Solvent Effects in the Nuclear Magnetic Resonance Spectra of Benzal Compounds

Abstract

Nuclear magnetic resonance spectra of benzal compounds, Z—C₆H₄—CH=C(X)Y, have been determined in carbon tetrachloride, chloroform, acetone, and benzene solutions. Group X, trans to the ring, was one of NO₂, COOEt, COCH₃, CONH₂, the series comprising mostly para-substituted (Z=NO₂, Cl, H, Me, OMe, NMe₂) β-nitrostyrenes, β-alkyl-β-nitrostyrenes, cinnamates, cyanocinnamates, malonates, acetoacetates, acetylacetones, and cyanocinnamides. The ambiguity in the assignments to the olefinic protons in the β-nitrostyrenes was resolved by studying deuterated compounds. The solvent shifts for the aromatic protons are shown to be additive, the olefinic side chains and the ring substituents making characteristic contributions. The solvent effect for the aromatic protons ortho to the side chain is most affected by variation of the latter and, when large, may contribute to the effect at the olefinic protons. The β-nitrostyrenes show the largest solvent shifts for the olefinic protons, and the reasons for the more marked effects at the β-proton are discussed.