The Structure and Properties of Benzene

The Resonance Representation The resonance picture of benzene is a natural extension of Kekuld s hypothesis. In a Kekuld structure, the C—C single bonds would be longer than the double bonds. Spectroscopic methods have shown that the benzene 

Benzene is actually a resonance hybrid of the two Kekule structures. This representation implies that the pi electrons are delocalized, with a bond order of 11 between adjacent carbon atoms. The carbon-carbon bond lengths in benzene are shorter than typical single-bond lengths, yet longer than typical double-bond lengths. 

Friedrich August Kekule von Stradoniz (1829-1896), pictured on a Belgian postage stamp. 

The resonance-delocalized picture explains most of the structural properties of benzene and its derivatives—the benzenoid aromatic compounds. Because the pi bonds are delocalized over the ring, we often inscribe a circle in the hexagon rather than draw three localized double bonds. This representation helps us remember there are no localized single or double bonds, and it prevents us from trying to draw supposedly different isomers that differ only in the placement of double bonds in the ring. We often use Kekuld structures in drawing reaction mechanisms, however, (o show the movement of individual pairs of electrons. 

1 Write Lewis structures for the Kekule representations of benzene. Show all the valence electrons. 

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As we have seen above, two formula types are conunonly used to depict benzene rings. The traditional bond-line representation allows easier depiction of mechanisms involving the tt electrons, as we shall need to do in upcoming chapters, whereas the circle in the hexagon notation better suggests the structure and properties of benzene rings. 

Using l,3-bis[bis(2-pyridylmethyl)amino]benzene, Schindler et al. investigated the structures and properties of complexes (738) linear (weak interaction with the tertiary amine)/distorted... 

In Part 11 we concentrate on aromatic systems, starting with the basics of structure and properties of benzene and then moving on to related ciromatic compounds. We even throw in a section of spectroscopy of aromatic compounds. Chapters 7 and 8 finish up this pcirt by going into detail about substitution reactions of aromatic compounds. You find out all you ever wanted to know (and maybe more) about electrophilic and nucleophilic substitutions, along with a little about elimination reactions. 

Resonance theory (Sections 2.4-2.5) accounts for the stability and properties of benzene by describing it as a resonance hybrid of two equivalea forms. Neither form is correct by itself the true structure of benzene is somewhere in between the two resonance forms but is impossible to draw with our usual conventions. Many chemists therefore represent benzene by drawing it with a circle inside to indicate the equivalence of the carbon-carbon bonds. This kind of representation has to be used carefully, however, because it doesn t indicate the number of w 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 jr electrons this way, but we must be aware of the limii tions of the drawing. . 

Experiments with Benzoquinone,—Benzoquinone was prepared by the oxidation of pure hydroquinone with chromic acid. It was recrystallized twice from benzene and finally sublimed. Only the purest crystals of the sublimed quinone were used for the reactions studied. Since benzoquinone was foimd to sublime very considerably at 110°, it was necessary to dissolve it first in anethol (this was done in a special apparatus filled with pure Ns), then the mixture added to the main apparatus. Benzoquinone was found to react readily with anethol yielding a deep red solution which on heating at 110°, in the absence of oxygen, deposited slowly small quantities of a deep brown amorphous solid, the structure and properties of... 

To change the structure and properties of the gel, complementary poly(A) was added to the 28c/benzene gel system [49]. Since poly(A) was virtually insoluble in organic solvents, the lipohilic poly(A)/cationic lipid complex was used instead. In the presence of an equal amount of the... 

W. Trochimezuk, Changes in Structure of Polyethylene/Styrene divinyl benzene System, presented at the Structure and Properties of Polymer Networks, Jablonna, Poland, April 1979. Polyethylene/poly(styrene-co-divinyl benzene) semi-II IPNs. Scanning electron microscopy of etched samples. Morphology goes from PS discontinuous to PS continuous as DVB level is increased passed 2%. Polyethylene crystal size is decreased with increasing DVB content. 

The MgClj-supported catalysts were developed by Xu et al. [62]. The catalyst was prepared by applying TiCl4 or Ti(OBu)4 to MgCl2 in combination with R3AI. The activity of supported catalysts was lower than that of soluble ones and increased with Lewis acidity of the cocatalyst. The polymer was found to contain benzene, the content of which depended on the temperature of the synthesis. The structure and properties of the polymers depend on the cocatalyst used. 

An interesting feature of the chemistry of monomeric organoplatinum(II) hydroxides and alkoxides is that CO inserts into the Pt—O bond more readily than into the Pt—C bond. Thus in 1993, Bennett and coworkers in an effort to examine the structures and properties of Pt(II)-hydroxycarbonyls with trans-arranged phosphines explored the reaction of the monomeric pincer Pt(II)-hydroxo species (14) with CO [9]. Hence, treating the (PCP)Pt(OH) pincer complex (14) with CO in benzene under ambient conditions caused the formation of the hydroxycarbonyl complex (15) as a... 

The block copolymer was complexed with Cu(II) ion in methanol, then the methanol was evaporated to dryness. The Cu complexed block copolymer was soluble in water. Its spectroscopic property showed that the structure and stability of the Cu complex were similar to those of the complex in a benzene solvent. But the reactivity of the Cu complex in the block copolymer could not be examined. The Cu complex was occuluded so tightly in the... 

The structures of cycloproparenes and the chemistry and properties of alkylidenecy-cloproparenes have been reviewed. Much experimental and theoretical work has been devoted to strain and stability and to bond fixation in such fused aromatic systems resulting in much controversy. The anellation of small rings to a benzene ring gives rise to substantial angular deformation and bond distance change. 

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