Substituent Effects in the ortho Position: Model Compounds with a Removed Reaction Centre

2'-Substituted phenylpropynoic acids 1 and - in less details - 2'-substituted (E)-phenyl-iminoacetic acids 3 were investigated theoretically as model for the ortho substitution, in which direct steric interaction is prevented by the extended side chain bearing the reaction centre. Energies of the acid molecules and of their deprotonated forms were calculated within the framework of the density functional theory at the level B3LYP/6-311+G(d,p)//B3LYP/6-311+G(d,p). The substituent effects were evaluated in terms of isodesmic reactions, on the one hand for the acidity, on the other separately for the acid molecules or for the anions. Contrary to the expectation, the steric interaction of the substituent with the side chain is rather strong. The substituent effects represent a blend of polar and steric effects, and cannot serve for deriving any set of standard ortho substituent constants free of proximity effects as it was originally believed.