Analysis of the Ortho Effect: Basicity of 2-Substituted Benzonitriles

Energies of 18 ortho-substituted benzonitriles and their protonated forms were calculated within the framework of the density functional theory at the level B3LYP/6-311+G(d,p). The substituent effects were evaluated in terms of isodesmic reactions as the effects on the basicity on the one hand and separately in the nitrile molecules and in the cations on the other. The ortho effect was defined as the difference when compared to the 4-substituted isomers. It is smaller than in the case of ortho-substituted benzoic acids but not negligible (up to 25 kJ mol-1). In unprotonated molecules it may be classified as purely steric effect, in spite of the small dimension of the CN group and estimates made with the calotte models. In the protonated forms, polar properties of the substituent are decisive. In addition, a new, quite important substituent effect was discovered with the substituents OH, SH, CHO and COOCH3, which is controlled by different conformation of protonated and unprotonated molecules. Substituents OH, SH and NH2 form also weak hydrogen bonds in unprotonated nitriles. In summary, 2-substituted benzonitriles allowed evaluating of several, partly new substituent effects but cannot serve as model molecules without an ortho effect, although the constant functional group CN is sterically little pretentious.