The effect of molecular geometry on the electronic effects of the azo and azoxy groups (1986)
AuthorsByrne, Christopher J.show all
The effect of molecular geometry on the electronic effect of the azo and azoxy groups has been examined in two ways. In Part I the pKa's of a series of substituted benzoic and phenylacetic acids were measured in 50% 0.1MKCl-ethanol at 25%. The results were interpreted in terms of the Hammett equation to derive σm, σp and σp° substituent constants. In addition the rates of solvolysis of a series of aryldimethylcarbinyl chlorides were measured in 87.5%, 80% and 72.5% acetone-water at 25°, with the results being interpreted in terms of σ p+ substituent constants. The electronic effect of the following groups was investigated in this way; phenylazo, (σm, σp, σp°, σp+); arylazo, (σp+); 2,6-dimethylphenylazo, (σp+); t-butylazo, (σm, σp, σp+); phenyl-ONN-azoxy, (σm, σp, σp+); and phenyl-NNO-azoxy, (σm, σp). The results suggested that the phenylazo group was in a coplanar configuration with respect to the ring bearing the reaction site. The electronic effect of these substituents in the non-planar configuration was also investigated by determining the effect on σp and σp+ when the coplanar configuration was hindered by two flanking methyl groups. The results showed that the non-planar configuration is intrinsicially more electron donating than the coplanar one. The relevance of these results to the different reported assessments of the activating power of the phenylazo group in electrophilic aromatic substitution is discussed. In Part II the ¹³C n.m.r. spectra of a series of 3' and 4'- substituted 2,6-dimethylazobenzenes have been measured in deuterochloroform. The effect of substituents in one ring on the chemical shift of the carbons in the other were interpreted in terms of their inductive and resonance effects by means of a dual substituent parameter analysis. The results showed that relative to the previously reported unhindered azobenzene series, a significant increase in the sensitivity to both inductive and resonance effects was observed at all positions. Possible explanations for this effect in terms of the modes of operation of the inductive and resonance effects are discussed.