Benzene Dewar forms

T orbital for benzene obtained from spin-coupled valence bond theory. (Figure redrawn from Gerratt ], D L oer, P B Karadakov and M Raimondi 1997. Modem valence bond theory. Chemical Society Reviews 87 100.) figure also shows the two Kekule and three Dewar benzene forms which contribute to the overall wavefunction Kekuleform contributes approximately 40.5% and each Dewar form approximately 6.4%. 

I should like to see a point of union between yours and Bamberger s views and the ideas I have developed that benzene has a Dewar form tautomeric with the two Kekule individuals. 29... 

Dewar benzene has actually been isolated, and found to revert only slowly to benzene (its half life is approximately 2 days at 25°C). This is remarkable given how similar its geometry is to that of benzene, and what is expected to be a huge thermodynamic driving force for the isomerization. The substituted Dewar benzene, 7, formed from photolysis of 1,2,4-tri-tert-butylbenzene, 6, is apparently even... 

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. 

A substituted Dewar form of benzene has also been prepared by the reaction of cyclobutadiene with an acetylenic compound.57... 

For benzene we have considered so far only the Kekule structures in which the bonds are formed between neighbouring atoms such forms represent the greatest possible interaction of the electron clouds, since the distance between neighbouring carbon atoms is only i 4 A, and will thus contribute more than other possible structures to the resultant state of the molecule. But it is necessary to consider all possible valency structures among which there is resonance and thus for benzene the three Dewar forms must be considered ... 

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 ... 

The existence of these intermediates has not been proven, and for heavily substituted fluoro-benzenes, Dewar isomeric forms arise from excited singlet states in apparent contravention of the symmetry considerations (253,254). The significance of this is not clear. 

A special Dewar benzene is formed by the reaction of perfluoroisobutene with chloroform in the presence of cesium (I) fluoride (20)22) ... 

Most prismanes were synthesized by photoreaction of benzenes. The formed intermediates were supposed to be Dewar benzenes which were cyclized by a (2- addition reaction. An early example is the photolysis of tri-fert-butylbenzene as mentioned before. In this case, a mixture of the prismanes was isolated. 

Fits. 1-9 The three, equivalent. "singly-excited-Kekuie" (or Dewar ) forms of benzene. 

Photolysis of hexafluorobenzene affords hexafluoro-Dewar-benzene which forms an adduct (2) with phenyl azide. This adduct... 

Okoyuma et al. have reported that the strained Dewar paracyclophanes (98) can be converted into the benzenoid parent compounds (99) by irradiation of the compounds in a diethylether-isopentane glass at 77 K using 365 nm light. Interestingly when (99, R = CN, = H) is allowed to thaw to room temperature there is a thermal reaction that converts it back into the Dewar form which provides the first observation of such a thermal cyclization. Irradiation at 254 nm of the tetraene (100), a hk-Dewar benzene, at 77 K in a matrix results in its conversion into the paracyclophane (101). Continued irradiation transforms (101) into the (4+4)-adduct (102). The products from this low temperature... 

When a compound is so constituted that more than one normal valency formula may be used to represent its molecule, as with benzene where the two Kekule forms and the three Dewar forms are possible, the energy of formation usually proves to be greater than that calculated for any of the possible individual formulae. The molecule thus appears to exist in a state which is more stable than that corresponding to any of the conventional valency-bond representations. Detailed evidence from many somces, indeed, suggests that these formulae are quite often inadequate. Benzene, for example, is best regarded not as possessing three single and three double carbon-carbon bonds, but six equal bonds of order approximately 1 -S. 

Scheme 1 Synthesis of a [6]annulene via its Dewar benzene isomer. The Dewar form is prepared via a [47H-27r] cycloaddition with [4]annulene and a substituted acetylene.

In the Dewar benzene structure, the bulky tert-butyl groups are farther apart and interact less with each other than they do in the aromatic structure and, in conjunction with other factors, render the Dewar form stable enough to be isolable. Reversion to the aromatic form occurs only on heating. The parent compound was reported by van Tamelen and Pappas in 1963 ... 

WORKED PROBLEM 13.3 There are three other resonance forms for benzene, which can be included to get a sUghtly better electronic description of the molecule than is provided by the pair of Kekule forms alone. In these other resonance forms, overlap between two p orbitals on across the ring carbons is taken into account. Draw these three resonance forms, called Dewar forms, showing the orbitals involved in the cross-ring bond. Be careful This problem uses bonding that you probably haven t seen before. 

PROBLEM 13.4 Do you think the Dewar forms will contribute strongly to the benzene structure Why or why not ... 

Dewar forms (Section 13.3) Resonance forms for benzene in which overlap between 2p orbitals on two para carbons is emphasized. These forms superficially resemble Dewar benzene (bicyclo[2.2.0]hexa-2,5-diene). 

Aromatic compounds are thermally very stable but they undergo many photochemical transformations. The photochemical transformation of benzene and its derivatives provides a lot of surprising changes. The solution phase photolysis of benzene with short wavelength, i.e., irradiation of benzene at 204 nm causes Sq -> S2 excitation and observation shows that bonding between the para position is favoured. Dewar benzene is formed. When irradiated at 254 nm the meta position favoured for to give benzvalene and fulvene. 

UV irradiation of hexafluorobenzene with indene or cycloalkenes gives high yields of 2+2 adducts, which undergo further intramolecular cycloaddition to form hexafluoropolycycloalkanes [754] (equation 38) Photolysis of fluormated deriva tives of vinylbenzenes afford benzocyclobutenes, whereas allyl benzenes yield Dewar benzene-type products [755]... 

Thermolysis of the hexamethyl-substituted Dewar benzene-phenylazide adduct 1 in refluxing Decalin affords the 1.2,3-triazonine derivative 2 as the major product in form of a yellow-orange powder.30 A bicyelic isomer is also isolated which arises from rearrangement of 2 by purification on alumina and is readily converted back to 2 on heating to 80 C. 

The Dewar benzene of hexafluorobenzene formed an adduct with pheny-lazide that gave a polyfluoro-l//-azepine on pyrolysis. R=C02Et (47) was obtained when ethylazidoformate was decomposed in C6F6 [82JCS(P1)2101]. Photolysis of (47) yielded a 2-aza-bicyclo(3.2.0.)hepta-3,6-diene, which, in contrast to its nonfluorinated analogue, showed excellent thermal stability (3 h, 200°C, 88% recovered) [82JCS(P1 )2105]. 

The Dewar structure (IV) has never been seriously considered since it suffers from the same defects, in an accentuated form, as the Kekule structure. Its advantage over the other structures is largely that it explains the intimate relationship that usually obtains between the para positions in the benzene ring. 

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. 

Ab initio (3-21G( )//STO-3G) calculations by Chandrasekhar and Schleyer163 on 1,4-disilabenzene 58, its Dewar benzene isomer 59, and a silylene isomer 60 showed that all three species exhibited approximately similar stabilities, the silylene 60 being 9.9 kcal mol-1 more stable than the planar aromatic form 58, which was 5.9 kcal mol-1 more stable than the Dewar benzene form 59. 

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