Kinetic Evidence Concerning the Geometry of Substituted Methyl Radicals

The result of Noyes et al. that cis or trans di‐iodoethylene will rapidly exchange I atoms with I2* via an I‐atom addition to the double bond (relative to much slower geometrical isomerization) is shown to be compatible with a planar structure for the intermediate free radical. A tetrahedral structure for the free‐radical end would lead to nearly equivalent rates of exchange and isomerization. The recent suggestion by Skell et al. that the stable form of the β‐iodo‐alkyl radical is I bridged is shown to be incorrect. His observations are explained in terms of the I atom catalyzed, concerted addition of I2 to the olefin.