Captodative effect

The captodative effect is the stabilization of radicals by a synergistic effect of an electron-withdrawing substituent and an electron-donating substituent. The name originates as the electron-withdrawing group (EWG) is sometimes called the "captor" group, whilst the electron-donating group (EDG) is the "dative" substituent.Olefins with this substituent pattern are sometime described as captodative. Radical reactions play an integral role in several chemical reactions and are also important to the field of polymer science.

When EDGs and EWGs are near the radical center, the stability of the radical center increases. The substituents can kinetically stabilize radical centers by preventing molecules and other radical centers from reacting with the center. The substituents thermodynamically stabilize the center by delocalizing the radical ion via resonance. These stabilization mechanisms lead to an enhanced rate for free-radical reactions. In the figure at right, the radical is delocalized between the captor nitrile (-CN), and the dative secondary amine (-N(CH₃)₂), thus stabilizing the radical center.

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. Delocalizing the radical ion stabilizes the transition state structure. As a result, the energy of activation decreases, enhancing the rate of the overall reaction. According to the captodative effect, the rate of a reaction is the greatest when both the EDG and EWG are able to delocalize the radical ion in the transition state structure.

Ito and co-workers observed the rate of addition reactions of arylthiyl 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 < CH₃ < OCH₂CH₃. 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:

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