Resonance, allyl anion/cation radical

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. 

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... 

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 ... 

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. 

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. 

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

Equation 4.52 can be used to calculate the charge density for each position of the allyl cation, radical, and anion, and the results are shown in Table 4.1. These values are comforting, because they agree with our chemical experience with allylic systems. The resonance description of the allyl cation and radical (Figure 4.13) suggests that exactly half of the charge or unpaired electron density is associated with each of the terminal carbon atoms, and the HMO result is the same. 

Allyl resonance is important in reactions of cations, radicals, and anions. Be sure you can manipulate the allyl system easily. 

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... 

Table 13.2 Weights w,- of resonant Lewis stmctures for the cationic, radical and anionic allyl molecules obtained with the two schemes HL-Cl and HL-P and compared with the standard NRT (B3LYP/6-31+G(d)) weights...

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... 

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. 

At its essence, a conjugated system involves at least one atom with ap orbital adjacent to at least one tt bond. The adjacent atom with the p orbital can be part of another ir bond, as in 1,3-butadiene, or a radical, cationic, or anionic reaction intermediate. If an example derives specifically from a propenyl group, the common name for this group is allyl. In general when we are considering a radical, cation, or anion that is adjacent to one or more TT bonds in a molecule other than propene, the adjacent position is called allylic. Below we show the formula for butadiene, resonance hybrids for the allyl radical and an allylic carbocation, and molecular orbital representations for each one. 

Although satisfactory for allyl cation. Figure 10.1 is insufficient for species with more than two tt electrons because the tt orbital in (c) can accommodate only two electrons. Molecular orbital (MO) theory, however, offers an alternative to resonance and valence-bond theory for understanding the structure and reactions of not only allylic cations, but radicals (three rr electrons) and anions (four tt electrons) as well. In a simplification known as the Hiickel, or ir-electron, approximation the tt MOs are considered as separate from... 

Each of the preceding three processes generates a reactive carbon center—a radical, a c -bocation, or a carbanion, respectively— Ihat is adjacent to the tt framework of a double bond. This arrangement seems to impart special stability. Why The reason is electron delocalization Each species may be described by a pair of equivalent contributing resonance forms (Section 1-5). These three-carbon intermediates have been given the name allyl (followed by the appropriate term radical, cation, or anion). The activated carbon is called allylic. 

Radical, cationic, anionic, and metal catalyzed polymerizations of butadiene are known—and there are three possible products of the polymerization (Figure 21.12). Initial addition of a radical to butadiene gives exclusively the allyl radical, which has two resonance forms. The more stable resonance form adds further butadiene to give 1,2-polybutadiene. The less inherently stable, but also less sterically hindered, form adds further butadiene to give cis- or tra s-l,4-polybutadiene. 

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