Substituent effects on 1H NMR spectra of 4‐and 4′‐substituted tras‐N‐benzylideneanilines in acidic solution

Various 4‐ and 4′‐substitued trans‐N‐benlizylidenanilines were synthesized and their 1H NMR spectra recorded at 30 °C in DMSO‐d6, and acidic media (TFA‐d and D2SO4). The effects of substituents on the chemical shifts of H‐α (proton on carbon of imine group) and H‐2′ (protons on benzaldehyde ring ortho to the imine group) were analysed by the Hammett single parameter and Taft dual substituent parameter (DSP) techniques. In the neutral solvent DMSO‐d6, there is transmission of the electronic effects of substituents on the benzaldehyde ring to the H‐α and H‐2′ protons, but no electronic transmission for substituents on the aniline ring. There is a dramatic change in acid media. Protonation of the imine group results in significant electronic transmission from substituents on the aniline ring to the H‐α and H‐2′ protons, as well as an enbancement in electronic transmission from substituents on the benzaldehyde ring. DSP analysis indicates that both inductive and resonance factors are responsible for the observed electronic transmission effects, with resonance being the major factor.