The Effects of Chemical Substitution and Polymerization on the pKa Values of Sulfonic Acids

The effects of ring substitution on the pK(a) value of benzenesulfonic acid (BSA) were investigated using a combined quantum mechanical and classical approach. Ring substitution with strong electron-withdrawing elements such as F, Cl, and Br is found to enhance the acidity of the BSA. More importantly, ring substitution with -NO2 groups which form an extended conjugated pi-system with the benzene ring exhibits the strongest enhancement of the acidity. The effects of polymerization on the styrenesulfonic acid (SSA) were also investigated by solving the classical Poisson-Boltzmarm equation. It is found that polyrnerization significantly decreases the acidity of SSA due to the alteration of the electrostatic environment surrounding the acid group upon polymerization. The average pK(a) value converges to 2.9 from the corresponding monomer value of -0.53 at a degree of polymerization of 8-12. These results shed significant light on how to design sulfonic-acid-based solid acid catalysts to achieve desired catalytic properties.