Benzene and related compounds

The term BTEX concerns benzene and its alkyl derivatives such as toluene, ethylbenzene and xylenes. These compounds are semivolatile and, therefore, the solution must be prepared with great attention in order to minimise evaporation loss. 

The shape of benzene and all alkylbenzenes UV spectra is characterised by a relatively fine structure. A general bathochromic effect can be observed according to the nature and position of the substituents with regard to benzene UV spectrum. The latter has a maximum at 256 nm, whereas the maximum is around 262 nm for toluene and ethylbenzene (Fig. 37) and around 266 nm for xylenes (Fig. 38). Concerning xylenes, the bathochromic shift is more important for the parasubstituted isomer (respectively, 4 and 3 nm compared to the spectra of 2-xylene and 3-xylene). In this case, a hypochromic effect is added to the bathochromic shift. 

Benzene could also be substituted by a chlorine atom (chlorobenzene) or a nitro group (nitrobenzene). The effect of the addition of one chlorine atom on the aromatic ring leads to a bathochromic shift of 5 nm (Fig. 39). 

Polycyclic aromatic hydrocarbons (PAHs) are highly toxic pollutants (LC50 mgL-1 for aquatic organisms), and some of them have proven to be carcinogenic [8,13]. They are thus monitored in potable water and wastewater. PAHs are also often present at high 

Benzo[k]fluoranthene (BkF ) Dibenzo[a,h]anthracene (dBA ) Benzo[a]pyrene (BaP ) 

Naphthalene, in contrast to benzene, did not show any NMR-spectra line-width narrowing up to its melting temperature of 353 K. The mean experimental second moment was 9.1 compared to 10.1 G, estimated for the rigid crystal. Measurement of spin-lattice relaxation times indicated, however, also a slow reorientational jump motion about an axis normal to the molecular axis An activation energy of 105 kJ/mol was derived. Molecular dynamics simulations suggest that this reorientation about the axis of greatest inertia occurs with a frequency of 100 MHz within 20 K of fusion (353.6 K) Still, no plastic crystal behavior as found in cyclohexane and related compounds (see Sect. 3.1.1) is indicated for benzene or naphthalane, even close to the melting temperature. 

Similar reorientations within the plane of the cyclic molecules have been discussed for pentafluorochlorobenzene (AS = R In 6) and other asymmetrically hexa-substituted benzenes In several of the latter molecules —CH3 and —Cl groups are 

Hexamethylbenzene itself shows, because of its high symmetry, again a narrowing of the second moment of the proton NMR spectrum independent of thermodynamic phase transitions In the triclinic low temperature phase III, up to 116 K, one 

Biphenyl is the first molecule of this series that may show rotation about the bond connecting the phenyl rings. On rotation out of the planar conformation the overall molecule remains linear because of the sp -bond-geometry, but it loses its center of inversion. In the gas and liquid phases the ortho-hydrogen repulsion is sufficient to yield 42 and 34 twisted conformations, respectively. Surprisingly X-ray diffraction shows a seemingly planar molecule (monoclinic, P2j/a, two molecules per unit cell) capacity measurements between 10 K and room tem- 

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The precise value of the resonance energy of benzene depends as comparisons with 13 5 cyclohexatriene and (Z) 13 5 hexatriene illustrate on the compound chosen as the reference What is important is that the resonance energy of benzene is quite large SIX to ten times that of a conjugated triene It is this very large increment of resonance energy that places benzene and related compounds m a separate category that we call aromatic... 

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. 

TABLE 12.9. Thermodynamic Data for Benzene and Related Compounds... 

It is important to note that the Kekule structure for benzene and related compounds has not been abandoned - it is still widely used. 

AROMATIC NUCLEUS. Six-carbon ring characteristic of all benzene and related compounds. ATMOSPHERE. 14.69 psi or 760 mm of mercury. 

Compounds in the two groups differ in a number of ways. The two differ chemically in that the aliphatic undergo free-radical substitution reactions and the aromatic undergo ionic substitution reactions. In this chapter you examine the basics of both ciromatic and heterocyclic ciromatic compounds, concentrating on benzene and related compounds. 

Antipyretic substance that reduces fever Aromatic Hydrocarbon an unsaturated cyclic hydrocarbon that does not readily undergo additional reaction, benzene and related compounds... 

Aromatic compounds have a special place in ground-state chemistry because of their enhanced thermodynamic stability, which is associated with the presence of a closed she of (4n + 2) pi-electrons. The thermal chemistry of benzene and related compounds is dominated by substitution reactions, especially electrophilic substitutions, in which the aromatic system is preserved in the overall process. In the photochemistry of aromatic compounds such thermodynamic factors are of secondary importance the electronically excited state is sufficiently energetic, and sufficiently different in electron distribution and electron donor-acceptor properties, ior pathways to be accessible that lead to products which are not characteristic of ground-state processes. Often these products are thermodynamically unstable (though kinetically stable) with respect to the substrates from which they are formed, or they represent an orientational preference different from the one that predominates thermally. 

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. 

Estimates of daily exposure to benzene from urban or suburban air range from 180 to 1300 /ig/person/day.1112 Urban air concentrations of the other aromatic hydrocarbons are similar to those of benzene and the vast majority of exposure of the general population to these other aromatic hydrocarbons will be due to road transport or solvent-containing products rather than food. A 1995 survey of these compounds in samples from the UK Total Diet Study showed that average dietary exposures to benzene and related compounds from food in the UK are low, and very much lower than estimated exposure from active smoking of tobacco or intakes from air by urban dwellers.13 The mean dietary exposure to benzene was estimated to be in the range 0.9-2.4 /ig/person/day. 

Huckel, E. Quantum-theoretical contributions to the benzene problem. I. The Electron configuration of benzene and related compounds, Z. Physik 1931, 70,204-288. 

In the ensuing sections many more examples will be cited, such as the cycloaddition reactions of alkenes to benzene and related compounds (Cantrell, 1977 Gilbert and Heath, 1979 Bryce-Smith et al., 1980b) where the quenching of fluorescence can be related to the redox properties of the quencher and yet it has proved impossible to detect the formation of fluorescent complexes or radical ions. On the basis of this negative evidence it has to be assumed that reaction proceeds via a non-relaxed exciplex. 

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. 

A Pulsed-Radiolysis Study of the Gas-Phase Reaction of Oxygen Atoms with Benzene and Related Compounds Rate Constants and Transient Spectra... 

The paper about benzene is part 1 of a series with the translated title Quantum chemical contributions to the benzene problem. I. The electron configuration of benzene and related compounds. Three other papers followed. The second paper [4] has the same series title and the translated subtitle IT. Quantum theory of induced polarizabilities. The study is an attempt to correlate chemical behaviour of substituted benzenes with the charge distribution in the ring which becomes disturbed by the substituents. The third paper [5] has a slightly different series title with no further subtitle Quantum theoretical contributions to the problem of aromatic and unsaturated compounds. ITT. The final paper in the series [6] has the subtitle Free radicals in organic chemistry ... 

As described earlier, the S Ar involves the reaction of an electrophilic species with an arene nucleophile. There are several types of arenes common to the S Ar reactions substituted benzenes, polycyclic aromatic compounds, and heterocyclic compounds. Substituent effects largely control the chemistry of substituted benzenes and related compounds. This includes both activating and directing effects of substituents on the S Ar reaction. 

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). 

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