Steric effects on mesomerism. XVI). Comparative investigations involving ultra‐violet absorption spectra, molecular refractions, rates of reaction, and basic strengths of aromatic amines

In several series of compounds with a more or less strong inhibition of the mesomerism, the variation in ultra‐violet spectrum, molecular refraction, rate of reaction, and basic strength has been investigated. The members of the present series are mainly derivatives of aniline, dimethylaniline, and their para‐nitro and para‐ethoxycarbonyl derivatives. They have been so chosen that any steric inhibition that may occur, concerns exclusively the mesomeric interaction between the amino group and the rest of the system. Upon comparison of the data it is found that none of the aniline derivatives examined shows any appreciable steric inhibition of the mesomerism. It is also on this basis that the relatively low basic strengths of ortho‐alkylanilines have been interpreted as due to steric hindrance to solvation. In the case of the dimethylaniline derivatives, where inhibition of the mesomerism occurs, linear relations are found between the extinction coefficient ϵ and the exaltation of the molecular refraction. Further the reaction rate data are in agreement with the linear relation between ϵ and the mesomeric energy found earlier. It is also on this basis that an estimation has been made of the magnitude of the anomalies in the basic strengths of ortho‐alkyl dimethylanilines. These have been interpreted as being at least mainly due to steric hindrance to solvation. By extrapolations of the straight lines found, data have been obtained on the exaltations of the molecular refractions in dimethylaniline and ethyl para‐dimethylaminobenzoate, and also on the relative importance of inductive and mesomeric effects for the alkaline hydrolysis of the latter compound.