Stability of the allyl radical

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 ... 

Ketones in which the double bond is located in the p,y position are likely candidates for a-cleavage because of the stability of the allyl radical that is formed. This is an important process on direct irradiation. Products then arise by recombination of the radicals or by recombination after decarbonylation. 

In molecular orbital terms, the stability of the allyl radical is due to the fact that the unpaired electron is delocalized, or spread out, over an extended 7T orbital network rather than localized at only one site, as shown by the computer-generated MO in Fig 10.3. This delocalization is particularly apparent in the so-called spin density surface in Figure 10.4, which shows the calculated location, of the unpaired electron. The two terminal carbons share the unpaired electron equally. 

This is generally attributed to resonance stabilization of the allylic radical ... 

The observation that in the case of PCSO there is no formation of propanol while allyl alcohol is formed from ACSO agrees with the resonance stabilization of the allyl radical and hence weaker bond for S-allyl than for S-propyl. The yield of allyl alcohol from irradiation of ACSO is considerably greater than that from S-allyl-L-cysteine, probably due to energy delocalization by the four p electrons of the S atom. 

The choice of diallylphtalate as the cross-linker is somewhat surprising, because allylic compounds are not very reactive in radical polymerizations due to the stability of the allyl radicals. 

Figure 13.1 The relative stability of the allyl radical compared to 1°, 2°, 3°, and vinyl radicals. (The stabilities of the radicals are relative to the hydrocarbon from which was formed, and the overall order of stability is allyl > 3° > 2° > 1° > vinyl).

The Stability of the Allyl Radical 13.3A Molecular Orbital Description of the Allyl Radical... 

Olefins. When the substrate molecule contains a double bond, treatment with chlorine or bromine usually leads to addition rather than substitution. However, for other radicals (and even for chlorine or bromine atoms when they do abstract a hydrogen) the position of attack is perfectly clear. Vinylic hydrogens are practically never abstracted, and allylic hydrogens are greatly preferred to other positions of the molecule, This is generally attributed41 to resonance stabilization of the allylic radical ... 

When chlorination or bromination of alkenes is carried out in the gas phase at high temperature, addition to the double bond becomes less significant and substitution at the allylic position becomes the dominant reaction.153-155 In chlorination studied more thoroughly a small amount of oxygen and a liquid film enhance substitution, which is a radical process in the transformation of linear alkenes. Branched alkenes such as isobutylene behave exceptionally, since they yield allyl-substituted product even at low temperature. This reaction, however, is an ionic reaction.156 Despite the possibility of significant resonance stabilization of the allylic radical, the reactivity of different hydrogens in alkenes in allylic chlorination is very similar to that of alkanes. This is in accordance with the reactivity of benzylic hydrogens in chlorination. 

An important distinction between dimerization and acrolein formation is that the selectivity of the former is evidently connected with a partially reduced state of the catalyst. It is commonly accepted, therefore, that cations like Bi3+, Sn4+, etc. play a role, presumably by adsorbing the allyl radical intermediate. Several authors assume that this is the case for allylic oxidation in general and that the role of a second oxide component is to promote dimerization byi stabilization of the allyl radical, or to direct the oxidation to aldehyde formation via a cationic allyl complex. Seiyama et al. [285] further suggest that the acidity of the promoting oxides is an important factor in this connection, and may, in part, explain why acidic oxides like Mo03 direct the oxidation to aldehydes, while basic compounds favour dimerization. 

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). 

In some way, then, the double bond affects the stability of certain free radicals it exerts a similar eflect on the incipient radicals of the transition state, and thus affects the rate of their formation. We have already seen (Sec. 5.4) a possible explanation for the unusually strong bond to vinylic hydrogen. The high stability of the allyl radical is readily accounted for by the structural theory specifically, by the concept of resonance. 

The relative stabilities of tertiary, secondary, and primary alkyl radicals are accounted for on exactly the same basis as the stability of the allyl radical ... 

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