Allyl radical dissociation

The degree to which allylic radicals are stabilized by delocalization of the unpaired electron causes reactions that generate them to proceed more readily than those that give simple alkyl radicals Compare for example the bond dissociation energies of the pri mary C—H bonds of propane and propene... 

We attributed the decreased bond dissociation energy in propene to stabilization of allyl radical by electron delocalization Similarly electron delocalization stabilizes benzyl rad ical and weakens the benzylic C—H bond... 

Resonance theory can also account for the stability of the allyl radical. For example, to form an ethylene radical from ethylene requites a bond dissociation energy of 410 kj/mol (98 kcal/mol), whereas the bond dissociation energy to form an allyl radical from propylene requites 368 kj/mol (88 kcal/mol). This difference results entirely from resonance stabilization. The electron spin resonance spectmm of the allyl radical shows three, not four, types of hydrogen signals. The infrared spectmm shows one type, not two, of carbon—carbon bonds. These data imply the existence, at least on the time scale probed, of a symmetric molecule. The two equivalent resonance stmctures for the allyl radical are as follows ... 

The spectroscopy, structure, photochemistry, and unimolecular reactions of allyl radical have been studied extensively and reviewed recently.145 Possible dissociation channels of allyl radical, their energetics, and the potential energy barriers of the C3H5 system are shown in Figs. 20 and 21.145,146... 

Fig. 20. Possible dissociation channels of allyl radical and their standard heats of formation relative to allyl. The loss of H2 generally proceeds via a high activation barrier and is thus considered unlikely. (From Fischer et ai.14B)...

Butler and co-workers have taken a unique approach to study the unimolecular dissociation of the vibrationally and rotationally hot allyl radical.150-152 They have examined the secondary C-H dissociation of the allyl radicals that are produced with high internal energies above the allyl dissociation thresholds in the primary photodissociation of allyl chloride and allyl iodide at 193 nm. The production of allene versus propyne (both at mass 40) from the secondary dissociation of the hot allyl radicals are... 

Assuming a reactive oxonium ylide 147 (or its metalated form) as the central intermediate in the above transformations, the symmetry-allowed [2,3] rearrangement would account for all or part of 148. The symmetry-forbidden [1,2] rearrangement product 150 could result from a dissociative process such as 147 - 149. Both as a radical pair and an ion pair, 149 would be stabilized by the respective substituents recombination would produce both [1,2] and additional [2,3] rearrangement product. Furthermore, the ROH-insertion product 146 could arise from 149. For the allyl halide reactions, the [1,2] pathway was envisaged as occurring via allyl metal complexes (Scheme 24) rather than an ion or radical pair such as 149. The remarkable dependence of the yield of [1,2] product 150 on the allyl acetal substituents seems, however, to justify a metal-free precursor with an allyl cation or allyl radical moiety. 

Another early acidity investigation of propene by the thermodynamic method involved the determination of the electron affinity of allyl radical by photodetachment from allyl anion34. Extrapolation of the data to a photodetachment threshold gave an electron affinity (EA) of allyl radical of 0.55 eV which, combined with a bond dissociation energy of allyl-H of 89 kcalmol-1, gave A//ac d = 390 kcalmol-1. 

Similarly to the triphenylmethyl system, captodative-substituted 1,5-hexa-dienes, which can be cleaved thermally in solution into the corresponding substituted allyl radicals [15], dissociate more easily than dicaptor-substituted systems (Van Hoecke et al., 1986). Since ground-state and radical substituent effects cannot be separated cleanly, not only because of electronic but also because of steric effects, a conclusive answer cannot be provided. 

The allylic radical has also by abstraction of the allylic hydrogen by the H2NC(0)CH2 radical in Thd [reaction (210] and its isomer has been produced from 6-chloromethyluracil by dissociative electron capture and also in low-tem-... 

As mentioned with benzyl groups, an allylic center is also quite susceptible to autoxidation chemistry (Fig. 109). The allylic hydrogen has a weak C-H bond dissociation energy due to the resonance stabilization energy of the resulting allylic radical (157). 

Arenesulfonyl iodide and bromide are rather unstable compounds because of low bond dissociation energies of their S02-I and S02-Br. Therefore, treatment of p-tosyl bromide (47) with alkene or allene (48) produces 1,2-adduct (49) through the addition of the formed p-tosyl radical onto the allene as shown in eq. 4.19 [52]. Here, the p-tosyl radical attacks the central sp carbon of the allene group to generate the stable allylic radical, and then it reacts with p-tosyl bromide to give 1,2-adduct (49) and a p-tosyl radical again, i.e., chain pathway. So, this is also an atom(group)-transfer reaction. 

The CH bond in propene is weaker than the CH bond of ethane because the allyl radical is stabilized by resonance. The ethyl radical has no such resonance stabilization. The difference between these bond dissociation energies provides an estimate of the resonance stabilization of the allyl radical 13 kcal/mol (54 kJ/mol). 

Although free-radical halogenation is a poor synthetic method in most cases, free-radical bromination of alkenes can be carried out in a highly selective manner. An allylic position is a carbon atom next to a carbon-carbon double bond. Allylic intermediates (cations, radicals, and anions) are stabilized by resonance with the double bond, allowing the charge or radical to be delocalized. The following bond dissociation enthalpies show that less energy is required to form a resonance-stabilized primary allylic radical than a typical secondary radical. 

Stability of Allylic Radicals Why is it that (in the first propagation step) a bromine radical abstracts only an allylic hydrogen atom, and not one from another secondary site Abstraction of allylic hydrogens is preferred because the allylic free radical is resonance-stabilized. The bond-dissociation enthalpies required to generate several free radicals are compared below. Notice that the allyl radical (a primary free radical) is actually 13 kJ/mol (3 kcal/mol) more stable than the tertiary butyl radical. 

In these two latter cases it appears with certainty from the heat of combustion and from spectra that there is nothing special about the bonds themselves. The low dissociation energy must, therefore, find its cause in the special stability of the products of dissociation, in these cases the allyl radical. The particular stability of this radical follows from the resonance which is possible here between two equivalent configurations. H2C=CH— GH2 H2C—GH=GH2... 

Perhaps the most severe test of the proposed scheme was encountered in our study of the radicaloid mode of benzene dissociation or, equivalently, a recombination of two allylic radicals [67]. Since the separated limit involves open shell subsystems, standatrd CCSD approaches an incorrect channel. However, when VB corrected CCSD is used, we obtain practically the FCI result. 

Dissociation into a pair of allylic radicals or into a cation-anion pairs followed by recombination... 

The bond dissociation energy for this process (87 kcal/mol) is even less than that for a 3° C-H bond (91 kcal/mol). Because the weaker the C-H bond, the more stable the resulting radical, an allyl radical is more stable than a 3° radical, and the following order of radical stability results ... 

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