Benzene resonance stabilization

The higher ratio is achieved by electron donation from the benzene ring to the otherwise 16-electron alkyl complex. The resulting 17 bonding is, however, not nearly as complete as in a normal 7r-allyl, because of the benzene resonance stabilization effect. 

The results of the derivation (which is reproduced in Appendix A) are summarized in Figure 7. This figure applies to both reactive and resonance stabilized (such as benzene) systems. The compounds A and B are the reactant and product in a pericyclic reaction, or the two equivalent Kekule structures in an aromatic system. The parameter t, is the reaction coordinate in a pericyclic reaction or the coordinate interchanging two Kekule structures in aromatic (and antiaromatic) systems. The avoided crossing model [26-28] predicts that the two eigenfunctions of the two-state system may be fomred by in-phase and out-of-phase combinations of the noninteracting basic states A) and B). State A) differs from B) by the spin-pairing scheme. 

Most of the resonance stabilization of benzene is lost when it is converted to the cyclohexadienyl cation intermediate In spite of being allylic a cyclohexadienyl cation IS not aromatic and possesses only a fraction of the resonance stabilization of benzene... 

Pyrrole is soluble in alcohol, benzene, and diethyl ether, but is only sparingly soluble in water and in aqueous alkaUes. It dissolves with decomposition in dilute acids. Pyrroles with substituents in the -position are usually less soluble in polar solvents than the corresponding a-substituted pyrroles. Pyrroles that have no substituent on nitrogen readily lose a proton to form the resonance-stabilized pyrrolyl anion, and alkaU metals react with it in hquid ammonia to form salts. However, pyrrole pK = ca 17.5) is a weaker acid than methanol (11). The acidity of the pyrrole hydrogen is gready increased by electron-withdrawing groups, eg, the pK of 2,5-dinitropyrrole [32602-96-3] is 3.6 (12,13). 

The most impressive example of resonance stabilization is benzene, in which the delocalization is responsible for a stabilization of 30-36 kcal/mol, the resonance energy of benzene. 

The isodesmic reaction approach (see Section 4.1) has also been applied to calculation of the resonance stabilization of benzene. 

The polycyclic aromatic hydrocarbons such as naphthalene, anthracene, and phenan-threne undergo electrophilic aromatic substitution and are generally more reactive than benzene. One reason is that the activation energy for formation of the c-complex is lower than for benzene because more of the initial resonance stabilization is retained in intermediates that have a fused benzene ring. 

Phenanthrene and anthracene both react preferentially in the center ring. This behavior is expected from simple resonance considerations. The c-complexes that result from substitution in the center ring have two intact benzene rings. The total resonance stabilization of these intermediates is larger than that of the naphthalene system that results if substitution occurs at one of the terminal rings. ... 

Aeeording to your Lewis structure(s) and to the actual geometry of the molecule, is the bonding in planar corannulene fully delocalized (as in benzene), or are some CC bonds long and some short Do your results support the notion that planar corrannulene is resonance stabilized Explain. 

The first generalization is illustrated by the behavior of the 2- and 4-vs. the 3-derivatives of pyridine, the second by the reactivity of 4- vs. 2-substituted pyridines, the third by the relation of 4- vs. 2-derivatives of pyrimidine, and the fourth by the appreciable reactivity of 3-substituted pyridines or 5-substituted pyrimidines compared to that of their benzene analogs. Various combinations of azine-nitrogens in other poly-azines supply further examples. Theoretical aspects of (1), (2) and (3) are discussed in Section II, B, 2. The effect involved in (4) is believed to be more the result of the inductive stabilization of an adjacent negative chaise in the transition state (cf. 251) than of the electron deficiency created in the ground state (cf. 252). The quantitative relation between inductive stabihzation and resonance stabilization is not precisely defined by available data. However, a... 

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. 

In aromatic combustion flames, cyclopentadienyl radicals (c-CgHj ) can be precursors for PAH formation. " At high temperatures, benzene is oxidized by reaction with an oxygen molecule to yield phenylperoxy (C6H5O2 ) radical, via the initial formation of the phenyl radical (by C-H bond cleavage) and then the rapid addition of O2 (reaction 6.16). After expulsion of CO from phenylperoxy radical, a resonance-stabilized cyclopentadienyl radical (c-CgHg ) is formed (reaction 6.16). 

A structure that prevents coplanarity prevents resonance stabilization to a corresponding degree. Thus the triptycyl radical has none of the stability of triphenylmethyl and the benzene rings are tied back in a position with the very maximum deviation from coplanarity.16... 

Regardless of how we wish to define the resonance stabilization of the n = 6 case of benzene, it is unequivocal that this substance enjoys considerable stabilization relative to classical expectations related to acyclic and/or less unsaturated precedent. Rather than discussing the plethora of models and even greater experimental evidence that documents this aromaticity , we consider benzene itself as the paradigm. We will return to olefinic paradigms later in this section. 

Resonance Raman scattering, 21 326-327 Resonance stabilization of benzene, 3 599 Resonance theory, 20 774 Resonant cavity, 14 851 Resonant-cavity enhanced structures,... 

The benzene molecule is a system of rich n electrons. Removal of n electron requires low energy and benzene cation is formed easily. Its positive charge is delocalized by the n molecular orbitals. Due to this resonance stabilization, further fragmentation of benzene cation requires considerable energy and therefore occurs with low probability. 

---