On the opposite effect of guanidinium chloride and guanidinium sulphate on the kinetics of the Diels-Alder reaction

Abstract

The transition state of Diels-Alder reactions resembles the contact-minimum configuration of pairwise hydrophobic interaction. On this ground, it has been possible to find a robust correlation between the experimental k2 values for the reaction between cyclopentadiene and methyl acrylate and the Gibbs energy estimates for the formation of this contact-minimum configuration. The enhancement of rate constant caused by Gdm2SO4 comes from the large density increase caused by the addition of this salt to water (i.e., the strong electrostrictive power of sulphate ions), that renders very large the Gibbs energy cost of cavity creation. The WASA decrease associated with the formation of the contact-minimum configuration leads to a decrease in the Gibbs energy cost of cavity creation with respect to the situation of the two molecules far apart from each other. This corresponds to a gain in translational entropy of ions and water molecules that is larger the greater the density or better the number density of the aqueous solution. This entropy gain in the case of Gdm2SO4 overwhelms the energy decrease associated with the loss of several attractive interactions between the Gdm+ions and the nonpolar solute molecules.