Symbols for Benzene

Two symbols are used to represent benzene. One is the Kekule structure, and the other is a hexagon with an inscribed circle, to represent the idea of a delocalized pi electron cloud. 

Regardless of which symbol is used, the hydrogens are usually not written explicitly, but we must remember that one hydrogen atom is attached to the carbon at each corner of the hexagon. 

The symbol with the inscribed circle emphasizes the fact that the electrons are distributed evenly around the ring, and in this sense, it is perhaps the more accurate of the two. The Kekule symbol, however, reminds us very clearly that there are six pi electrons in benzene. For this reason, it is particularly useful in allowing us to keep track of the valence electrons during chemical reactions of benzene. In this book, we will use the Kekule symbol. However, we must keep in mind that the double bonds are not fixed in the positions shown, nor are they really double bonds at all. 

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Despite the completely different approach to chemical interaction, which has been followed here, the conventional standard symbols which are used to define the connectivity in covalent molecules, can also be applied, without modification, to distinguish between interactions of different order. However, each linkage pictured by formulae such as H3C-CH3, H2C=CH2, HC=CH, represents the sharing of a single pair of electrons with location unspecified. The number of connecting fines only counts bond order and may be established from the classical valence rules, e.g. v(C,N,0,F)=(4,3,2,l). Special symbols are used for non-integral bond orders, as in the symbol for benzene ... 

In aromatic compounds there is a nucleus Cg which is present in benzene, formed by piling (Schichtung) as in V, but since it was not then possible to reach a definitive conclusion it was regarded as a 6-valent (sechsstelliges) element and represented by a circle VI, benzene VII being in the phenyl series what marsh gas CH4 is in the methyl series. Loschmidt gave the formula O3 for ozone, recognised 5-valent N in ammonium compounds (he sometimes has 7-valent N) and 6-valent S in sulphuric acid. His symbol for benzene V may C—C—C-... 

We do not need to think in terms of an oscillation between two structures (Kekule) or of a resonance hybrid for benzene. The tt bonds are not localized between specific carbon atoms but are spread out around the six-membered ring. To represent this delocalized tt bonding, the symbol for benzene is often written as a hexagon with an inscribed circle (Fig. 11-30c). 

Repeat the proeedure using HMO. HMO requires entry of the entire lower semimatrix, ineluding the diagonal and all zero elements. Beeause the matrix element format is II, only one symbol ean be entered for eaeh element. The numbers 0.5 and 1.2 eannot be entered in this format instead enter 1, whieh will be modified later. The initial unmodified input for pyridine is the same as that for benzene, 010010001000010100010 henee, we ean make a trial run on benzene to see if everything is working properly. 

Figure 7. Experimental data (symbols) for TNB s viscosity [78] superimposed on the results of the fitting procedure (line) from Lubchenko and Wolynes [47] are shown. Ta is diown by a tickmark. (TNB = trinaphthyl benzene). The temperature Ter signifies a crossover from activated to collisional viscosity, dominant at the lower and higher temperatures, respectively (see text). The temperature is varied between the boiling point and the glass transition. The right-hand side panel depicts the temperature dependence of the length scales of cooperative motions in the liquid. The thick solid and dashed lines are the critical radius and the cooperativity length respectively. Taken from Ref. [47] with permission.

FIGURE 8.14 Critical sooting equivalence ratio l c at 2200K as a function of the number C—C bonds in hydrocarbon fuels. +, 0, and - indicate ethane/l-octane mixtures in molar ratios of 5 to 1, 2 to 1 and 1 to 2, respectively x, acetylene/benzene at a molar ratio of 1 to 3. The O symbol for 2 to 1, falls on top of the butene symbol. 

Figure 5. Rate-energy curves for benzene ion dissociation. Circles are TRPD points (corrected for IR-radiative relaxation) from Ref. 24 squares are REMPI points from Ref. 10. The two points with ( ) symbols are TRPD points uncorrected for radiative relaxation. The curves are from variational RRKM, with Eq = 3.88 eV, and simple RRKM, with Eo = 3.81 eV.

For the heterohelicenes, where isomers are possible, a new way for abbreviation is introduced by giving after the number in brackets the correct sequence of benzene and hetero rings, using symbols B = benzene, S = thiophene, NH = pyrrole, N = pyridine (the position of the hetero atom is indicated by the position number), O = furan etc. 

Dithiolenes are best considered to be a resonance hybrid of the limiting structures (1)—(3). In both bis- and tris-dithiolenes the electron delocalization is not limited to the ligand, but includes the metals to give rise to cyclic delocalization ( aromaticity ). To symbolize this electron delocalization in dithiolenes, they can be represented, in a manner similar to that used for benzene, by formulas containing a ring inside the framework given by the metal, sulfur and carbon atoms. We will use this notation, shown in (4), throughout this chapter. 

Although the canonical forms for benzene are imaginary and do not exist, the structure of benzene will be represented by one of the Kekule structures throughout this book. This is common practice. A circle within a hexagon as in 10, symbolic of the 7C-cloud, is sometimes used to represent benzene. 

Fig. 17. Concentration dependence of the self-diffusion coefficient Da for benzene in two ZSM-5 samples. Filled symbols, Si/Al = 135 crosses, Si/Al > 1000 (9).

Figure 1.17. Symbol for phenyl, the benzene molecule with one hydrogen removed.

Benzene is typically thought of as a combination of two equivalent resonance structures. These could be written as the SMILES C1=C-C=C-C=C1 and C1-C=C-C=C-C=1. In order to have just one representation for benzene and other aromatic systems, SMILES handles these aromatic systems specially, treating the atoms in an aromatic ring as a special aromatic type and the bonds as a special aromatic type. The lowercase symbol is used to denote an aromatic atom in SMILES and SMARTS. The SMILES for benzene then becomes clcccccl. A bond between aromatic atoms is an aromatic bond, unless otherwise spelled out. For example, biphenyl can be written as clcccccl-clcccccl. 

FIGURE 55. The ouroboros—a symbol for completeness, cycle of life, and even the conservation of matter. The ouroboros continuously devours itself as it regenerates. Did August Kekule actually dream about the ouroboros when he postulated that benzene was a cyclic compound (From Atalanta Fugiens, from The Roy G. Neville Historical Chemical Library, a collection in the Othmer Library, CHF.)... 

Table 1 Double-ionizations of the benzene molecule to singlet dication states, predicted in the standard enhanced ADC(2) approximation [5] and in the diagonal approximation described in the text. In this and subsequent tables, Term indicates the term symbol for a transition. Character the sum of the squares of the normalised transition eigenvector coefficients associated with the dominant basis configuration, and AE the small symmetry-breaking energy splitting in degenerate irreducible representations introduced by the diagonal approximation (see text)...

Fig. 9 Effect of sample quantity and nature of purge gas on ZLC response curves for benzene in 50- jim crystals of NaX zeolite at 250 °C. a Desorption curves. Note that when the sample is sufficiently small, desorption is rapid and the curves for He and N2 purge coincide, but for a larger sample we see slower desorption with a significant difference between the curves for He and N2, indicating the intrusion of external diffusional resistance. b Apparent diffusional time constants showing the variation with sample mass. Filled symbols denote He purge, open symbols denote N2 purge. Note that when the sample mass is sufficiently small, the time constants for He and N2 become coincident and independent of sample mass, showing the absence of external diffusional resistance. From Brandani et al. [55]...

Figure 9.8 Log k vs. log Pow (a) and k vs. log (b) relationships predicted by eqs. 9.23 and 9.24 (solid lines), and experimental values (symbols) for a series of monosubstituted benzenes acetanilide, acetophenone, benzaldehyde, benzene, benzonitrile, benzyl alcohol, benzylamine, bromobenzene, butyrophenone, he phenone, methyl benzoate, methyl phenyl ether, nitrobenzene, propiophenone, toluene, and valerophenone. Molar concentrations of SDS in mobile phase (1,a) 0, (2, ) 0.016, (3,0) 0.05, (4,°) 0.1, and (5) 0.15. Reprinted from Ref 21 with permission of Elsevier.

Kekules model for the structure of benzene is nearly, but not entirely, correct. Kekules two structures for benzene differ only in the arrangement of the electrons all of the atoms occupy the same positions in both structures. This is precisely the requirement for resonance (review Sec. 1.12). Kekule s formulas represent two identical contributing structures to a single resonance hybrid structure of benzene. Instead of writing an equilibrium symbol between them, as Kekul did, we now write the double-headed arrow (<->) used to indicate a resonance hybrid ... 

A final note about nomenclature is in order for benzene. Drawing a benzene ring occupies a lot of space, so a shorthand representation is used in many structures. In 96, the shorthand symbol Ph is used to represent a phenyl substituent. In the older chemical literature, the Greek symbol ( ) (phi) was sometimes used. The Ph representation will be used often in this book, and compound 96 is named 5-chloro-2,6-diphenyloct-2-ene. 

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