Strain energy from equilibrium constants

In suitable cases such as cis/trans-isomers, the difference in strain energies can be determined from equilibrium constants or heats of hydrogenation. It is apparent from Table 20 that both methods give similar results. The strain energy of the Bredt-olefin 69 has been estimated from the heat of reaction with acetic acid, with the calculated AHR of the corresponding reaction of 2-methyl-2-butene (257a) as standard (219). 

The principle behind calculating the isomer distribution from the strain energies is the classical thermodynamic relationship between the difference in free energy (G) and equilibrium constant K,... 

The equilibrium constants for a series of cycloalkenes decrease in the order norbomene > c -cyclooc-tene > cyclopentene > cycloheptene > cyclohexene, which correlates with the calculated strain energies as well as the kinetically determined relative adsorption constants on Pt (Table 2). Tolman states that electron donation from a filled metal rf-orbital to an empty alkene Tr -orbital is extremely important in determining the stability of these complexes. Steric effects of substituents are relatively unimportant compared to electronic effects, and resonance is more important than inductive interactions. The ability of the metal to back bond is lowered progressively in the series Ni° > Pt° > Rh > Pt" > Ag which reduces the importance of resonance and decreases the selectivity of the metal for different substituted alkenes. 

Here Ki are the reaction equilibrium constants, y come from the additional strain contribution to the total excess free energy of mixing [5]. 

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