Benzene and related molecules

In the final section of this chapter, we discuss the vibrational spectra of benzene, its isostructural species CeOg- (rhodizonate dianion), and dibenzene metal complexes. 

Historically, the cyclic structure of benzene with symmetry, as shown in Fig. 7.3.15, was deduced by enumerating the derivatives formed in the mono-, di-, tri-substitution reactions of benzene. The structure can also be established directly using physical methods such as X-ray and neutron diffraction, NMR, and vibrational spectroscopy. We now discuss the infrared and Raman spectral data of benzene. 

So four bands are expected in the IR spectrum (one with A2u symmetry and three sets with E u symmetry), while there should be seven bands in the Raman spectrum (two with A211 symmetry, one set having Fig symmetry, and four sets with F2g symmetry). 

The 20 vibrational modes of benzene are pictorially illustrated in Fig. 7.3.16. Also shown are the observed frequencies. In this figure, i (CH) and i (CC) represent C-H and C-C stretching modes, respectively, while 8 and n denote in-plane and out-of-plane bending modes, respectively. For each E mode, only one component is shown. 

The 20 vibrational modes illustrated in Fig. 7.3.16 may be broken down into various types of vibrations. In the following tabulation, the representation generated by the 12 vibrations of the carbon ring skeleton is denoted by V c6, and V c-c and V c-h are the representations for the stretching motions of C-C and C-H bonds, respectively. 

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We start with some biographical notes on Erich Huckel, in the context of which we also mention the merits of Otto Schmidt, the inventor of the free-electron model. The basic assumptions behind the HMO (Huckel Molecular Orbital) model are discussed, and those aspects of this model are reviewed that make it still a powerful tool in Theoretical Chemistry. We ask whether HMO should be regarded as semiempirical or parameter-free. We present closed solutions for special classes of molecules, review the important concept of alternant hydrocarbons and point out how useful perturbation theory within the HMO model is. We then come to bond alternation and the question whether the pi or the sigma bonds are responsible for bond delocalization in benzene and related molecules. Mobius hydrocarbons and diamagnetic ring currents are other topics. We come to optimistic conclusions as to the further role of the HMO model, not as an approximation for the solution of the Schrodinger equation, but as a way towards the understanding of some aspects of the Chemical Bond. 

Electrophilic attack on benzene and related molecules proceeds by an addition-elimination mechanism. Initial attack generates a carbocationic intermediate from which loss of a proton restores the aromatic system. The carbocation intermediate is stabilized by resonance. 

Even these three types do not cover all the carbon-carbon bonds found in nature, however. In benzene (CgHg), the experimental carbon-carbon bond length is 1.397 A, and its bond dissociation energy is 505 kj mol . This bond is intermediate between a single bond and a double bond (its bond order is l ). In fact, the bonding in compounds such as benzene differs from that in many other compounds (see Chapter 7). Although many bonds have properties that depend primarily on the two atoms that form the bond (and thus are similar from one compound to another), bonding in benzene and related molecules, and a few other classes of compounds, depends on the nature of the whole molecule. 

In short, the notion that a double bond consists of one bond having properties very similar to those of the corresponding bond in a saturated compound, while the other has very peculiar properties, is suggested, so to speak, by experimental evidence the second bond seems to be responsible for the high reactivity of olefins, the chemical behaviour of conjugated compounds, the aromaticity of benzene and related molecules, the physical properties characteristic of unsaturated and aromatic com-... 

Still another aspect of Pauling-type approximations (5.50-5.54) is inconsistent with empirical resonance concepts. For benzene and related molecules, it was recognized empirically that resonance hybridization is associated with unusual stabihty, such that the energy is lower than (rather than an average of ) its localized constituents. However, for Pop = Hop. where (5.54) could he expected to follow from (5.51), the Pauhng formulation... 

In the preceding chapter, we looked at aromaticity—the stability associated with benzene and related compounds that contain a cyclic conjugated system of An + 2 77 electrons. In this chapter, we ll look at some of the unique reactions that aromatic molecules undergo. 

IN the past twenty years the electronic structures of many organic molecules, particularly benzene and related compounds, have been discussed in toms of the molecular orbital and valence bond methods.1 During the same period the structures of inorganic ions have been inferred from the bond distances f a bond distance shorter than the sum of the conventional radii has been attributed to the resonance of double bonded structures with the single bonded or Lewis structure. 

Chapter 16 is the first of three chapters that discuss the chemistiy of conjugated molecules—molecules with overlapping p orbitals on three or more adjacent atoms. Chapter 16 focuses mainly on acyclic conjugated compounds, whereas Chapters 17 and 18 discuss the chemistiy of benzene and related compounds that have a p orbital on every atom in a ring. 

Pyrazine may be represented as a resonance hybrid of the canonical structure illustrated (21a -<+ 21d). The molecule is planar, and Pauling (114) states that it is stabilized by about 40kcal/mol as in benzene and pyridine, but resonance energies derived by different methods show considerable variation. Some of these resonance energies together with values for benzene and related heterocycles are summarized in Table 1.2 (115-117). More recent measurements of heats of hydrogenation are given in Section IV. 1B. 

The Ea are considered according to the methods used. Those for hydrocarbons determined by other methods are compared with the ECD values. The Ea for ben-zaldehydes, acetophenones, benzophenones, bezonitriles, esters, and nitrobenzenes have been determined by the ECD, TCT, and/or NIMS methods. These compounds and related molecules obtained from reduction potentials and charge transfer complex energies will be evaluated. Based on these data, the substitution effects for CH3C=0, N02, HC=0, NH2, F, Cl, and CF3 and the alkyl groups are determined. The multiple substitution effects of N02 and CN on benzene and ethylene are also established. 

The delocalized n bonding system confers stability on the structure and also gives benzene and related compounds a particular chemistry. The presence of a stable structure means that reactions involving benzene will tend to retain or restore this structure where possible. As in alkenes, the presence of such an electron-rich region in the molecule makes arenes susceptible to electrophilic attack. However, alkenes undergo electrophilic addition, but the interaction of benzene and other arenes with electrophiles results in electrophilic substitution reactions, in which the structure of the aromatic ring is preserved (Figure 20.34). 

In previous sections, ruthenium-catalyzed cycloadditions of alkynes leading to benzenes and related heterocycles were surveyed along with their underlying mechanisms. As demonstrated by the examples above, the past decade has witnessed significant development of efficient catalytic protocols and synthetic methodology, which provides chemo- and regioselective routes to polycyclic benzenes and heterocycles. In this section, applications of these methods to the constmction of unnatural functional molecules are outlined. For synthetic applications to natural products, see Chapter 7. 

The most common ingredient in shampoos is also the most common detergent in use in other products a class of surfactants known as straight-chain alkyl benzene sulfonates. Examples are ammonium lauryl sulfate, its sodium relative, and the slightly larger but related molecule ammonium lauryl ether sulfate (sometimes abbreviated as ammonium laureth sulfate). 

Fig. 27. Packing relations in the crystal structure of 47 benzene (1 1) 64>. Stereo drawing of complementary stick style and space filling representations of host and guest molecules, respectively (atomic radii of the corresponding guest atoms in the space filling style are set to about half of their common van der Waals values the H atoms of the host molecules are omitted)...

This conclusion, nevertheless, should not be considered categorical but it points to the necessity of careful consideration of the correlation between the AEdis value and the part of it that relates to cyclic electron delocalization. It has been shown by use of TRE calculations of aromatic benzene and antiaromatic cyclobutadiene molecules that in the case of benzene the distortion into a Kekule-type structure is characterized by a change of the aromatic cyclic Tr-electron delocalization energy in an opposite direction... 

For pyridine, pyrazine, and related six-membered heterocyclic molecules Kekul6 resonance occurs as in benzene, causing the molecules to be planar and stabilizing them by about 40 kcal/mole. The interatomic distances observed in these molecules,106 C—C = 1.40 A, C—N = 1.33 A, and N—N 1.32 A, are compatible with this structure. The resonance energy found for quinoline, 69 kcal/mole, is about the same as that of naphthalene. 

This expression relates the composition of the vapor (in terms of the mole fraction of A in the vapor) to the composition of the liquid (in terms of the mole fraction of A in the liquid, remembering that xB = 1 — xA). It is plotted for benzene and toluene in Fig. 8.39, and we see that xbaaent jVapor > Xbenzenejiqui just as we anticipated. For instance, for the mixture we have been considering in which one-third the molecules are benzene in the liquid, the mole fraction of benzene in the vapor is found to be 0.619, or nearly twice as large. 

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