Benzene structural forms

The alternating ring stretch of the benzene CC bonds is illustrated in Fig. 4.22(f). During this vibration the electronic structure can change and approach the Kekul6 benzene structure form where the electrons have partially... 

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. 

Chemists sometimes represent the two benzene resonance forms by using a circle to indicate the equivalence of the carbon-carbon bonds. This hind of representation has to be used carefully, however, because it doesn t indicate the number of tt electrons in the ring. (How many electrons does a circle represent ) In this book, benzene and other aromatic compounds will be represented by a single line-bond structure. We ll be able to keep count of tt electrons this way but must be aware of the limitations of the drawings. 

In 1-benzoxepins the benzene oxide form is energetically unfavorable. Thus, the adducts 5 formed with dienophiles such as ethenetetracarbonitrile arise from the oxepin structure with the nonaromatic double bonds as diene fragment.233 The yields of these reactions arc almost quantitative. 

Only four years after Kekule proposed the ring structure for benzene, and before the configuration of salicylic acid was established, Kraut concluded on the basis of sound chemical evidence that the products obtained on heating acetylsalicylic acid possess chain structures formed through intermolecular esterification. He assigned the dimeric and tetrameric formulas... 

Fig. 18. Stereoview of the channel-type clathrate structures formed by host 12 with benzene (shown schematically) or o-xylene...

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. 

Thiadiazole and selenodiazole rings are isoelectronic with benzene and form a fully conjugated, essentially planar structure when fused to porphyrazines at the p-pyrrolic position (168). Compared to pc, the incorporation of sulfur and selenium in addition to nitrogen atoms to the peripheral heterocyclic rings affects the electronic charge density and influences the interactions between adjacent molecular stacks. 

A recent example of diasteromeric amplification with achiral guests and a racemic library can be seen in the work of Iwasawa and coworkers. The library members consisted of a racemic polyol and l,4-benzenedi(boronic acid) [2], When these components were mixed in an equimolar ratio in methanol, a precipitate formed, which was insoluble in other organic solvents and thought to be a polymeric boronate. However, when the library members were mixed in the presence of toluene or benzene, a precipitate again formed, but it was soluble in several (nonprotic) organic solvents where boronic ester exchange is slow. With toluene a [2 2] complex of the polyol and diboronic acid formed, as evidenced by NMR and FAB-MS data. X-ray crystallography confirmed that a cyclic structure formed with... 

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... 

It is interesting that benzene and naphthalene form monolayer surface structures on the Pt(l 11) crystal face at 300 K and higher temperatures while monolayer surface structures form only at low temperatures ( 200 K) on the Ag(l 11) crystal face While these aromatic molecules are held by strong chemical bonds to the platinum, their heats of adsorption must not be greater than the heats of sublimation... 

From the decomposition mechanism and the products formed it can be deduced that DCP primarily generates cumyloxy radicals, which further decompose into highly reactive methyl radicals and acetophenone, having a typical sweet smell. Similarly, tert-butyl cumyl peroxide (TBCP) forms large quantities of acetophenone, as this compound still half-resembles DCP. From the decomposition products of l-(2-6 rt-butylperoxyisopropyl)-3-isopropenyl benzene ( ), it can be deduced that the amount of aromatic alcohol and aromatic ketone are below the detection limit (<0.01 mol/mol decomposed peroxide) furthermore no traces of other decomposition products could be identified. This implies that most likely the initially formed aromatic decomposition products reacted with the substrate by the formation of adducts. In addition, unlike DCP, there is no possibility of TBIB (because of its chemical structure) forming acetophenone. As DTBT contains the same basic tert-butyl peroxide unit as TBIB, it may be anticipated that their primary decomposition products will be similar. This also explains why the decomposition products obtained from the multifunctional peroxides do not provide an unpleasant smell, unlike DCP [37, 38]. 

To determine the effect of adsorption on the secondary porous structure of the pellets and on the external surface of the crystals on their size variation, benzene adsorption was measured. The voids or micropores of the zeolite NaA are inaccessible to benzene molecules. Adsorption occurs only on the external surface of the crystals and in the secondary porous structure formed by the gaps between the contacting zeolite crystals. In Figure 3 the relative pressure is plotted on the z-axis for better comparison. Variations in the sizes of the pellets of zeolite NaA up to p/p9 = 0.93 are negligibly small, and the points coincide with the x-axis. Consequently, adsorption on the external surface of crystals and at the sites of their contact—i.e., in transitional and macropores of the secondary porous... 

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,... 

The picture is less clear for molecular crystals when the molecules deviate strongly from a globular form. NMR data and tracer diffusion data are then often in disagreement. Diffusion profiles (In c, vs. distance) are found to be curved, which is usually attributed to additional heterogeneous and fast diffusion pathways. For plastic crystals, this could indicate that many of them possess a highly defective structure. Even for the aromatic ring molecule benzene, which forms a non-plastic crystal, one finds a D (NMR)/D (tracer) ratio on the order of 103. This cannot be understood unless one invokes other than bulk lattice mechanisms of diffusion. 

Tolman has shown that the equilibrium constants for the reactions of 38 substituted ethylenes with Ni[P(0-o-tolyls)]3 in benzene, to form (ENE)bis-(tri-o-tolylphosphite)nickel complexes, is sensitive to the ethylene s structure, eqn. (2) (ref. 7). Values of Ki at 25° vary from 10 for cyclohexene to... 

Mercuric nitrate combines with benzene to form a deep-brown or black addition compound, the probable structure of which is indicated below. This material when warmed with nitric acid is oxidized with the production of red fumes and the formation of... 

The solubility of small molecules like rare gases or CH4- -C q in water decreases with TM 116 but increases with Tin cyclohexane. This indicates too that the solubility mechanism in water depends on a special ordered structure forming of water. Benzene and derivatives show a minimum of the T-dependence of the solubility in water. In homologous series the solubility in water decreases with increasing radius of the solute87 (Fig. 18) and becomes very low at a radius of more than 6 A. This... 

The crystal packing of the molecules of thiepine 19 in the unsolvated form and the benzene clathrate is very closely related, and the / -axial lengths are essentially identical. The packing along the a-axis of the unsolvated form is very similar to the c-axis packing in the benzene clathrate. A least squares superposition of the molecule of thiepine 19 in its unsolvated and benzene-complexed form showed that their structures and conformations match almost perfectly. 

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