Benzene resonance

The submitters have not been successful in isolating tert-butylcyanoketene by any method. If the solvent is removed, the ketene polymerizes. The spectral properties of the product are as follows infrared (benzene) cm.-1 2220 (C=N), 2130 (C=C=0) proton magnetic resonance (benzene) <5, multiplicity, assignment 0,75 [singlet, C(CH3)3],... 

After 1 hour, the reaction is complete as monitored by gas chromatographic analysis on an Hewlett-Packard F M 5751 research chromatograph (1.85 m. x 0.313 cm. stainless-steel column of 10% Apiezon L on Chromosorb W AW/DMCS column temperature 250°). Thin-layer chromatography was found to be of little use in monitoring the reaction, as the Rf values for benzyl disulfide and benzyl sulfide are virtually identical for a variety of solvent systems tried. Proton magnetic resonance (benzene) shows that these two compounds have coincidental chemical shifts for the benzylic protons 8 3.4 (singlet). 

Fig. 3. Heat of hydrogenation of hypothetical 1,3,5-cyclohexatriene to benzene (kcal/mole) (a) cyclohexane (b) non-resonating cyclohex-atriene (c) non-resonating benzene (d) normal (resonating) benzene. (Taken from Turner, ref. 21).

The success of the HL model and its relation to Lewis model, posed a wonderful opportunity for the young Pauling and Slater to construct a general quantum chemical theory for polyatomic molecules. They both published, in the same year, 1931, several seminal papers in which they each developed the notion of hybridization, the covalent-ionic superposition, and the resonating benzene picture.Especially effective were Pauling s papers that linked the new theory to the chemical theory of Lewis, and that rested on an encyclopedic command of chemical facts. In the first paper, Pauling presented the electron-pair bond as a superposition of the covalent HL form and the two... 

Because all six carbon atoms and all six p orbitals in benzene are equivalent, it s impossible to define three localized it bonds in which a given p orbital overlaps only one neighboring p orbital. Rather, each p orbital overlaps equally well with both neighboring p orbitals, leading to a picture of benzene in which all six tt electrons are free to move about the entire ring (Figure 15.3b). In resonance terms (Sections 2.4 aud 2.5), benzene is a hybrid of two equivalent forms. Neither form is correct by itself the tme stmcture of benzene is somewhere in between the two resonance forms but is impossible to draw with our usual conventions. Because of this resonance, benzene is more stable and less reactive than a typical alkene. 

Structurally benzene is the simplest stable compound having aromatic character, but a satisfactory graphical representation of its formula proved to be a perplexing problem for chemists. Kekule is usually credited with description of two resonating structures which. 

The radical and ions are exceptionally stable due to resonance the free electron or charge is not localized on the methyl carbon atom but is distributed over the benzene rings. 

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. 

Benzene was probably the fust compound in chemical history where the valence bond concept proved to be insufficient. Localizing the nr-systems, one comes up with two equivalent but different representations. The true bonding in benzene was described as resulting from a resonance between these two representations (Figure 2-46). 

Benzene has already been mentioned as a prime example of the inadequacy of a connection table description, as it cannot adequately be represented by a single valence bond structure. Consequently, whenever some property of an arbitrary molecule is accessed which is influenced by conjugation, the other possible resonance structures have to be at least generated and weighted. Attempts have already been made to derive adequate representations of r-electron systems [84, 85]. 

Let us illustrate this with the example of the bromination of monosubstituted benzene derivatives. Observations on the product distributions and relative reaction rates compared with unsubstituted benzene led chemists to conceive the notion of inductive and resonance effects that made it possible to explain" the experimental observations. On an even more quantitative basis, linear free energy relationships of the form of the Hammett equation allowed the estimation of relative rates. It has to be emphasized that inductive and resonance effects were conceived, not from theoretical calculations, but as constructs to order observations. The explanation" is built on analogy, not on any theoretical method. 

Twentieth century theories of bonding m benzene gave us a clearer picture of aromatic ity We 11 start with a resonance description of benzene... 

The two Kekule structures for benzene have the same arrangement of atoms but differ m the placement of electrons Thus they are resonance forms and neither one by Itself correctly describes the bonding m the actual molecule As a hybrid of the two Kekule structures benzene is often represented by a hexagon containing an inscribed circle... 

Because the carbons that are singly bonded m one resonance form are doubly bonded m the other the resonance description is consistent with the observed carbon-carbon bond distances m benzene These distances not only are all identical but also are intermediate between typical single bond and double bond lengths... 

The precise value of the resonance energy of benzene depends as comparisons with 13 5 cyclohexatriene and (Z) 13 5 hexatriene illustrate on the compound chosen as the reference What is important is that the resonance energy of benzene is quite large SIX to ten times that of a conjugated triene It is this very large increment of resonance energy that places benzene and related compounds m a separate category that we call aromatic... 

Members of a class of arenes called polycyclic aromatic hydrocarbons possess subslanlial resonance energies because each is a colleclion of benzene rings fused logelher... 

In general the most stable resonance structure for a polycyclic aromatic hydro carbon is the one with the greatest number of rings that correspond to Kekule formula tions of benzene Naphthalene provides a fairly typical example... 

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