A Web Tool for Calculating Substituent Descriptors Compatible with Hammett Sigma Constants**

Abstract

The electron-donating and -accepting power of organic substituents is an important parameter affecting many properties of parent molecules, most notably their reactivity and pKa of ionisable groups. These substituent properties are described by Hammett σ constants obtained by measuring ionization constants of substituted benzoic acids. Although values of the Hammett σ constants have been measured for the most common functional groups, data for many important substituents are not available. In the present study, a method to calculate substituent descriptors compatible with the Hammett σ constants using quantum-chemically derived parameters is described. On this basis, a free web tool allowing to calculate electronic and hydrophobic substituent descriptors is made available at https://bitly.com/getsigmas.