Captodative effect

Certain substituents are better at stabilizing radical centers than others. This is influenced by the substituent's ability to delocalize the radical ion in the transition state structure. Ito and co-workers observed the rate of addition reactions of aryl thiol radical to disubstituted olefins. The olefins contained an EWG nitrile group and varying EDGs and the effect of varying EDGs on the rate of the addition reactions was observed. The process studied was: : The rate of the addition reaction was accelerated by the following EDGs in increasing order: H 3 2CH3. When R = OCH2CH3, the rate of the reaction is the fastest because the reaction has the smallest energy of activation (ΔG‡). The ethoxy and cyano groups are able to delocalize the radical ion in the transition state, thus stabilizing the radical center. The rate enhancement is due to the captodative effect. When R = H, the reaction has the largest energy of activation because the radical center is not stabilized by the captodative effect. The hydrogen atom is not able to delocalize the radical ion. Thus, the reaction is slow relative to the R = OCH2CH3 case. When R = CH3, the rate of the reaction is faster relative to when R = H because methyl groups have more electron donating capability. However, the reaction rate is slower relative to when R = OCH2CH3 because the radical ion is not delocalized over the methyl group . Thus, the captodative does not influence the reaction rate if the radical ion is not delocalized onto both the EWG and EDG substituents. Each of these cases is illustrated below: : ==Uses in synthesis==

The term "captodative ethylenes" has been used in the context of cycloaddition reactions involving captodative radical intermediates – for example, the thermal [2+2] head-to-head dimerization of 2-methylthioacrylonitrile occurs readily at room temperature; formation of the equivalent cyclobutane derivative of acrylonitrile is "sluggish". Intramolecular [2+2] cyclizations have also been reported to be enhanced by captodative effects, Effects have also been reported in cases like Diels-Alder and Friedel-Crafts reactions in cases where nucleophilic olefins react inefficiently, attributed to the transition state being close to a biradical and thus stabilized. These studies have revealed a direct dependence on Δω, difference in electrophilicity, and the polar nature of the reaction. They have been used because of their highly reactive, stereoselective, regioselective nature within these reactions. : Captodative olefins in reactions also show interfering effects with the typical kinetic isotope effect, allowing atypical reactions to occur with isotope-labeled molecules and demonstrating that the mechanisms and transition states of these reactions have been influenced. ==Polymer science application==

Free-radical polymerization, where radicals are the chain carriers in the propagation of the process, accounted for 40 billion of the 110 billion pounds of polymers produced in the United States in 2001. Captodative olefins have a specific advantage of being responsive to solvent effects without the effect of destabilizing the radical. ==References==