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

Przypomnijmy, że dodanie anty-Markownikowa obserwuje się tylko w przypadku HBr. Radykalna addycja HCl i HI jest niekorzystna termodynamicznie.
Ale dlaczego?
Przyjrzyjmy się termodynamice dla każdego etapu propagacji zaangażowanego w mechanizm rodnika.
Aby reakcja była spontaniczna, zmiana energii swobodnej Gibbsa, ∆G, musi być ujemna.
W pierwszym kroku propagacji znak ∆G jest podyktowany konkurencją między entalpią a składnikami entropii.
W przypadku HBr i HCl w niskich temperaturach dominuje człon entalpii, a ∆G staje się ujemne, co oznacza reakcję korzystną termodynamicznie.
Jednak w przypadku HI zarówno warunki entalpii, jak i entropii są dodatnie, co oznacza, że ∆G jest dodatnie, niezależnie od temperatury, co wskazuje na termodynamicznie niekorzystną reakcję.
W drugim etapie propagacji składnik entropii jest bliski zeru, ponieważ liczba reagentów i cząsteczek produktu jest równa. Tak więc termin entalpii dyktuje znak ∆G.
Jak widać, ∆G jest dodatni dla HCl, co sprawia, że reakcja jest niekorzystna termodynamicznie.
Ogólnie rzecz biorąc, dwa etapy propagacji są termodynamicznie korzystne tylko w przypadku HBr.

rodnikowa addycja anty-markownikowa do alkenów: termodynamika

Radical Anti-Markovnikov Addition to Alkenes: Thermodynamics

Addycja halogenowodorów do alkenu anty-Markownikowa jest termodynamicznie wykonalny tylko w przypadku HBr. Reakcja addycji rodnikowej z innymi halogenowodorami, takimi jak HCl i HI, jest termodynamicznie niekorzystna.

<img alt="Figure1" src="/files/ftp_upload/12904/12904_PT_Figure_1.svg" style="text-align: center; height: 125px;">

Czynniki termodynamiczne

Temperatura wpływa na spontaniczność reakcji, którą można ocenić na podstawie zmiany energii swobodnej Gibbsa ΔG. Jeśli zmiana energii swobodnej Gibbsa, ΔG, jest ujemna, reakcja zachodzi samoistnie. Jak pokazano poniżej, ΔG można oszacować bezpośrednio na podstawie wartości dwóch składników, ΔH i -TΔS.

<img alt="Figure2" src="/files/ftp_upload/12904/12904_PT_Figure_2.svg" style="text-align: center; height: 14px;">

Termodynamiczną wykonalność addycji anty-Markownikowa za pomocą HBr można przewidzieć na podstawie każdego etapu propagacji zaangażowanego w mechanizm rodnikowy.

W pierwszym etapie propagacji reakcji rodnikowej znak ∆G jest wyznaczany przez konkurencję między wyrazami entalpii i entropii. W niższych temperaturach w określeniu entalpii dominuje addycja rodnikowa z HBr i HCl. W konsekwencji ΔG jest ujemne, co wskazuje, że reakcja jest korzystna termodynamicznie. Jednakże w wysokich temperaturach reakcja jest endotermiczna i niekorzystna termodynamicznie. Natomiast w przypadku HI, niezależnie od temperatury, zarówno entalpia, jak i entropia są dodatnie. W rezultacie ∆G staje się dodatnie, co oznacza, że reakcja jest termodynamicznie niekorzystna.

W drugim etapie propagacji człon entropii jest bliski zeru, ponieważ liczba reagentów i cząsteczek produktu jest równa, co oznacza, że znak ∆G zależy od składnika entalpii. Ponieważ w przypadku HBr i HI dominuje termin entalpia, reakcja jest egzotermiczna i korzystna termodynamicznie. Jednakże w przypadku HCl ∆G jest dodatnie, co oznacza, że reakcja jest endotermiczna i niekorzystna termodynamicznie.

Podsumowując, dwa etapy propagacji w reakcji są korzystne termodynamicznie tylko w przypadku HBr.

Rodnikowa addycja anty-Markownikowa do alkenów: termodynamika

Anty-Markownikowa dodanie halogenków wodoru do alkenu jest termodynamicznie wykonalne tylko przy użyciu HBr. Reakcja radykalnego dodawania z innymi ...

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

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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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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...

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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

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HBr Selectivity in Alkene Addition Thermodynamics

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