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Organic Chemistry -- GOC 05 : Hyperconjugation Effect in Carbocation , Free Radical JEE MAINS/NEET

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  1. Catalog Record: Dicoordinated carbocations | HathiTrust Digital Library.
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Peter J. Stang (Author of Dicoordinated Carbocations)

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Optimization of all coordinates, except for the interionic distance, gives the geometry depicted in Figure 2. The cation I1 in the ion-pair I1-LiH2- has characteristics intermediate between those expected for the free ion and the 41 Electron correlation by MP2 was successfully applied in previous studies of counterion effects on the structure of carbocations see ref No local minimum is calculated for I1, which isomerizes barrierless to I2.

Specific interactions between an anion and I1 are thus capable of stabilizing I1 enough to make it a metastable species. The presence of a dielectric is expected to stabilize it even more, since this has been shown to flatten the PES for hydride migration for the bare ion cf. Figure 1. Excessive computational costs of such calculations refrained us from verifying this. Mechanism of Photolysis. Nature of the Reactive Excited State. Its position and extinction coefficient are equal within experimental error in methanol, acetonitrile, and n-pentane.

See: a Scaiano, J. Effect of Wavelength of Excitation. The photochemistry of 1E in n-pentane at room temperature was studied at wavelengths of excitation ranging from to nm the absorption onset is at nm; Table 3. The lack of sensitivity of the photoreaction to the wavelength of irradiation contrasts with the photochemistry of other vinyl halides. Mechanism 2 suggests parallel formation of both vinylic radicals and vinylic cations from electronically excited vinylic halides. Since the formation of radicals and ions requires very different charge distributions in the reactive excited state, it is unlikely that they will simultaneously be formed from a single excited state.

The absence of wavelength dependence therefore argues against mechanism 2 Chart 2. Effect of Solvent Variation. The photoreactivity of 1E changes only slightly upon variation of the solvent Table 4. This trend is not expected for a reaction which 46 Klessinger, M.

Essentials of Molecular Photochemistry; Blackwell: London, ; p TABLE 5. Mechanism 1 Chart 1 predicts a substantial solvent effect on the relative efficiency of formation of cation- and radical-derived products. A simple relation is expected: the more polar the solvent, the more ionic products.

Catalog Record: Dicoordinated carbocations | HathiTrust Digital Library

This implies that if ET is one of the reaction steps of the initially formed intermediate, it cannot be the only one next to diffusion apart as in mechanism 1. For mechanism 1, no obvious other reaction is available for the radical pair. The vinylic radicals are stable enough to diffuse in competition with ET. Effect of Variation of the Reaction Temperature.

Lowering of the temperature yields lower amounts of all products. The activation energies of the formation of photoproducts from 1E should give important information. Especially in an apolar solvent, such as n-pentane, mechanisms 1 and 2 Charts 1 and 2 are expected to give very different activation energies for the formation of cationderived 2 and radical-derived 3.

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  • The activation barriers for formation of 2 and 3 are equal within experimental error. This observation disagrees with mechanisms 1 and 2 Charts 1 and 2. In the case of mechanism 1, an endothermic ET step this step is especially endothermic in n-pentane forms the cation-derived product 2, after intial homolysis. In this mechanism, a significiantly higher activation energy on the formation of 2 than on the formation of 3 is thus expected.

    It is extremely unlikely that the formation of an ion pair and a radical pair from a single excited state, as in the case of mechanism 2, have equal activation energies. CHART 3. Relative rates of formation of products from 1E and 1E-2D. On the basis of the above results, we propose mechanism 3 Chart 3 for the photodissociation of the C-Br bond in 1E. In this state a direct heterolysis of the C-Br bond occurs, yielding an ion pair consisting of a vinylic cation and a bromide anion. The components of the resulting radical pair diffuse apart and the vinyl radical yields 3 as stable product by abstraction of a hydrogen atom from the solvent.

    Mechanism 3 Chart 3 agrees with the solvent effects. The formation of ionic species in the first reaction step accounts for the higher quantum yields of disappearance in polar solvents. Branching between proton loss and ET occurs, which is hardly affected by the solvent polarity. This conclusion is in line with the absence of nucleophilic substitution products.

    Mechanism 3 also agrees with the activation energies for 2 and 3.

    Exchange Discount Summary

    Formation of the ion pair is the rate-determining step and the subsequent ET step leading to formation of 3 is almost without barrier.