Benzene bonds

Before going into a detailed account of the chemistry of phanes, the author will touch on 3,4,7,8-tetrasilacycloocta-l,5-diyne briefly, since the compound illustrates the importance of a—it mixing. The ionization potential of the Si-Si bond is estimated by photoelectron spectroscopy to be 8.69 eV (9). Thus, the HOMO level of the Si-Si is comparable to most HOMOs of tt systems. Consequently, the Si-Si bond can conjugate efficiently with carbon-carbon double and triple bonds, benzene rings, and other tt systems. Most Si-Si bonds are stable enough to construct sophisticated structures by themselves and with organic molecules (10). 

If you completed the Concept Check activity on page 12, you drew a possible structure for benzene. For many years, scientists could not determine the structure of benzene. From its molecular formula, CeHe, scientists reasoned that it should contain two double bonds and one triple bond, or even two triple bonds. Benzene, however, does not undergo the same reactions as other compounds with double or triple bonds. 

The associative mechanism resembles a conventional radical (hydrogen atom) substitution reaction where the 7T-bonded benzene molecule is attacked by a hydrogen atom formed by the dissociative adsorption of water or hydrogen gas. The activation energy in this process is essentially due to the partial localization of one tt electron in the transition complex 21, 31). The transition state differs, however, from conventional substitution reactions by being 77-bonded to the catalyst surface ... 

Recently Harper and Kemball 90) have elaborated the associative mechanism in a manner which avoids the above geometrical limitations. Much of the reasoning upon which they base their ideas is recorded in papers by Gault et al. 91) and by Rooney 80). They postulate that TT-bonded benzene or molecular species such as B can combine either with a hydrogen atom from the surface or with a hydrogen molecule from the gas or physically adsorbed phase ... 

Cyclohexane, C6Hi2> is a carbon ring with two hydrogen atoms attached to each carbon. It resembles benzene, but there are no double bonds. Benzene is the feed to a cyclohexane plant, which is just a hydrogenation process. 

The heat of hydrogenation of cyclohexene is about -120 kJ/mol (kilojoules per mole). If the reaction of one double bond releases this amount of energy, then the reaction of two double bonds (1,3-cyclohexadiene) should release about twice this cimount of energy. The classical, three double-bond benzene should... 

A further result of the delocalization of the p-electrons is the merging of single and double bonds benzene is a perfect hexagon with all C-C bond lengths the same (0.140 nm). 

A true picture of benzene s structure was not determined until the 1930s when Linus Pauling produced his work on the chemical bond. Benzene does not exist as either of its resonance structures, and its structure should not be considered as either one or the other. A more appropriate model is to consider the... 

The classic example of resonance is provided by the tt bonding of benzene. This compound was shown in Chapter 1 to have the molecular formula C6H6, to be planar and hexagonal with bond angles of 120°, and to possess six equivalent C-C bonds and six equivalent C-H bonds. Benzene usually is written with a structural formula proposed by Kekule ... 

In the early days of organic chemistry, the word aromatic was used to describe certain fragrant substances from fruits, trees, and other natural sources. Chemists soon realized, however, that substances grouped as aromatic behaved in a chemically different manner from most other organic compounds. Today, the term aromatic refers to the class of compounds that can be represented as having a six-membered ring with three double bonds. Benzene is the simplest aromatic compound, but aspirin, the steroid sex hormone estradiol, and many other important compounds also contain aromatic rings. 

These, to some extent, are related to redistribution reactions but, of course, do not lie within the scope of this review. Probably one of the first redistribution reactions of ir-bonded compounds is the exchange of carbonyl groups with 7r-bonded benzene (85) on chromium leading to the mixed compound, benzenechromium tricarbonyl. 

Aihara J-I, Ishida T (2010) Aromaticity, benzene, and the role of sigma and pi bonds alternating-bond benzene is still aromatic. J Phys Chem A 114 1093... 

Aromatic compounds have ring structures with alternate single and double bonds. Benzene has a molecular formula of C6H6. The carbon atoms are joined together in a hexagonal ring and have alternate single and double bonds. 

Lithium naphthalenide (prepared from lithium and 1.33 equivalents of naphthalene) also reductively cleaves benzyl ethers [Scheme 4.143],262 Some functionalities survive the reaction conditions like carbon-carbon double bonds, benzene rings, THP ethers, stlyl ethers and methoxymethyl ethers. A ketone group can be present but its prior conversion to an enolate is necessary. A similar transformation, but with a catalytic amount of naphthalene, has been reported.263 Although allyl ethers are also cleaved by the procedure, the selective deprotec-... 

For transition metal-containing metallacarbaboranes, reactions at the metal center with no concomitant change to the cluster core are possible. The simplest type of reaction is ligand substitution. In equation (26), the three carbonyl ligands (each a two-electron donor) are replaced by a a--bonded benzene ring (a six-electron donor). When the net... 

The described deviation of the data for PC from the linear relationship (Fig. 3.11) can be explained by the fact that the chemical structure of these polymers differs significantly from that of polyolefines instead of CH2-CH2 bonds, benzene rings... 

Having just seen a resonance description of benzene, let s now look at the alternative molecular orbital description. An orbital view of benzene makes clear the cyclic conjugation of the benzene molecule and the equivalence of the. six carbon-carbon bonds. Benzene is a planar molecule with the shape of a regular hexagon. All C-C-C bond angles are 120 , all six carbon atoms are sp -hybridized, and each carbon has a p orbital perpendicular to the plane of the six-membered ring. 

The absence of rapid isotropic motion of the benzene molecules allows efficient transfer of proton magnetization, not only to the directly bonded benzene carbons, but also to the C60 carbons. For this reason, the CP/MAS spectrum consists of a single narrow line at 143.7 ppm surrounded by a set of spinning sidebands (Fig. 27). 

Molecules with double bonds next to each other and aromatic molecules based on benzene contain more than two tt orbitals on adjacent atoms. The bonds, as well as the entire molecules, are described as being conjugated. Electrons in these molecules are free to move from one bond to the next on the same molecule and so are delocalized. In 0014, we saw electron delocalization extending throughout the entire substance in materials with metallic bonds. Benzene (CeHe) has the following resonance forms ... 

Although it contains double bonds, benzene does not share most of the properties of alkenes. It is unusually resistant to hydrogenation, whereas most alkenes readily become hydrogenated. To account for this inertness, chemists have suggested that the extra electrons are shared equally by all six carbons in the ring rather than being located between specific carbon atoms. The currently accepted structure for the benzene molecule is shown in Figure 18.9. 

As a consequence, it is not surprising that there appears to be only one isolated structure claimed. This is of the indium(I) and thallium(I) complexes with a single M—C bond from a o--bonded benzene anion that carries two bulky tri-substituted benzene substituents in ortho positions these partially block approach of other potential donors to the metal cation (Figure 4.3). An X-ray crystal structure has been determined, defining the shape. It is a rare observation, as other complexes (such as Mn(I) and Fe(I)) of the same ligand bind another ligand at the open side of the molecule, as expected. In any case, this coordination number is trivial in the sense that it can have only one shape - a linear M—L arrangement. 

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