AbstractThe thermal cis‐trans‐isomerization of benzylidene aniline was investigated in the aprotic solvents cyclohexane (ϵ = 2), and acetonitrile (ϵ= 37) and the protic solvents ethanol (ϵ = 25) and methanol (ϵ= 36), and also in 98% H2SO4.Whereas the isomerization in the organic solvents proceeds via an inversion mechanism this reaction is drastically hindered or even blocked in 98% H2SO4.It is shown that the experimental activation energy for the cis‐trans‐inversion is composed of three parts: 1. rehybridization energy E*rehyb, 2. difference between the dipole field energies of the cis‐isomer and the kinetic transition state due to different dipole moments of these species and 3. difference in the hydrogen bond solvation energies of the cis isomer and the kinetic transition state. It can be shown that the second dielectric term vanishes in solvents with high dielectric constants. By comparison of the activation energies of the cis‐trans isomerization in cyclohexane and acetonitrile we obtain μsp < μcis. By comparison of the activation energies as measured in acetonitrile and methanol the hydrogen bond solvation energy of the cis‐isomer in methanol is estimated to be approximately ‐ 2 kcal/mol. Additionally the cis‐trans‐isomerization of 10 substituted benzylidene anilines was investigated in cyclohexane.The frequency factors for the reaction rates of the substituted benzylidene anilines are all within the narrow range from 4 · 1012 to 4 · 1013 sec‐1, whereas the activation energies vary from 14.6 to 18.8 kcal. The kinetic results agree qualitatively with those, obtained by Wettermark in ethanol. The equilibrium constant K and reaction enthalpy δ H° for the reaction trans‐benzylidene aniline ⇌ cis‐benzylidene aniline is estimated for the solvent chlorobenzene by combining appropriate kinetic data:magnified image.