Accedi

The thermodynamic favorability of a reaction is determined by the change in Gibbs free energy (ΔG). ΔG has two components- enthalpy (ΔH) and entropy (ΔS). The entropy component is negligible for alkane halogenation because the number of reactants and product molecules are equal. In this case, the ΔG is governed only by the enthalpy component. The most crucial factor that determines ΔH is the strength of the bonds. ΔH can be determined by comparing the energy between bonds broken and bonds formed.

Based on the thermodynamics of the reaction, radical halogenation of alkanes has a different order of reactivity for fluorination, bromination, and iodination. The ΔH for radical iodination is positive (+55 kJ/mol), which suggests that the ΔG value is also positive for this reaction. Therefore, iodination is thermodynamically unfavorable, and the reaction does not take place. On the other hand, the overall ΔH for the radical fluorination of methane is large and negative (-431 kJ/mol), making the reaction thermodynamically favorable but highly exothermic and not having any practical use. The ΔH value for chlorination and bromination is -104 kJ/mol and -33 kJ/mol, respectively, making these reactions thermodynamically favorable and practically feasible. The reaction rate comparison between chlorination and bromination shows that bromination is slower than chlorination. The rate-determining step for this reaction is the first propagation step or the hydrogen abstraction step. The first propagation step for chlorination reaction is exothermic, and the energy of activation is small, while for bromination, this step is endothermic, and the energy of activation is large, which explains why bromination is slower than chlorination.

Tags

Radical HalogenationThermodynamicsGibbs Free EnergyEnthalpyEntropyAlkane HalogenationBond StrengthFluorinationBrominationIodinationReaction FavorabilityExothermic ReactionEndothermic ReactionHydrogen Abstraction StepEnergy Of Activation

Dal capitolo 20:

article

Now Playing

20.17 : Radical Halogenation: Thermodynamics

Radical Chemistry

3.6K Visualizzazioni

article

20.1 : Radicali: struttura elettronica e geometria

Radical Chemistry

3.8K Visualizzazioni

article

20.2 : Spettroscopia di Risonanza Paramagnetica Elettronica (EPR): Radicali Organici

Radical Chemistry

2.3K Visualizzazioni

article

20.3 : Formazione radicale: panoramica

Radical Chemistry

2.0K Visualizzazioni

article

20.4 : Formazione radicalica: Omolisi

Radical Chemistry

3.4K Visualizzazioni

article

20.5 : Formazione radicale: astrazione

Radical Chemistry

3.3K Visualizzazioni

article

20.6 : Formazione radicale: addizione

Radical Chemistry

1.6K Visualizzazioni

article

20.7 : Formazione radicale: Eliminazione

Radical Chemistry

1.6K Visualizzazioni

article

20.8 : Reattività radicale: panoramica

Radical Chemistry

1.9K Visualizzazioni

article

20.9 : Reattività radicale: effetti sterici

Radical Chemistry

1.8K Visualizzazioni

article

20.10 : Reattività radicale: effetti della concentrazione

Radical Chemistry

1.5K Visualizzazioni

article

20.11 : Reattività radicale: radicali elettrofili

Radical Chemistry

1.8K Visualizzazioni

article

20.12 : Reattività radicale: radicali nucleofili

Radical Chemistry

2.0K Visualizzazioni

article

20.13 : Reattività radicalica: intramolecolare vs intermolecolare

Radical Chemistry

1.7K Visualizzazioni

article

20.14 : Autossidazione radicale

Radical Chemistry

2.1K Visualizzazioni

See More

JoVE Logo

Riservatezza

Condizioni di utilizzo

Politiche

Ricerca

Didattica

CHI SIAMO

Copyright © 2025 MyJoVE Corporation. Tutti i diritti riservati