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On the Way to Physical Interpretation of Hammett Constants: How Substituent Active Space Impacts on Acidity and Electron Distribution in p-Substituted Benzoic Acid Molecules

Sadlej-Sosnowska

Polish Journal of Chemistry

2007

Vol. 81, nr 5-6

1123-1134

Electron distribution was investigated in molecules of p-substituted benzoic acids by means of Natural Population Analysis. The analysis offers a valuable insight into the impact of a given substituent on the individual atoms’ charges in the molecules, on the excess of pi electrons in the ring and on the uniformity of their distribution. The more uniform the distribution, measured by means of standard deviation of pi electrons amounts localized on individual atoms in the ring, themore acidic is the molecule. On the other hand, an increase of the pi electron number in the ring brings about an opposite effect. No correlation was found between the total substituent charges and the Hammett constants. However, a new variable, composed of charges of substituents, added to charges of the ring Cipso atoms, was highly correlated with charges of acidic hydrogens, with the pi-electron populations on the individual ring atoms, and eventually, with the Hammett constants. On that basis a notion of the active substituent space was introduced, which comprises the substituent itself and the ring ipso carbon atom. It has been found that it is the charge of the active space, but not the charge of the substituent itself, which is well correlated with the acidity and other properties of p-substituted benzoic acids.