Bonding in Aromatic Compounds

FIGURE 3.14 Image of a single suspended sheet of graphene taken with TEAM 0.5, showing individual carbon atoms (yellow) on the honeycomb lattice. (Courtesy of Steven Pennycook.) 

We can account for this large difference in energies by looking at the relevant molecular orbitals. The o-skeleton of benzene is constructed by using sp hybrid orbitals at each carbon 

FIGURE 3.17 Energy diagram of the molecular orbitals of benzene. 

FIGURE 3.18 Lowest-energy jt-molecular orbital of benzene (oblique view). 

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All double bonds are perceived as possible dienophile synthons by the notation package. The screening involves only the elimination of all double bonds in aromatic compounds (WLN symbol R ). 

The contents of Sections II, III, and IV show that the activation of C—H bonds in alkanes by transition metal compounds has much in common with the activation of C—H bonds in aromatic compounds. It appears, therefore, to be more profitable at the present time to draw mechanistic parallels between alkanes and aromatic systems, as has been done here, than, say, between alkanes and molecular hydrogen, although, of course, much work has been done on hydrogen activation (59). 

Now the bond lengths of the CC bonds in aromatic compounds vary over only a small range it is reasonable to suppose that the bond energies of their a components will all be much the same. This should also be true of the bonds in arenonium ions such as I. 

Dipole moment internal H bonds in aromatic compounds and benzene solutions. 

Katagi, T. Theoretical studies on the photo-Fries rearrangement of O-aryl N-methylcarbamates. Nippon Noyaku Gakkaishi 1991, 16, 57-62. Grimme, S. MO theoretical investigation on the photodissociation of carbon-oxygen bonds in aromatic compounds. Chem. Phys. 1992,163, 313-330. 

The 77 bonds in aromatic compounds are also reactive toward electrophiles, although not nearly so much as alkenes. The aromatic ring attacks an electrophile to give an intermediate carbocation. The carbocation then undergoes fragmenta-tive loss of H+ (sometimes another cation) from the same C to which the electrophile added to re-form the aromatic system and give an overall substitution reaction. Thus, the predominant mechanism of substitution at aromatic rings under acidic conditions is electrophilic addition-elimination, sometimes referred to as SpAr. The reaction of toluene and nitric acid is indicative. 

As far as activation of the C-H bond in aromatic compounds with palladium(ll) is concerned, the features of these catalytic reactions are the following (1) hydrogen atom is departed at the first step as a proton (2) electrons of the activated C-H bond are involved in the formation of organopalladium intermediate (3) palladium(ll) is reduced into palladium (O), which then undergoes oxidation with an external oxidant into palladium(ll) acetate, thus providing a cyclic catalytic process (Scheme 4). 

Bonds in aromatic compounds tend not to alternate but bonds in antiaromatic compounds alternate. 

The available evidence is entirely in accord with this prediction. The bonds in aromatic compounds such as benzene or [18]annulene all have similar lengths, close to 1.40 A, whereas the bonds in antiaromatic compounds seem to alternate. A good example of the latter statement is provided by cyclo-... 

Owing to the convex shape of the wall of Cgo, all the 30 double bonds, located exocyclic to the pentagons, are strained and thus more reactive than formal double bonds in aromatic compounds. Because the electron density is much higher on the [6,6] ring junctions than on the [5,6] jnnctions, most of the chemical reactions of fullerenes tend to occur across these sites. Cycloaddition reactions (widely applied for functionalization of fullerenes), as well as additions of nucleophiles and free radicals, provoke a hybridization change in the carbon atoms involved from a trigonal sp to a tetrahedral sp. This release of the double bond strain is the driving force of such reactions. 

Resonance is a very 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 aromatic compounds, such as benzene, are equivalent and that benzene is best represented as a hybrid of two resonance forms. Although each individual resonance form seems to imply that benzene has alternating single and double bonds, neither form is correct by itself. The tme benzene stmcture 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. 

Due to the relatively high strength of C-H bond in aromatic compounds (113 kcal mol ) and N-H bond in ammonia (107 kcal moF ) [63], C-H/N-H dehydrogenative nitrogenation of arene with ammonia was usually proceeded in the presence of metal catalyst under harsh reaction conditions (Eq. 2.1). In 1917, the first example of this transformation using nickel/iron catalyst in the temperature range from 550 to 600 °C had been reported by Wibaut [64]. 

In presence of one carbon-nitrogen triple bond —C—C=N In compounds with tendency to dipole formation, e.g., C=C—C=0 In aromatic compounds... 

Although the above structures satisfy the molecular formula, double bonds do not in reality exist in aromatic compounds. Thus, aromatic rings are usually depicted by a hexagon with a circle in it. It is understood that a hydrogen is at each corner. 

Hydrazines and compounds that can be hydrogenated to hydrazines undergo N-N bond rupture on noble metal catalysts. Pd seems to be the best catalyst for the hydrogenolysis of the N-N bonds in aromatic hydrazines, whereas Rh seems best for the aliphatic ones. Ni is also useful for the hydrogenolysis of N-N bonds in hydrazines. 

The requirements necessary for the occurrence of aromatic stabilisation, and character, in cyclic polyenes appear to be (a) that the molecule should be flat (to allow of cyclic overlap of p orbitals) and (b) that all the bonding orbitals should be completely filled. This latter condition is fulfilled in cyclic systems with 4n + 2n electrons (HuckeVs rule), and the arrangement that occurs by far the most commonly in aromatic compounds is when n = 1, i.e. that with 6n electrons. IO71 electrons (n = 2) are present in naphthalene [12, stabilisation energy, 255 kJ (61 kcal)mol-1], and I4n electrons (n = 3) in anthracene (13) and phenanthrene (14)—stabilisation energies, 352 and 380 kJ (84 and 91 kcal) mol- respectively ... 

Cycloaddition with nitrile oxides occur with compounds of practically any type with a C=C bond alkenes and cycloalkenes, their functional derivatives, dienes and trienes with isolated, conjugated or cumulated double bonds, some aromatic compounds, unsaturated and aromatic heterocycles, and fullerenes. The content of this subsection is classified according to the mentioned types of dipolarophiles. Problems of relative reactivities of dienophiles and dipoles, regio- and stereoselectivity of nitrile oxide cycloadditions were considered in detail by Jaeger and... 

Arylation of C-H bonds is achieved by coupling reactions of C-H bonds with aromatic compounds such as halides, triflates, and organometallic reagents. Early works in this field involve the reaction of aryl halides with norbornene. As shown in Scheme 5, the coupling reaction of bromobenzene with norbornene in the presence of Pd(PPh3)4 as a... 

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