We report herein a means to modify the reactivity of alkenes;
and particularly to modify their selectivity toward reactions with nonpolar reactants (e.g;
nonpolar free radicals) in supercritical carbon dioxide near the critical point. Rate constants for free radical addition of the light hydrogen isotope muonium to ethylene;
vinylidene fluoride;
and vinylidene chloride in supercritical carbon dioxide are compared over a range of pressures and temperatures. Near carbon dioxide's critical point;
the addition to ethylene exhibits critical speeding up;
while the halogenated analogues display critical slowing. This suggests that supercritical carbon dioxide as a solvent may be used to tune alkene chemistry in near-critical conditions. © 2014 American Chemical Society;