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Substituent Effects in the ¹³C NMR Chemical Shifts of Para-(para-substituted benzylidene amino)benzonitrile and Para-(ortho-substituted benzylidene amino)benzonitrile

G.F. Fadhil^a and A.H. Essa^b,*

^aDepartment of Chemistry, College of Science, University of Dohuk, Dohuk, Iraq
^bDepartment of Chemistry, College of Science, University of Basrah, Basrah, Iraq

¹³C NMR chemical shifts were measured in CDCl₃ for two series of substituted benzylidene anilines. The substituted benzylidene anilines p-X-C₆H₄CH=NC₆H₄-p-CN p-X-C₆H₄CH=NC₆H₄-o-CN (X = NO₂, F, Cl, Br, H, Me, MeO, NMe₂). The substituent dependence of ×_C(C=N) was used as a tool to study electronic substituent effects on the azomethine unit. The benzylidene substituents X have a reverse effect on ×_C(C=N): electron-withdrawing substituents cause shielding, while electron-donating ones do the reverse, the resonance effects clearly predominating over the inductive effects. Additionally, the presence of a specific cross-interaction between X and C=N could be verified. The electronic effects of the neighboring aromatic ring substituents systematically modify the sensitivity of the C=N group to the electronic effects of the benzylidene substituents. These results can be rationalized in terms of the substituent-sensitive balance of the electron delocalization (mesomeric effects).

Keywords: N-Benzylideneaniline, Substituent effects, DSP analysis, Reynolds' model