Kekule structure reactions

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. 

Ozonolysis was once used to locate the position of a double bond (or bonds) in unsaturated compounds of unknown structure—largely because of the ease of characterisation of the carbonyl products— but has now been superseded by physical methods, e.g. n.m.r. spectroscopy, which are easier and quicker. Benzene forms a triozonide which decomposes to yield three molecules of glyoxal, OHC—CHO the sole reaction of benzene that suggests it may contain three real double bonds in a Kekule structure Alkynes also undergo ozonolysis, but at a much slower rate than alkenes. 

The second mechanism, due to the permutational properties of the electronic wave function is referred to as the permutational mechanism. It was introduced in Section I for the H4 system, and above for pericyclic reactions and is closely related to the aromaticity of the reaction. Following Evans principle, an aromatic transition state is defined in analogy with the hybrid of the two Kekule structures of benzene. A cyclic transition state in pericyclic reactions is defined as aromatic or antiaromatic according to whether it is more stable or less stable than the open chain analogue, respectively. In [32], it was assumed that the in-phase combination in Eq. (14) lies always the on the ground state potential. As discussed above, it can be shown that the ground state of aromatic systems is always represented by the in-phase combination of Eq. (14), and antiaromatic ones—by the out-of-phase combination. 

As a result of these substituent-induced polarizations, the complementary conjugative interactions at each ring site become somewhat imbalanced (so that, e.g., the donor-acceptor interaction from C3—C4 to C5—C(, is 23.1 kcal mol-1, but that in the opposite direction is only 16.4 kcal mol-1). From the polarization pattern in (3.133) one can recognize that excess pi density is accumulated at the ortho (C2, C6) and para (C4) positions, and thus that the reactivity of these sites should increase with respect to electrophilic attack. This is in accord with the well-known o, /(-directing effect of amino substitution in electrophilic aromatic substitution reactions. Although the localized NBO analysis has been carried out for the specific Kckule structure of aniline shown in Fig. 3.40, it is easy to verify that exactly the same physical conclusions are drawn if one starts from the alternative Kekule structure. 

These polarizations are seen to be in the opposite direction to those in aniline (3.133), so that higher pi density remains at the Ci (junction) and C3 and C5 (meta) positions. These polarity shifts are again consistent with the well-known m-directing effect of nitro substituents in electrophilic aromatic substitution reactions, and the results are again quite independent of which starting Kekule structure is selected for the localized analysis.63... 

Kekule, a German chemist, was the first to propose a structure for benzene In 1865. It was a cyclic structure of alternating single and double bonds. The Kekule structure, however, does not fit all the evidence that chemists have since collected. For example, when benzene Is added to a bromine solution, rapid decolourlsatlon does not take place. This Implies that benzene resists addition reactions and Is much more stable than a typical unsaturated hydrocarbon. The reason for this becomes clear when we examine more closely the structure and bonding In benzene. 

Scheme 41. (a) Transition State for the Degenerate Semibullvalene Rearrangement, (b) Avoided Crossing of the Corresponding Kekule Structures Along the Reaction Coordinate Mode (t>2), and the Generation of the Twin States by Avoided Crossing, (c) Frequencies of the b2 Mode in the Two States (Ref 225). Delocalized Bonds in c Are Removed for Clarity... 

Hydrogenation of the catalyst during the reaction and subsequent decrease in activity was observed with dehydrochlorinated polyvinylidene chloride 62 i and also with Mg-phthalocyanine 72 >. It has been pointed out before that on pyrolysis of organic polymers quinonic structures may be formed, and if we consider the Kekule structure of the phthalocyanine molecule... 

The resonance-delocalized picture explains most of the structural properties of benzene and its derivatives—the benzenoid aromatic compounds. Because the pi bonds are delocalized over the ring, we often inscribe a circle in the hexagon rather than draw three localized double bonds. This representation helps us remember there are no localized single or double bonds, and it prevents us from trying to draw supposedly different isomers that differ only in the placement of double bonds in the ring. We often use Kekule structures in drawing reaction mechanisms, however, to show the movement of individual pairs of electrons. 

The most thoroughly studied reactions of 1,2.3-triazines 1 and 1,2,3-benzotriazincs 3 are pyrolysis and photolysis. Since in all Kekule structures of these compounds an N-N double bond is present, they may be precursors of azacyclobutadienes (azetes) 2 or benzazetes 4 by elimination of nitrogen (ef. Section 2.1.2.5.). 

The Kekule structures of benzene account for the molecular formula of benzene and for the number of isomers obtained as a result of substitution. However, they fail to account for the unusual stability of benzene and for the observation that the double bonds of benzene do not undergo the addition reactions characteristic of alkenes. That benzene had a six-membered ring was confirmed in 1901, when Paul Sabatier (Section 4.11) found that the hydrogenation of benzene produced cyclohexane. This, however, still did not solve the puzzle of benzene s structure. 

CsHi4 CsHs + 4H2 Benzene is the simplest aromatic hydrocarbon. It shows characteristic electrophilic substitution reactions, which are difficult to explain assuming a simple unsaturated structure (such as Kekule s (1865) or Dewar s (1867) formulae). This anomolous behavior can now be explained by assuming that the six pi electrons are delocalized and that benzene is, therefore, a resonance hybrid. It consists of two Kekule structures and three Dewar structures, the former contributing 80% and the latter 20% to the hybrid. Benzene is usually represented by either a Kekule structure or a hexagon containing a circle (which denotes the delocalized electrons). 

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