Bell-Evans-Polanyi principle

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Energetic consideration of the course of the reaction

The Bell-Evans-Polanyi principle is a model that establishes an energetic relationship between the activation enthalpy and the reaction enthalpy of a chemical reaction . It says that within a number of similar reactions there is a linear relationship between the above reaction constants. The activation enthalpy is therefore lower the lower the reaction enthalpy. The basics were described in publications by RP Bell , MG Evans and M. Polanyi in the 1930s .

The Bell-Evans-Polanyi principle allows both thermodynamic and kinetic statements to be formulated about a series of chemical reactions:

  1. Exothermic reactions have an energetically more favorable (earlier, more educt- similar) transition state .
  2. An energetically more favorable product causes a lower reaction barrier and thus a faster reaction (see also Arrhenius equation ).

The general derivation from the Bell-Evans-Polanyi principle is the Hammond postulate .

A good example of the Bell-Evans-Polanyi principle is the pyrolysis of azo compounds . Here, molecular nitrogen is released from the azo compound used , with the radicals of the corresponding organic radicals being formed. The enthalpy of reaction ΔH R depends on the energy content of the starting materials and the stabilization of the radicals formed. The activation enthalpies ΔH A behave relatively like the reaction enthalpies (see figure).

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  1. ↑ Entry on Bell – Evans – Polanyi principle . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.B00628 Version: 2.3.1.
  2. ^ RP Bell: The Theory of Reactions Involving Proton Transfers , in: Proc. Roy. Soc. London 1936 , 154A ; doi : 10.1098 / rspa.1936.0060 .
  3. ^ MG Evans, M. Polanyi: Inertia and driving force of chemical reactions , in: Trans. Faraday Soc. 1938 , 34 , 11-24; doi : 10.1039 / TF9383400011 .

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