Benzene Kekule structure

A flaw m Kekule s structure for benzene was soon discovered Kekule s structure requires that 1 2 and 1 6 disubstitution patterns create different compounds (isomers)... 

At ceremonies in Berlin in 1890 celebrating the twenty fifth anniversary of his proposed struc ture of benzene August Kekule recalled the origins of his view of the benzene structure... 

F, A. Kekule s structural formulae for organic compotmds ring structure of benzene 1865. 

Analogies are dangerous, but one point may be emphasized, and possibly overemphasized. Is not the art of emulsion at a point similar to that where Kekule found the art of organic chemistry some 100 years ago His contribution of the benzene structure began the tremendous advances in this field with which all are familiar. It is unlikely that on the basis of present knowledge any simple formula will suffice however, any correlative efforts must be of help. 

It is wrong (but common) to see a reversible reaction written with a double-headed arrow, as A B. Such an arrow implies resonance, e.g. between the two extreme valence-bond structures of Kekule benzene. 

This leads to modifications of the localized it orbitals. In benzene, for example, a Kekule localization which mixes the a and ir orbitals to form double banana bonds is preferred over the other equivalent ir localizations discussed. 60) In naphthalene a Kekule type structure is found similar to the one presently discussed, but different in that the (jtE2) are hybridized with corresponding o-CC bonding orbitals to form banana bonds, whereas the (ttC2 ) remains a pure jt orbital. 61 > While this is of interest in the discussion of the whole molecule, it is clear that certain intrinsic properties of the ir-electrons are more readily recognized by the localization which has been discussed here. We hope to discuss elsewhere localized orbitals involving a bonds in organic molecules. 

Solving the simultaneous equations (2a) and (2b) leads to y = 3 and t = n — 2, implying the presence of three B-B bonds and n — 2 B-B-B bonds in the boron skeleton. Since a deltahedron with n vertices has In — 4 faces, the n — 2 B-B-B bonds cover exactly half of the faces. In this sense a Kekule-type structure for the deltahedral boranes B H 2- has exactly half of the faces covered by B-B-B bonds just as a Kekule structure for benzene has half of its edges covered by C=C double bonds. In 1977 Lipscomb and co-workers [29] reported a variety of such Kekule-type localized bonding structures with the lowest energies for deltahedral boranes. These structures were computed using wave functions in the differential overlap approximation. 

In 1890, Kekule was honored by his friends, his former students, and the German Chemical Society at a "Benzol Fest" held at the Berlin City Hall. The date chosen, March 11, 1890, was the anniversary of the publication date of the first benzene structure paper, and, coincidentally, it was the kaiser s birthday. The celebration was well attended by the press, industrialists, and government representatives as well as by colleagues from Germany and... 

The sequential removal of H and H+ from isobutene-type structural units (so-called H2+ abstraction ) was also used to generate the radical anion of non-Kekule benzene , i.e. l,3-dimethylenecyclobutane-l,3-diyl (39) (Scheme 11). As shown by Hill and Squires161, this highly unusual, distonic C(,II(, isomer can be produced in pure form by reaction of O with 1,3-dimethylenecyclobutane (38). Working in a flowing afterglow mass spectrometer, subsequent reactions were again used to characterize this radical anion and differentiate it from other ( VdL, isomers. 

However, in recent years this basis has been somewhat undermined due to a critical reappraisal of experimental data on the benzene structure which, surprisingly, showed that a rigorous experimental proof of the generally accepted D6h structure of benzene is actually nonexistent It turned out that the X-ray structural data for benzene are compatible not only with the crystallographically ordered Dbh structure but also with the disordered Dih model associated with superposition of Kekule-type benzene molecules rotated by 60° with respect to each other about the threefold axis, both static and dynamic types of disorder being conceivable [87AG(E)782]. It has been shown by very simple calculations that if the difference between the C—C and C=C bond lengths in the D3h form is... 

This conclusion, nevertheless, should not be considered categorical but it points to the necessity of careful consideration of the correlation between the AEdis value and the part of it that relates to cyclic electron delocalization. It has been shown by use of TRE calculations of aromatic benzene and antiaromatic cyclobutadiene molecules that in the case of benzene the distortion into a Kekule-type structure is characterized by a change of the aromatic cyclic Tr-electron delocalization energy in an opposite direction... 

The material in this chapter traced the history of organic chemistry from Wohler s synthesis of urea through Kekule s structure of benzene. The millions of organic chemicals known to exist can be classified into a relatively small number of families, each defined by a common functional group. During the last century, chemists have discovered how to mimic nature to synthesize organic chemicals. A multitude of familiar products are natural or synthesized organic chemicals. 

The 30 Kekul ikc structures for this molecule are shown below. The groups of hexagons with circles in their centers represent the corresponding set of Kekul61ike structures two for benzene, three for naphthalene, and four for anthracene. The number under each structural formula is the number of KekulSlike structures corresponding to it. 

This, in turn, generated a massive further theoretical controversy over just how 13 should be interpreted, which, for a time, even became apart of Cold-War politics 4 We shall examine experimental and theoretical aspects of the benzene structure in some detail later. It is interesting that more than 100 years after Kekule s proposal the final story on the benzene structure is yet to be told." ... 

Pairing schemes 9 and 10 correspond to Kekule s structures, whereas 11,12, and 13 are called Dewar structures because J. Dewar suggested, in 1869, that benzene might have a structure such as 14 ... 

The concept of electronic delocalization has germinated in the pre-electron period to Kekule s structural theory and its application to benzene as a prototype of a family of compounds so-called aromatics . Kekule had to address two major properties of benzene revealed from substitution experiments. The first was the empirical equivalence of all positions of benzene, what is called today the Dfjh symmetry of both geometry and electronic structure, and second the persistence of the aromatic essence in chemical reactions, what we recognize today as aromatic stability . Thus, Kekule postulated that there is a Ce nucleus and the four valences of the carbons are distributed to give two oscillating structures, which when cast in our contemporary molecular drawings look like part a in Scheme 2.39-44 One of the many alternative hypotheses on the nature of... 

In a similar way Kekule s theory of the benzene structure has been very completely established by the whole development of aromatic chemistry. The direct physical verification of the presumably planar arrangement was in this case more delayed. The crystal structure of graphite was examined almost as soon as that of diamond, but the early results were inconclusive. The structure is not determined by the symmetry alone, and the later detailed investigation by Bernal (1924) showed that the carbon atoms in the hexagonal net must be coplanar to within at least 0-38 A. Later work by Ott (1928) narrowed this limit still further. Although it is generally assumed that the atoms are coplanar, the exactness with which this can be established depends on... 

Although Kekule s structure accounted for the modes of unsaturation in benzene, it did not account for benzene s reactivity. 

Figure6.30 Illustration from a spoof issue of Berichte derDurstigen Chemischen Geseilscha/i (Berlin, 1886) showing (a) monkeys grasping one another s limbs, an image that originally inspired Kekule s structure of benzene and (b) a facetious benzene tautomer in which the monkeys are allowed to use their tails instead of their feet for bonding. The purported effect was termed a caudal residual affinity .

Whereas Loschmidt s work was not much publicized, Kekule s structure of benzene immediately became well known, criticized, and controversial. Various other stmctures were proposed as substitution on benzene was shown to be easier than addition, which conflicted with the cyclohexatriene stmcture. Claus and Dewar proposed alternative stmctures in 1867, and Claus formula was adopted by Koener in 1874. 

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