Dewar structures of benzene

FIGURE 7.4 (a) Kekule and Dewar structures of benzene, and their symmetry... 

In his first published paper on the structure of benzene in 1922, Ingold stated his aim to unify aliphatic and aromatic chemistry by studies of ring formation of unsaturated systems. He argued that chemists must accept the Dewar formulation, using a bridged phase of benzene, in order to relate the properties of aromatic compounds to the facts of aliphatic chemistry. 31... 

They are estimated by calculation to account for about 75% of the resonance stabilization of benzene with Dewar structures accounting for most of the remaining stabilization. However, such resonance structures have no separate physical reality or independent existence. A dotted circle is often used to indicate the resonance-stabilized bonding of benzene. Nonetheless, the most frequently appearing structures of benzene are the two Kekule structures. 

Proof for the existence of benzene isomers in irradiated benzene has been obtained in several ways. These will not be discussed in detail, but they may be classified broadly as physical and chemical. Nuclear magnetic resonance has been used by Wilzbach and Kaplan to identify benzvalene.39 Prismane has also been identified by NMR and by vapor-phase chromatography. The Dewar form has been synthesized in several steps which start with ris-1,2-dihydrophthalic anhydride. Photochemically this compound yields bicyclo(2,2,0)hexa-5-ene-2,3-dicarboxylic aqid anhydride. This was followed by catalytic reduction and oxidative decarboxylation to give the Dewar form of benzene.39 The method of synthesis alone provides some basis for structure assignment but several other bits of supporting evidence were also adduced. Dewar benzene has a half-life of about 48 hr at room temperature in pyridine solution and its stability decreases rapidly as the temperature is raised. 

The structure of benzene with its six C—H groups has been known for decades. However, arriving at the ling structure of the compound required much research and imagination in the early days of organic chemistry. Both Kckult and Dewar and, a hit later, Claus proposed ingenious structures. 

The product shown in Equation 13.66 is a derivative of Dewar benzene the unsubstituted analog, in a planar conformation, was originally suggested by Dewar as the structure of benzene.102 Once formed, such highly strained molecules cannot undergo photochemical reversion because they lack a low-lying... 

In the mid-1800s, chemists were struggling to determine the structure of benzene. One of the structures that was proposed at that time is known as Dewar benzene, after the chemist who suggested it ... 

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. 

The X-ray crystal structure of benzene, proving the equivalence of the six C-C bonds, appeared in 1929 and 1932, and Pauling reported its electron-diffraction data in 1931. Note that several of the structures proposed in the 19th century, such as Dewar benzene (non-planar) and Ladenburg s prismane, which are valence isomers of benzene, have now actually been prepared from benzene derivatives photochemically. They are kinetically stabilized, since they do not spontaneously revert to benzene or its derivatives [17-20]. 

A primary resonance structure includes only the two main structures of benzene, not the Dewar benzene forms, etc. 

Isocyclic compounds. For all aromatic molecules there is resonance between different electron configurations, the number of which becomes rapidly greater for larger molecules. In particular the number of possible so-called excited configurations increases, in which, analogous to the Dewar structures in benzene, one, two or more formal bonds are present. 

The five structures are the two Kekul and the three Dewar forms. It is possible, however, to construct other structures of benzene e.g. by joining atoms in the meta position with a tt bond or by making all three it bonds of the Dewar type ... 

In the course of the study of the structure of benzene, several structural formulas have been proposed for it. Well known structures are Kekule s cyclohexatriene, Ladenburg s prismane, Thiele s partial bond structure, and Dewar s bicyclohexadiene. Later, Huckel proposed benzvalene as another structure of benzene. Since Kekule s structure was gradually accepted as the structure of benzene, other structures are only of historical importance. 

Baker and Rouvray 11 argued that the term Dewar benzene should be abandoned for the honour of Dewar, since he never believed that the bicyclohexadiene could be a structure of benzene. He only showed some possibilities of compounds by molecular model, which included Kekule structure and others. The authors of this article don t agree with this proposal since we should pay more attention to Dewar s proposal of a highly strained compound. Furthermore, the term Dewar benzene has become too familiar to chemists to be abandoned. Thus, we use this term in this review,... 

Considering what nineteenth-century chemists knew about benzene, which is a better proposal for the structure of benzene, Dewar benzene or Ladenburg benzene Why ... 

Dewar structure A representation of the structure of BENZENE in which there is a single bond between two opposite corners of the hexagonal ring and two double bonds at the sides of the ring. The Dewar structures contribute to the resonance hybrid of benzene. 

By a careful compilation of the number of isomers of the known isomers of various aromatic compounds at that time, Kekuld postulated the benzene ring. Several other structures of benzene have been proposed, e.g., the Dewar benzene structure, theLadenburg benzene structure. 

To be sure, Baeyer said, what we are talking about are pictures or representations (Bilder), which must never be confused with real things themselves, but Hertz s comment applies whenever our theoretical pictures approach the unseen reality. These same considerations apply to the benzene theory. Baeyer proceeded to summarize recent research on the structure of benzene. The benzene formula proposed years earlier by James Dewar had quickly been ruled out as unable to explain the... 

Kekule, who was the first to draw localized bond structures of benzene, was criticized by his contemporaries because the alternating single bond/double bond structure did not explain the number of isomers found in substituted benzenes (e.g., only one isomer of o-substituted benzene). A. Claus [12], J. Dewar [13], L. Meyer [14], H. E. Armstrong [15], A. v. Baeyer [16], and A. Ladenburg [17] proposed alternative structures (Figure 13.2). 

Dewar structure A proposed structure of benzene, having a hexagonal ring of six carbon atoms with two opposite atoms joined by a long single bond across the ring and with two double C-C bonds, one on each side of the hexagon. Dewar structures contribute to the resonance hybrid of benzene. 

It s vital to keep clear the difference between resonance forms and molecules in equilibrium. In this chapter, that difference is exemplified by Dewar benzene, bicyclo[2.2.0]hexa-2,5-diene (Problem 13.5), and the Dewar resonance forms contributing slightly to the structure of benzene. As always, resonance forms are related only by the movement of electrons, whereas... 

Historical developments of the structure of benzene is important part of history of chemistry. Structures considered were Dewar benzene 486, Ladenberg s pris-mane 487 and benzvalene 488 (Fig. 20). Photochemisfly of fluorinated aromatic systems has made an important contribution to the study of valence isomers because it has been possible to isolate and characterize some species on which there had previously only been speculation. 

DEPT-NMR spectrum. 6-methyl-5-hepten-2-ol, 451 Detergent, structure of, 1065 Deuterium isotope effect, 386-387 El reaction and, 392 E2 reaction and, 386-387 Dewar benzene. 1201 Dextromethorphan, structure of, 294 Dextrorotatory, 295 Dextrose, structure of. 973 Dialkylamine, pKa of, 852 Diastereomers, 302-303 kinds of, 310-311 Diastereotopic (NMR), 456... 

Bicyclo[2.2.0]hexadienes and prismanes are valence isomers of benzenes. These compiounds actually have the structures that were proposed for benzenes in the nineteenth century. Prismanes have the Ladenburg formula, and bicyclo[2.2.0]-hexadienes have the Dewar formula. Because of this bicyclo[2.2.0]hexadiene is often called Dewar benzene. On page 32 it was mentioned that Dewar formulas are canonical forms (though not very important) of benzenes. Yet, they also exist as separate compounds in which the positions of the nuclei are different from those of benzenes. 

One of the most popular of the semi-empirical LCAO methods is that of Hfickel. It is applicable to planar molecules which have n -electron systems. The delocalization of these systems, as treated by this method, has particular chemical significance. The traditional application is to the benzene molecule. Historically, different structures of this molecule were suggested by K6kuI6+ and by Dewar that are described in virtually all textbooks of organic chemistry. These structures represented the first efforts to represent the delocalization of the 3r orbitals in such systems. In the present context the delocalization can be better specified with the use of the method of Hilckel. 

---