Steric effects on mesomerism. XV. Comparative investigations involving ultra‐violet absorption spectra, molecular refractions, basic strengths, and rates of reaction of derivatives of nitrobenzene

Abstract

AbstractFor several series of compounds the variation in ultra‐violet spectrum, molecular refraction, basic strength, and rate of reaction, due to the presence of a more or less strong inhibition of mesomerism, has been investigated. The members of these series are derivatives of nitrobenzene, para‐nitroaniline, and para‐nitroacetanilide, substituted in such a way that the steric inhibition concerns exclusively the mesomeric interaction between the nitro group and the rest of the system.When comparing these data, it appears that the percentual decrease of the importance of the mesomerism according to each of the criteria in question is approximately the same; several linear relations are found. The non‐linearity between the extinction coefficients of nitrobenzenes and para‐ nitroanilines is interpreted as the consequence of a decrease of the average angle of twist of the nitro group upon substitution of an amino group in para position.By extrapolation of the straight lines found, data are obtained about: the exaltation in the molecular refraction of nitrobenzene resulting from the mesomeric interaction between benzene ring and substituent. the energy of mesomerism in para‐nitroaniline due to the interaction of the substituents. the relative importance of mesomeric and inductive effects of the nitro group for the basic strength of para‐nitroaniline, and for the rate of deacylation of para‐nitroacetanilide. the meta/para ratio of the inductive effect in benzene derivatives. Finally the conclusions resulting from the data with regard to the effective sizes of alkyl groups are discussed.