Reversal of substituent effect on electronic absorption spectra of N-(4-substituted phenyl)-benzamides in different solvents

摘要

Absorption spectra of eleven N-(4-substituted phenyl)-benzamides have been recorded in ten solvents in the range 200-400 nm. The substituents at the phenyl nucleus are as follows: N (CH_3)_2, OCH_3, CH_3, H, Cl, Br, F, CN, CF_3, COCH_3 and NO_2. The effects of substituents on the absorption spectra of investigated compounds are interpreted by correlation of absorption frequencies with simple and extended Hammett equation. When the electron-releasing substituents are attached to the nitrogen atom substituents effects are transmitted through the amide bond by the usual mechanism. However, the effect of electron withdrawing substituents appears to be quite opposite. The effect of solvent polarity and hydrogen bonding on the absorption spectra are interpreted by means of linear solvation energy relationships using a general equation of the form v = v_o + s#pi + b#beta# + a#alpha#, where #pi# is a measure of the solvent polarity, #beta# is the scale of the solvent hydrogen bond acceptor basi-cities and #alpha# is the scale of the solvent hydrogen bond donor acidities. The results obtained for N-(4-substituted phenyl)-benzamides are compared with the results for N-(4-substituted phenyl)-2-phenylacetamides under the same experimental conditions.