Benzene, structure

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... 

The usual syntheses of quinazolines make use of an o-disubstituted benzene structure (46) from which the quinazoline skeleton is completed by adding C-2 and N-3 in various ways. Substituents could either be in (a) the pyrimidine ring or (b) the benzene ring or in both rings. The syntheses will be described in this order and the methods used for (a) apply equally well to quinazolines substituted in both rings. 

Resonance is an extremely useful concept that we ll return to on numerous occasions throughout the rest of this book. We ll see in Chapter 15, for instance, that the six carbon-carbon bonds in so-called aromatic compounds, such as benzene, are equivalent and that benzene is best represented as a hybrid of two resonance forms. Although an individual resonance form seems to imply that benzene has alternating single and double bonds, neither form is correct by itself. The true benzene structure is a hybrid of the two individual forms, and all six carbon-carbon bonds are equivalent. This symmetrical distribution of electrons around the molecule is evident in an electrostatic potential map. 

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. 

Cyclohexadiene and benzene form identical structures on Pt(l 1 1) at low pressures (Figures 7.23 and 7.24). 1,3-Cyclohexadiene dehydrogenates to form benzene on the surface, while benzene adsorbs molecularly. Figure 7.24b schematically shows the adsorbed benzene structure at low pressure. The STM images of the C6 cyclic hydrocarbons show three different adsorbed structures on Pt(l 1 1). Cyclohexene and cyclohexane partially dehydrogenate to form rc-allyl, 1,4-cyclohexadiene adsorbs in a boat configuration, and both 1,3-cylohexadiene and benzene adsorb as molecular benzene on the surface. 

This picture of the benzene structure, which thus results from Thiele s conceptions, seems still to-day the most suitable for deducing and understanding the aromatic character of benzene in relation to the nature of the olefines. 

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... 

Pure computational results indicate that it should be possible to observe bond fixation in angularly distorted benzenes. Structural studies, either experimental or high-level computational, show that simple cyclic annelations are ineffective at inducing such bond fixation. Computational investigations of arenes with bicyclic annelations predict that these systems will show the desired bond fixation, trisbi-cyclo[2.1.1]hexabenzene, is the favored structure. 

The ring structure with the rapidly moving double bonds explained many of the facts known about benzene at the time. However, as more information became available and as chemistry advanced, it became obvious that more was going on in this system than just rapidly interconverting structures. Chemists determined that only one benzene structure existed, not an equilibrium between two related structures. 

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... 

Nonbenzenoids These compounds generally have two or more rings fused together, but none of the rings is a benzene structure, and they conform to HiickeTs rule, i.e. they have (4u + 2) tt electrons, and are aromatic compounds, e.g. azulene. 

If we think of a hybrid of these two structures, then the C—C bonds in benzene are neither single bonds nor double bonds. They should have a bond order between a single (1.47 A) and a double bond (1.33 A). It has actually been proven that benzene is a planar molecule, and ah of its C—C bonds are of equal length (1.39 A). The bond order (1.39 A) is indeed in between a single and a double bond Thus, instead of drawing the benzene structure using alternative single and double bonds, a hybrid structure can be drawn as follows. 

Benzene structure in the light of molecular orbital theory... 

N2,N4,NkTripicrylmelamine (TPM) [Structure (2.31)] has been reported to be a moderately thermally stable explosive with performance a little better than that of TNT [75]. Subsequently, heterocyclic nitrogen atoms of TPM were systematically replaced [76, 77] with the C-nitro function to give nitro-substituted tris(picrylamino) derivatives of pyrimidine [Structure (2.32)], pyridine [Structure (2.33)] and benzene [Structure (2.34)] given in Figure 2.2. 

Thus there is ample evidence that photochemically benzene will form adducts with several molecules and that these adducts indicate that the benzene structure has been transformed to an isomeric form,... 

Perfluoro(3,5-dialkvlpyridazines) 35 rearrange photolytieally to the 2,6-disubstituted pyra/ines 36.40 The mechanism that best explains these results involves rearrangement of an intermediate Dewar benzene structure 37 to 38, followed by rearrangement to the pyrazine structure (Scheme 12).40... 

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." ... 

The Lewis structure of borazine is nearly identical to that of benzene. It is obtained by replacing alternating C atoms in the benzene structure with B and N as shown. The orbitals at each B and N atom are sp2 hybridized. 

The ether extract of cane molasses yields an acidic substance with the characteristic odor of raw sugar.128 The steam distillation of molasses is stated to yield a rum oil. 129 Fractionation of cane final molasses on fuller s earth clay produces a concentrate with a strong molasses odor.70 The infrared spectra of the volatile portion of this concentrate indicated the absence of hydroxyl and carbonyl and the presence of a substituted benzene structure, of paraffinic methylene and methyl groups, of an acetate group, and of the > C=C < and —C=C— linkages. The presence of a sulfur function is probable. Further chromatography indicated complexity in this volatile concentrate.180... 

The conformation of the host is very different in the yellow 2 1 (host guest) benzene clathrate, 19-1/2 benzene. Structure 19 in benzene clathrate does not have approximate mirror symmetry. The aroyl groups have an asymmetric transoid arrangement and the torsion angles C(3)-C(2)-C(16)-C(17) and C(6)-C(7)-C(23)-C(24) are 155.24(12)° and 158.45(12)°,... 

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... 

X-ray investigations of the structure of benzene began in 1923 when Broome took the first X-ray powder photographs of the molecule. Later, Cox (1928) determined the cell dimensions and space group and showed that the molecule was at least centrosymmetric. The development of the work on the benzene structure has been reviewed by Cox (1958). A more detailed paper on the crystal structure of benzene at — 3°C (Cox et al., 1958) has established that the benzene molecule does not deviate significantly from the 6jmmm symmetry predicted by chemical theory, the maximum deviation of the carbon atoms from the mean molecular plane being 0-0013 A. 

Modem descriptions of the benzene structure combine resonance theory with molecular orbital theory. 

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