User:DgsinUM/sandbox

Captodative Effect
Captodative Effect is the phenomenon associated with the stabilization of radicals by their substituents. These substituents, one being electron-withdrawing groups (EWG), the "captor", and the other electron-donating groups (EDG), the donor or ”dative” substituent. These two work in tandem to enhance the stabilization of the radical center of molecules in free radical reactions. Radicals play an integral role in several chemical reactions and also have been 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 can also thermodynamically stabilize the center by delocalizing the radical ion via resonance. These stabilization mechanisms lead to an enhanced rate for free radical reactions.



In figure 1, the radical is delocalized between the EWG, a nitrile (-CN), and the EDG, a secondary amine (-N(CH3)2), thus stabilizing the radical center. Radical intermediates play an integral role in chemical reactions and also have been important to the field of polymer science.

Substituent Effect on Reaction Rates
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.

Osamu Ito (et al) observed the rate of addition reactions of Arylthiyl Radical to Disubstituted Olefins. 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. 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.

Polymer Science Application
Captodative olefins have a specific advantage of being responsive to solvent effects without the effect of destabilizing the radical. They have also shown to undergo their radical transformation spontaneously which allows them to be useful in polymerization mechanism elucidation and better understood through NMR Studies. Furthermore captodative ethanes are initiators with unique properties giving higher molecular weight distribution and forming block copolymers through the known radical mechanisms. The Polymers obtained from captodatively substituted starting materials exhibit “desirable” properties such as optical activity, differences in polarity, solvent affinity, thermal and mechanical stabilities.


 * 1) Polymers with polar substituents are known to have interesting applications including within electrical and optical materials.
 * 2) These polymers are typically transparent.
 * 3) The Tdi (initial decomposition) of these polymers are relatively low compared to their analogues, but have relatively higher Tdm (maximum rate of weight change temperatures). Meaning although they will start to melt quicker, they will take longer to fully change phases.
 * 4) Polymers with large captodative stabilizations starting materials can quickly “unzip” to their starting monomer upon heating.
 * 5) Bifunctional polymers, with two different functional groups at every monomer unit, are commonly formed from the captodative monomers.
 * 6) Dative groups substantially alter the solubility through  Hydrogen bonding in specific bifunctional polymers( see figure above). However no clear correlation has been developed at this time, since not all combinations of substituents and solubilities have been investigated.
 * 7) Captodative polymer is highly functional in chelates with certain metals.

Uses in Synthesis
Because of the duality of nucleophilicity and electrophilicity of captodative ethylenes, they exhibit the ability to undergo polar cycloadditions such as [2+2], [3+2], [4+2] and [3+4] Cycloaddition and in Friedel-Craft reaction mechanisims in higher yields than their unstabalized counterparts. 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. Additionally they have shown interfering effects with the typical kinetic isotope effect allowing atypical reactions to occur with isotope labeled molecules.