Resonance, allyl anion/cation

To account for 1,2-addition in polar solvents, Szwarc (1959) proposed an ion-pair as the reactive species. This was visualized as a metal cation and a resonating allyl anion (XLVIII), the latter existing in the more favorable trans conformation. Since a secondary carbanion is more reactive than a primary one, 1,2-ad-... 

It has been contended that here too, as with the benzene ring (Ref 6), the geometry is forced upon allylic systems by the a framework, and not the 7t system Shaik, S.S. Hiberty, P.C. Ohanessian, G. Lefour, J. Nouv. J. Chim., 1985, 9, 385. It has also been suggested, on the basis of ab initio calculations, that while the allyl cation has significant resonance stabilization, the allyl anion has little stabilization Wiberg, K.B. Breneman, C.M. LePage, T.J. J. Am. Chem. Soc., 1990, 112, 61. 

We may perform the same analysis for the allyl radical and the allyl anion, respectively, by adding the energy of 4>2 to the cation with each successive addition of an electron, i.e., H (allyl radical) = 2(a + V2/3) + a and Hn allyl anion) = 2(a + s/2f) + 2a. In the hypothetical fully 7T-localized non-interacting system, each new electron would go into the non-interacting p orbital, also contributing each time a factor of a to the energy (by definition of o ). Thus, the Hiickel resonance energies of the allyl radical and the allyl anion are the same as for the allyl cation, namely, 0.83/1. 

The resonances due to unsaturated 29Si nuclei of cyclotetrasilenylium cation 70 appear at + 77.3 (terminal Si) and +315.7 ppm (central Si). The chemical shifts are independent of the solvent such as dichloromethane, benzene, and toluene, implying no covalent interaction between the cation part and solvent molecules. Rather unusually, the central silicon in the cation part is more deshielded than the terminal silicons.42 The terminal and central 29Si resonances of anion 71 are -31.5 and + 273.0 ppm in toluene-terminal silicon atoms are equivalent, indicating that the lithium cation is fluxional and 71 adopts an allylic anion-type structure in solution. Similarly to allyllithium, the central nucleus of 71 is deshielded, while the terminal nuclei are highly shielded. 

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. 

Remember that no resonance form has an independent existence A compound has characteristics of all its resonance forms at the same time, but it does not resonate among them. The p orbitals of all three carbon atoms must be parallel to have simultaneous pi bonding overlap between Cl and C2 and between C2 and C3. The geometric structure of the allyl system is shown in Figure 15-10. The allyl cation, the allyl radical, and the allyl anion all have this same geometric structure, differing only in the number of pi electrons. 

For C=C-C, the allyl cation, there would be two electrons in the pi system, filling just ipi, leaving the others empty. The shape and energy of the MOs are independent of the occupancy. The allyl anion has four electrons in the pi system, filling both ipi and t )2- The valence bond resonance forms, C-C C C C=C, describe the electron... 

Resonance structures (top) and resonance hybrid (bottom) for (a) allyl cation, (b) allyl radical, and (c) allyl anion. 

Electronegativity is important. In the allyl anion (Rg. 1.34a), for example, there are two equivalent resonance forms. As in the allyl cation, the charge resides... 

PROBLEM 12.20 Add electrons to both the resonance and molecular orbital descriptions in Figure 12.47 to form the allyl anion, radical, and cation. 

Like allyl cation and allyl radical, allyl anion is planar and stabilized by electron delocalization. The unshared pair plus the two tt electrons of the double bond are shared by the three carbons of the allyl unit. This delocalization can be expressed in resonance terms... 

We first show in Sect. 13.3.1 that the space-based HL-P and energy-based HL-CI methods give comparable results in the description of the resonance of the allylic systems (cation, radical and anion), except for the radical case, where the HL-P method is more reliable as it respects symmetry. 

Resonance in the Allyl Series Cation, Radical and Anion... 

Resolution (enantiomers), 307-309 Resonance, 43-47 acetate ion and, 43 acetone anion and. 45 acyl cations and, 558 allylic carbocations and, 488-489 allylic radical and, 341 arylamines and, 924 benzene and, 44. 521 benzylic carbocation and, 377 benzylic radical and, 578 carbonate ion and. 47 carboxylate ions and, 756-757 enolate ions and, 850 naphthalene and, 532 pentadienyl radical and. 48 phenoxide ions and, 605-606 Resonance effect, 562 Resonance forms, 43... 

Unfortunately, while it is clear that the allyl cation, radical, and anion all enjoy some degree of resonance stabilization, neither experiment, in the form of measured rotational barriers, nor higher levels of theory support the notion that in all three cases the magnitude is the same (see, for instance, Gobbi and Frenking 1994 Mo el al. 1996). So, what aspects of Hiickel theory render it incapable of accurately distinguishing between these three allyl systems ... 

In the allyl cation, with two tt electrons, and in the anion, with four -n electrons, there are two in M(V Note that the nonbonding >Pmo2 is concentrated at the ends of the chain the molecular orbital pictures for these species thus correspond closely to the resonance pictures (see 8, 9, 10, p. 6), which show the charge or unpaired electron to be concentrated at the ends. 

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