änderung der freien Gibbs-Energie ΔG bewertet werden kann. Ist die Änderung der freien Gibbs-Energie ΔG negativ, läuft die Reaktion spontan ab. Wie unten gezeigt, kann ΔG direkt aus den Werten der beiden Terme ΔH und -TΔS berechnet werden.

<img alt="Figure2" src="/files/ftp_upload/12904/12904_PT_Figure_2.svg" style="text-align: center; height: 14px;">

Die thermodynamische Durchführbarkeit der Anti-Markovnikov-Addition mit HBr lässt sich anhand der einzelnen am Radikalmechanismus beteiligten Propagationsschritte vorhersagen.

Im ersten Ausbreitungsschritt der Radikalreaktion wird das ∆G-Vorzeichen durch die Konkurrenz zwischen dem Enthalpie- und dem Entropie-Term bestimmt. Bei niedrigeren Temperaturen dominiert die Radikaladdition mit HBr und HCl den Enthalpieterm. Folglich ist ΔG negativ, was darauf hinweist, dass die Reaktion thermodynamisch günstig ist. Bei hohen Temperaturen ist die Reaktion jedoch endotherm und thermodynamisch ungünstig. Im Gegensatz dazu sind bei HI, unabhängig von der Temperatur, sowohl der Enthalpie- als auch der Entropieterm positiv. Infolgedessen wird ∆G positiv, was bedeutet, dass die Reaktion thermodynamisch ungünstig ist.

Im zweiten Ausbreitungsschritt ist der Entropieterm nahe Null, da die Anzahl der Reaktanten und Produktmoleküle gleich ist, was bedeutet, dass das Vorzeichen von ∆G vom Enthalpieterm abhängt. Da der Enthalpieterm bei HBr und HI dominiert, ist die Reaktion exotherm und thermodynamisch günstig. Bei HCl hingegen ist ∆G positiv, was bedeutet, dass die Reaktion endotherm und thermodynamisch ungünstig ist.

Zusammenfassend lässt sich sagen, dass die beiden Fortpflanzungsschritte der Reaktion nur mit HBr thermodynamisch günstig sind.

Radikale Anti-Markovnikov-Addition an Alkene: Thermodynamik

Die Anti-Markovnikov-Addition von Halogenwasserstoff-Halogeniden an ein Alken ist thermodynamisch nur mit HBr machbar. Die radikale Additionsreaktion mit anderen ...

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

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Organic Chemistry

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Radical Chemistry

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KEY TERMS AND CONCEPTS

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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

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EPR Hyperfine Splitting in Organic Radicals

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Radical Halogenation: Chiral Products

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radical substitution: allylic chlorination

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Allylic Chlorination by Radical Chain Reaction

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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

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Hydrogen Bromide Addition to Alkenes by Radicals

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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

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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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