The E T parameter represents the charge-transfer absorption of the solvent in pyridinium N -phenolbetaine and serves as a different measure of solvent polarity than the dielectric constant . For example, methyl formamide has an extremely high dielectric constant but is of similar polarity to methanol as characterized by its E T value. It appears from these studies that the rates of β-scission reactions can be generalized to increase with some measure of solvent polarity such as E T . However, since only reactions of cumyloxyl and alkyloxyl radicals have been investigated, and mostly in water, acetonitrile or mixtures of the two, it is perhaps dangerous to infer kinetic solvent effects for the entire reaction family. Like the other reaction families discussed thus far, the rates of Diels-Alder reactions in solution, in general, depend on both solute and solvent properties . Breslow et al. described a large increase in rate of Diels-Alder reactions in water and in stereoselectivity between endo and exo products . They explain the acceleration in terms of the reactant structures; the diene and dienophile engage in hydrophobic stacking. Experimental studies with methanol show that it is indeed this hydrophobic effect, rather than a polarity effect, which increases the rate in water; the rates of some reactions actually decrease in methanol, which is unexpected.