The Role of meta and para Benzene Derivatives in the Evaluation of Substituent Effects: a Multivariate Data Analysis.

Abstract

Five aromatic reactivity data sets, selected to be balanced and representative for different types of substituent effect have been investigated with respect to eight extrathermodynamic relationships (the Hammett equation and various extensions). The five data sets all refer to properties of meta and para substituted phenyl compounds and comprise ionization in solution and in the gas phase of some acids and bases, NMR shifts, IR spectral frequencies and melting points. The results of principal components analysis (PCA) and projection to latent structures (PLS) analysis of the data give evidence that the effects of meta and para substituents are essentially different in nature. The behaviour of para derivatives is more complex, requiring more terms in the correlation equation: this complexity is due mainly to the donor substituents. Modelling meta and para derivatives together has mainly historical reasons; it may be acceptable in the simplest cases or when the small number of data prevent a more correct treatment.