Resonance stabilization energies allyl

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). 

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. 

An Important feature of butene pyrolysis Is the existence of free radicals with resonant stabilization energy. The allyl and the 2-butenyl (methylallyl) radicals are stabilized by the fact that they have delocalized electrons. Added stability of these free radicals Is about 10 to 13 kcal/mole (16), which significantly increases the rate of their production and decreases the rate of their consumption. 

Formation of 5-bromo-2-hexene requires reaction of the diene with HBr to give a secondary, nonallylic carbocation by protonation at C3. The activation energy for formation of this less stable 2° carbocation is considerably greater than that for formation of the resonance-stabilized 2° allylic carbocation therefore, formation of this carbocation and the resulting 5-bromo-2-hexene does not compete effectively with formation of the observed products. 

Allyl and benzyl radical are substantially stabilized, as anticipated from the resonance structures (see Section 1.3.6). Comparing the BDEs of propene and toluene to an appropriate reference such as ethane suggests resonance stabilization energies of 12.4 and 14.1 kcal / mol, respectively. An alternative way to estimate allyl stabilization is to consider allyl rotation barriers (Eq. 2.12). Rotating a terminal CH2 90° out-of-plane completely destroys allyl resonance, and so the transition state for rotation is a good model for an allylic structure lacking resonance. For allyl radical the rotation barrier has been determined to be 15.7 kcal / mol, in acceptable agreement with the direct thermochemical number. 

Table 4 MMP2 Non-bonded Resonance Stabilization Energies. noho (kcalmor ), in Allyl Cations and 1,3 n -Density Matrix Elements of the Allyl, l-Methylallyl, and 1,3-DimethyI-allyl Cations at MMP2 (Edl3(MMP2)) and at MP2(full)/6-31G (ED13(MP2))...

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 ... 

Some fundamental structure-stability relationships can be employed to illustrate the use of resonance concepts. The allyl cation is known to be a particularly stable carbocation. This stability can be understood by recognizing that the positive charge is delocalized between two carbon atoms, as represented by the two equivalent resonance structures. The delocalization imposes a structural requirement. The p orbitals on the three contiguous carbon atoms must all be aligned in the same direction to permit electron delocalization. As a result, there is an energy barrier to rotation about the carbon-carbon... 

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 radical60 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. 

When considering the stability of spin-delocalized radicals the use of isodesmic reaction Eq. 1 presents one further problem, which can be illustrated using the 1-methyl allyl radical 24. The description of this radical through resonance structures 24a and 24b indicates that 24 may formally be considered to either be a methyl-substituted allyl radical or a methylvinyl-substituted methyl radical. While this discussion is rather pointless for a delocalized, resonance-stabilized radical such as 24, there are indeed two options for the localized closed shell reference compound. When selecting 1-butene (25) as the closed shell parent, C - H abstraction at the C3 position leads to 24 with a radical stabilization energy of - 91.3 kj/mol, while C - H abstraction from the Cl position of trans-2-butene (26) generates the same radical with a RSE value of - 79.5 kj/mol (Scheme 6). The difference between these two values (12 kj/mol) reflects nothing else but the stability difference of the two parents 25 and 26. 

Shaik and Bar102 demonstrated that allyl anion has a distortive jr-component but at the same time exhibits a rotational barrier. This analysis was reaffirmed later for allyl radical.5 Subsequently, Gobbi and Frenking93 pointed out that the total distortion energy of allylic species is very small because it reflects the balance of jr-distortivity opposed by the a-symmetrizing propensity. They further argued that along with this jr-distortivity, the allylic species enjoys resonance stabilization which is the source of the rotational barrier. A detailed VB analysis by Mo et al.149 established the same tendency. 

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). 

The trae stracture of the allyl carbocation is a hybrid of the two resonance stractures. In the hybrid, the Jt bond is delocalized over all three atoms. As a result, the positive charge is also delocalized over the two terminal carbons. Delocalizing electron density lowers the energy of the hybrid, thus stabilizing the allyl carbocation and making it more stable than a normal 1° carbocation. Experimental data show that its stability is comparable to a more highly substituted 2° carbocation. 

A further, most important outcome of the resonance theory is this as a resonance hybrid, the allyl radical is more stable (i.e., contains less energy) than either of the contributing structures. This additional stability possessed by the molecule is referred to as resonance energy. Since these particular contributing structures are exactly equivalent and hence of the same stability, we expect stabilization due to resonance to be large. 

To avoid the energy-rich cyclopropyl cation 4 as an intermediate, the cationic cyclopropyl to allyl rearrangement occurs as an orbital symmetry controlled synchronous reaction, leading directly to the allyl cation 2, as long as there is no substituent capable of pronounced resonance stabilization of positive charge at the reaction center. Note, cyclopropyl products have been observed for R = F however, these may not have formed via a cyclopropyl cation, but via an intermediate cyclopropene (elimination-addition mechanism). No cyclopropyl products were observed under different conditions. ... 

However, the activation energy drops considerably when at least two phenyl substituents on the cyclopropyl radical provide strong resonance stabilization to the allylic radical resulting from ring opening. Therefore, diphenylcyclopropane peresters or peranhydrides undergo cyclopropyl radical to allyl radical rearrangement, followed by dimerization, in the liquid phase (benzene, ethylbenzene, mesitylene, decaline, benzonitrile etc.) " at 80-... 

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