Reactivity of the silyl, germanyl, and stannyl radicals in addition reactions: The effect of the atomic radius on the activation energy

Abstract

Experimental data on the addition of silyl, germanyl, and stannyl radicals to olefins are analyzed in the framework of a three intersecting parabolas model. The parameters characterizing these reactions are calculated. The activation energy of the thermally neutral reaction for this class of reactions depends on both the strength of the formed bond and the radius of the atom bearing the free valence. The dependence is the following: E e, 0 1/2 ∼ αD e + bD e 3/2 r C-X 1/2 , where D e is the strength of the formed bond and r C-X is its length. Steric repulsion is observed in the reactions of the silyl radicals with symmetrically substituted ethylene derivatives. The presence of a π-bond or aromatic ring near the attacked double bond increases E e, 0. The increments are calculated that characterize the contribution to the activation energy from the following factors: the enthalpy of the reaction, triplet repulsion, steric hindrance, and effect of adjacent π electrons.