Effect of Solvent on Free Radical Reactions

Abstract

Solvent viscosity has a substantial influence on the elementary reactions for which the rates are diffusion-limited. Such reactions include recombination of atoms and reactions between free radicals that have rate constants on the order of 10⁹ or 10¹⁰ liter/mole·sec. A decrease in the diffusion coefficient of iodine atoms with increasing viscosity was demonstrated in [422]. Often used to calculate k is the Stokes-Einstein formula D = kT/3πησ, which is substituted into the formula k = 4·10^-3πσ D N to give k = 8 R T/3000 η. For atoms, this formula is, strictly speaking, inapplicable, as is evident from Table 96: The product kη/T changes from solvent to solvent instead of remaining constant.