Benzene isomers

UV irradiafion of perfluoro o, m-, and p-xylenes in the gas phase gives a mixture of all possible Dewar benzene isomers Prismane valence bond isomers are proposed to be intermediates [148]... 

In contrast to the spontaneous reaction, the catalyzed process seldom gives the 1,2,3-trisubstituted benzene isomer from an acetylene RC=CH. The chief product is usually the 1,2,4-isomer, with lesser amounts of the 1,3,15-isomer also generally obtained, but little if any of the 1,2,3-isomer. The mechanism of the catalyzed... 

The orbital phase theory can be applied to the thermodynamic stability of the disubstituted benzene isomers. The cyclic orbital interaction in the benzene substituted with two EDGs is shown in Scheme 21. The orbital phase is continuous in the meta isomer and discontinuous in the ortho and para isomers (Scheme 22, cf. Scheme 4). 

Polymethyl-substitated benzene isomers usually are referred to by trivial names such as pseudocumene (1,2,4-trimethylbenzene), mesitylene (1,3,5-trimethylbenzene), and durene (1,2,4,5-tetramethylbenzene). These names were given to these compounds when aromahc compounds were hrst isolated from coal and their structures were as yet not proven. 

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. 

Benzene is commonly used as solvent in photochemical reactions nevertheless one has to take into consideration that arenes do undergo photorearrangements 330a,b). Benzene isomers as Dewar benzene331 (3.20) or benzvalene 332) (3.21) have been prepared by photolysis of benzenes. 

Figure 1. The Kekule oscillation hypothesis of two rapidly equilibrating benzene isomers constructed of van t Hoff tetrahedra used by Mills and Nixon.

A number of semibullvalenes and some barbaralanes, which undergo a facile Cope rearrangement via bicyclooctadienyl diradicals, produce various cyclic peroxides on exposure to triplet oxygen ". Thus, cyclopentano semibullvalene 82a is peroxidated with air to give a mixture of 5-membered 83a and 6-membered endoperoxide 84a in a 1 2 ratio, whereas the cyclohexano derivative 82b yields only the 6-membered cyclic peroxide 84b (Scheme 15). The energy in the highly strained mono(Dewar benzene) isomer of... 

The above reaction is predicted by the 3-2IG calculations to be exothermic by 42 kcal/mol. This reduces the energy difference between benzene and Dewar benzene to approximately 38 kcal/mol (80 kcal/mol - 42 kcal/mol). Thermodynamics still very much favors the crowded benzene isomer over the less-crowded Dewar benzene alternative. 

Benzvalene (18) is a tricyclic benzene isomer containing a bicyclobutane ring system bridged by an ethylene moiety its radical cation is accessible by PET or radiolysis. CIDNP indicated negative hfcs for the alkene protons (H ), strong positive hfcs for the non-allylic bridgehead protons (Hy), and negligible hfcs for the... 

It should also be noted that Leach and Migirdician37 have produced adducts of hexatriene in mixtures of benzene with certain other molecules in glassy matrices at low temperatures. Thus, along with benzene isomers which retain the empirical formula of the monomer, there may be formed photochemically other products which result from a rupture of the ring. Just possibly some of the cuprenelike polymer formed at very short wavelengths20 may be made in this way. We will return to this point in later sections. 

The benzene isomers which retain one hydrogen atom on each carbon atom are given the names (a) Dewar benzene, (b) prismane,... 

The present difficulty confronting theoreticians who desire to treat the photochemical formation of benzene isomers lies in the fact that the data are neither sufficiently abundant nor sufficiently precise to distinguish among theories. 

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 reports in 1966 by Haller18 and by Camaggi and co-workers,19 that photolysis of hexafluorobenzene (1) yields the hexafluorinated Dewar benzene isomer 2, triggered intensive in terest in the valence isomers of polyfluoroaromatics and polyfluoroheteroaromatics. 

The results obtained in the photolysis of perfluoroxylenes do not require intervention of benzvalenc isomers, as is favored for the hydrocarbon case, nor are structures 12 14 necessarily involved. A previous study21 reported that photolysis of ofyi-xylcnc 8 leads to Dewar benzene isomers 11 and 14. 

It is interesting to compare the dehydrocyclization activity of platinum with that of chromia-alumina. Pines and Goetschel reacted different butyl-benzene isomers over acidic and nonacidic chromia-alumina catalysts between 480°C and 492°C (41). Dehydrocyclization is much slower over... 

Several classes of polycyclic polysilanes, such as the octasilacubanes and the hex-asilaprismanes, have been synthesized100, but little is known about the photochemical behavior of the silacubanes which appear to be stable toward ultraviolet radiation. The silaprismanes, such as 195, on the other hand, were observed to rearrange to the Dewar benzene isomer 196, using 340-380 nm radiation101. The Dewar benzene reverted to the... 

Bats, 54 Batteries, 54 Benzene isomers, 55 Benzyl compounds, 55 Bis(arenediazo) oxides, 56 Bis(arenediazo) sulfides, 56 Bis(difluoroamino)alkyl nitrates, 56 Bis(fluorooxy)perhaloafkanes, 57 Bis(sulfur)diimides, 57 Bitumen, 57 Bleaching powder, 57 Bleve, 58... 

Analytical Properties Marginal effectiveness in separating disubstituted benzene isomers Reference 13... 

Analytical Properties Separation of dimethylbenzene isomers, dihalobenzene isomers (Cl, Br), haloketone benzene isomers, dimethoxybenzene isomers Reference 14... 

Analytical Properties Separation of disubstituted benzene isomers Reference 15... 

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