Cis Equilibrium Research Articles

Proton NMR Studies of Substituent Effects. The Hammett-type Relationship in meta- and para-Substituted Formanilides

Abstract The spectral analyses of 23 meta- and para-substituted formanilides in dimethyl sulfoxide have proved that they exist in an equilibrium of cis- and trans-forms of restricted rotational isomers in the solvent. The chemical shifts of the protons on the formanilides are strongly dependent on the polar nature of the substituent. In the cis-form, the chemical shifts of both the imino and the formyl protons correlate with the Hammett σ0. In the tarns-form, a plot of the chemical shifts of the formyl protons against the σ0-constants reveals a good correlation in the case of meta-derivatives, but serious deivations for the para-substituents, both for the electron-releasing conjugative substituents and for the electron-withdrawing conjugative substituents. The deviations for both types of substituents are in the direction to be anticipated for any important extra conjugative interaction between the para-substituent and the side-chain. The Yukawa-Tsuno equation is precisely applicable to these shifts when we assume that this system has the same γ-value for both types of substituents; the relation is expressed as: Δδ=−12.0(σ0+1.12Δ\barσR)+0.5. where Δ\barσR=σ+−σ0 for the electron-releasing conjugative substituents and where Δ\barσR=σ+−σ0 for the electron-withdrawing conjugative substituents (r=0.993, s=0.8). From the solvent effect on the substituents, such as NH2, OH, and COOH, their σ0-values in dimethyl sulfoxide are proposed. It is also deduced that this solvent effect affects principally the part of the inductive contribution in the overall substituent effect.