Quantification and classification of substituent effects in organic chemistry: a theoretical molecular electrostatic potential study

Abstract

Substituent effects in organic chemistry are generally described in terms of experimentally derived Hammett parameters whereas a convenient theoretical tool to study these effects in π-conjugated molecular systems is molecular electrostatic potential (MESP) analysis. The present study shows that the difference between MESP at the nucleus of the para carbon of substituted benzene and a carbon atom in benzene, designated as ΔV_C, is very useful to quantify and classify substituent effects. On the basis of positive and negative ΔV_C values, a broad classification of around 381 substituents into electron withdrawing and donating categories is made. Each category is again sorted based on the magnitude of ΔV_C into subcategories such as very strong, strong, medium, and weak electron donating/withdrawing. Furthermore, the data are used to show the transferability and additivity of substituent effects in π-conjugated organic molecules such as condensed aromatic, olefinic, acetylenic, and heterocyclic systems. The transferability properties hold good for ΔV_C in all these molecular systems. The additive properties of substituent effects are strongly reflected on ΔV_C and the predictive power of the data to assign the total substituent effects of multi-substituted systems is verified. The ΔV_C data and the present classification of substituents are very useful to design π-conjugated organic molecular systems with desired electron rich/poor character.