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Captodative Effect

Captodative Effect describes how electron-donating groups (EDG) and electron-withdrawing groups (EWG) work in tandem to stabilize the radical center of molecules in free radical reactions ^[1]. When EDGs and EWGs are near the radical center the stability of the radical center increases ^[2] . The Substituents can kinetically stabilize radical centers by preventing molecules and other radical centers from reacting with the center ^[1] . The substituents can also thermodynamically stabilize the center by delocalizing the radical ion via resonance ^[1] ^[2]. These stabilization mechanisms lead to an enhance rate for free radical reactions.

The radical ion is delocalized between the Electron-Widthdrawing Group, CN, and and the Electron-Donating Group N(CH₃, thus stabilizing the radical center.

Substituent Effect on Reaction Rates

Certain substituents are better at stabilizing radical centers than others substituents ^[3] . This is influenced by the substituent’s ability to delocalize the radical ion in the transition state structure ^[1] . 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 ^[4] .

Osamu Ito (et al) observed the rate of addition reactions of Arylthiyl Radical to Disubstituted Olefins. ^[3] The Olefin contained an EWG, a cyan group and varying EDGs. The authors observed how the varying EDGs effect the rate of the addition reactions.

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 -OCH₂CH₃ and the cyano group are able to delocalize the radical ion in the transition, 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 able to delocalize the radical ion. Thus, the reaction is slow relative to when R=OCH₂CH₃. When R=CH₃, the rate of the reaction is faster relative to when R=H because methyl groups have more electron donating capability.^[3] However, when the reaction rate is slower relative to when R=OCH₂CH₃ because the radical ion is not delocalized the methyl group groups. Thus, the captodative does not influence the reaction rate if the radical ion is not delocalized onto both the EWG and EDG substituents.

Uses in Synthesis

^[1]

^ ^a ^b ^c ^d ^e Viehe, Heinz G. (1985). "The Captodative Effect". Accounts of Chemical Research. 18 (5): 148–154. doi:10.1021/ar00113a004. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ ^a ^b Eric V. Anslyn, Dennis A. Dougherty (2006). Modern physical organic chemistry (Dodr. ed.). Sausalito, Calif.: University Science Books. ISBN 9781891389313.
^ ^a ^b ^c Ito, Osamu (1988). "Captodative Effects on Rate of Addition Reactions of Arylthiyl Radical to Disubstituted Olefins". Journal of Chemical Society, Perkin Transaction II (6): 869–873. doi:10.1039/p29880000869. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ Creary, Xavier (1985). "Captodative Rate Enhancementin the Methylenecyclopropane". The Journal of Organic Chemistry. 51: 2664–2668. doi:10.1021/jo00364a009. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[The_Captodative_Effect-1] Viehe, Heinz G. (1985). "The Captodative Effect". Accounts of Chemical Research. 18 (5): 148–154. doi:10.1021/ar00113a004. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[Physical_Organic_Book-2] Eric V. Anslyn, Dennis A. Dougherty (2006). Modern physical organic chemistry (Dodr. ed.). Sausalito, Calif.: University Science Books. ISBN 9781891389313.

[Rate_Enhancement_Olefins-3] Ito, Osamu (1988). "Captodative Effects on Rate of Addition Reactions of Arylthiyl Radical to Disubstituted Olefins". Journal of Chemical Society, Perkin Transaction II (6): 869–873. doi:10.1039/p29880000869. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[Rate_Enhancment_Methylenecyclopropane-4] Creary, Xavier (1985). "Captodative Rate Enhancementin the Methylenecyclopropane". The Journal of Organic Chemistry. 51: 2664–2668. doi:10.1021/jo00364a009. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

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