Allylic-type resonance

Fig. 8. (a) Homoaromatic lit-electron conjugation in silyl cation 3. (b) Allyl-type resonance in Sit-electron radical 26. ... 

This is called allyl type resonance because it can be drawn for allylic carbocations, allylic carb-anions, and allylic radicals. 

As shown in Figure 27, an in-phase combination of type-V structures leads to another A] symmetry structures (type-VI), which is expected to be stabilized by allyl cation-type resonance. However, calculation shows that the two shuctures are isoenergetic. The electronic wave function preserves its phase when tr ansported through a complete loop around the degeneracy shown in Figure 25, so that no conical intersection (or an even number of conical intersections) should be enclosed in it. This is obviously in contrast with the Jahn-Teller theorem, that predicts splitting into A and states. 

The energies of this Cl and of the other ones calculated in this work are listed in Table III. The calculated CASSCF values of the energies of the two lowest electronically states are 9.0 eV (5i, vertical) and 10.3 eV ( 2, vertical) [99]. They are considerably higher than the expenmental ones, as noted for this method by other workers [65]. In all cases, the computed conical intersections lie at much lower energies than the excited state, and are easily accessible upon excitation to Si. In the case of the H/allyl Cl, the validity confirmation process recovered the CHDN and 1,3-CHDN anchors. An attempt to approach the third anchor [BCE(I)] resulted instead in a biradical, shown in Figure 43. The bhadical may be regarded as a resonance hybrid of two allyl-type biradicals. 

The allylic-type furylic radical 6 is resonance stabilized to such a degree that its reactivity in promoting propagation by adding onto another furan ring is minimal. The fate of these radicals will simply be to couple with another radical present in the reaction medium (primary or secondary) or to disproportionate to regenerate the furan character of the ring26. ... 

In a polymerization system, not only tertiary alkyl ions but also ions of the allyl type, because of their stabilizing resonance, would be formed readily. Hence, some hydrogenation and dehydrogenation of the primary polymer (e.g., RCH2CH2CH=CHR ) would occur in the following manner ... 

Problem 6.42 Use the concepts of (a) resonance and b) extended tt orbital overlap (delocalization) to account for the extraordinary stability of the allyl-type radical. 

Two products are formed because an allylic type of carbocation is a resonance hybrid... 

Allyl-type monomers do not yield high polymers. The substituent on the carbon in the / position with respect to the double bond is easily eliminated (especially hydrogen, halogenides, etc.). The generated radical is resonance-stabilized. It reacts much more readily with growing radicals than with the monomer. The low probability of long chain formation is a consequence of these terminating and transfer reactions. 

Because this is an example of an allyl-type system p<=Y—Z ), a second resonance structure can be drawn that moves the lone pair and the jt bond. To delocalize the lone pair and make the system conjugated, the labeled carbon atom must besp hybridized with the lone pair occupying ap orbital. 

These kinetic parameters take into account the formation of a five-membered ring intermediate, the H-abstraction reaction of two H-atoms of allyl type and the formation of the resonantly stabilized l-hexen-3-yl radical. These facts explain why the reverse isomerization reaction requires greater activation energy. As clearly shown in Fig. 6, there is a new class of important reactions, i.e. ring decomposition (e.g., cyclo-hexyl to form hexenyl radical) and the reverse cyclo-addition reaction. The activation energies of ring decomposition to form primary radicals are 31,500 and 28,000 kcal/kmol respectively for the... 

This particular carbocation is a resonance-stabilized one of the allylic type. It is a cyclo-hexadienyl cation (often referred to as an arenium ion). 

A and B characterized by the VB-like structures shown in Scheme 2.4. State A is an allyl-type structure, whereas state B has a bond between 1 and 1 . States A and B will be degenerate at geometries where the sides of the triangle formed by the three centers are approximately equivalent. This geometry will be our model conical intersection. We use two additional resonance structures, Loc and Loc which have bonds between centers 1 and 2, and 1 and 2, respectively. These resonance structures or states can be constructed as linear combinations of the two states A and B. 

When a gas-phase thermal reaction involves molecules of the type YH which are likely to form free radicals of the allylic type Y, which are stabilized by resonance, by the abstraction of an H atom, the terminations involving these radicals must be taken into account, i.e. ... 

As mentioned earlier, the allyl cation can be captured at either carbon sharing the positive charge. Table 12.3 gives some average rates of SnI solvolysis for a few common structural types. Primary allylic halides react much faster than primary alkyl halides. Secondary and tertiary allylic halides also ionize faster than their non-allylic counterparts. Resonance stabilization does have an accelerating effect on the ionizations. 

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