Activation energies for iodine-exchange systems containing organic iodine compounds

In studies on the non-equilibrium isotopic exchange method for determining iodine in organic iodine compounds, activation energies have been measured to find systems having appropriate rate of exchange reactions. Activation energies are discussed by considering the effect of the structure of organic iodine compounds, the concentrations of reactants and solvent, etc. In homogeneous systems, activation energy is found to become larger in the order of CH3I<C2H5I<n-C3H7I<n-C4H9I<n-C6H13I<iso-C4H9I<CH2I2. As to the effect of solvents, activation energy is less in 100% acetone than in 90% acetone solution. In heterogeneous systems, e.g. Org. I(aq.)--I2(CCL4 or C2H4Cl2), activation energy increases in the order of 3,5-diiodotyrosine<3-iodotyrosine<5-iodouracil. The catalytic effect of I2 is large, and the iodine ratio between I2 and organic iodine is a predominant factor in determining the rate of the exchange reaction. (auth.)