Thermodynamic and Kinetic Aspects of Reactions

Correlations between Thermodynamic and Kinetic Aspects of Reactions 

Is there any inherent relationship between the free energy of a reaction, AG, and the rate of reaction, which is governed by AG Are more exothermic reactions 

d Angelo, Tetrahedron, 32, 2979 (1976) H. O. House, Modem Synthetic Reactions, 2nd Edition, W. A. Benjamin, Menlo Park, CA, 1972. 

Bell-Evans-Polyani Relationship. The Bell-Evans-Polyani formula deals with the relationship between reaction rate and exothermicity for a series of related single-step reactions. Evans and Polyani pointed out that for a series of homolytic atom transfer reactions of the general type 

This relationship can be explored, for example, by considering the rates of hydrogen abstraction reactions. Eor example, the E for hydrogen abstraction from simple hydrocarbons shown below can be compared with the AH of the reaction, as derived from the bond energy data in Table 3.2. The plot is shown in Eigure 3.10. 

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The equations for both the reaction of NADH to NAD and that of ethanol to acetaldehyde have been written as oxidation half reactions. If ethanol and NADH were mixed in a test tube, no reaction could take place because there would be no electron acceptor. If, however, NADH were mixed with acetaldehyde, which is an oxidized species, a transfer of electrons could take place, producing ethanol and NAD. (This reaction would take place very slowly in the absence of an enzyme to catalyze it. Here we have an excellent example of the difference between the thermodynamic and kinetic aspects of reactions. The reaction is spontaneous in the thermodynamic sense but very slow in the kinetic sense.)... 

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