Kekule symbol

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. 

The circle m a hexagon symbol was first suggested by the British chemist Sir Robert Robinson to represent what he called the aromatic sextet —the six delocalized TT electrons of the three double bonds Robinson s symbol is a convenient time saving shorthand device but Kekule type formulas are better for counting and keeping track of electrons especially m chemical reactions... 

Write structural formulas for toluene (CeHsCHj) and for benzoic acid (C6H5CO2H) (a) as resonance hybrids of two Kekule forms and (b) with the Robinson symbol... 

Mahdihassan, S. Kekule s dream of the Ouroboros and the significance of this symbol. Scientia 55 (1961) 187-195. 

Mahdihassan, S. The probable origin of Kekule s symbol of the benzene ring. Scientia 54 (1960) 1-6. 

Figure 11.16 The Kekule structures of benzene (C6H6) (upper structures), showing the two possible arrangements of the double bonds around the ring. Below is the aromatic resonance structure with the rotation of the double bonds symbolized by a ring, and the hydrogen atoms assumed to be present at each of the six corners .

Practitioners of quantum chemistry employed both the visual imagery of nineteenth-century theoretical chemists like Kekule and Crum Brown and the abstract symbolism of twentieth-century mathematical physicists like Dirac and Schrodinger. Pauling s Nature of the Chemical Bond abounded in pictures of hexagons, tetrahedrons, spheres, and dumbbells. Mulliken s 1948 memoir on the theory of molecular orbitals included a list of 120 entries for symbols and words having exact definitions and usages in the new mathematical language of quantum chemistry. 

Kekule structure functions by the symbols and Kf , the wave function at the saddle point is... 

Kekule forms of benzoic acid Robinson symbol... 

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. 

In the nineteenth century we have seen several trends of graphical representation in chemistry. The first was the circle, followed by circle with the letter symbol inside it next was the letter alone. The letters were arranged in linear fashion. The lines were either single and all-inclusive or stacked one upon the other. At times the letter symbols were joined by brackets—other times dashes, and other times arcs. From time to time various systems were proposed which were synthesized from several previous ideas. Kekule proposed elimination of all letters and circles and the use of only lines and arcs. Others tried to develop working models of the compounds. As each of these proposals fell short of expectations, chemists tried new modifications of established precepts. 

The papers that follow aided in the construction of a remarkable symbolic language, permitting not only the association of a different symbol (a structural formula) with almost every distinct chemical substance, but also, from the details of the formula, a prediction as to the likely chemical and physical behavior of the substance. The formulas of Kekule, Gouper, van t Hoff and le Bel worked they turned out to be eminently useful. Why they worked, what held atoms together and oriented valence bonds in space, remained a mystery whose unravelling took more than fifty years. The developments of that later period are surveyed in the Epilogue. 

Two symbols are commonly used to represent the benzene ring. The representation in Figure 12.6b is the structure proposed by Kekule. The structure in Figure 12.6d represents the tt clouds. 

K (symbol for equihbrium constant) relation to AG°, 106—107 Karplus, Martin, 544 Kazan, University of, 3 Kekule, August, 3, 399-402 Kendrew, John C., 1087 a-Keratin, 1085 Ketals. See Acetals Ketene, 783... 

Could the ouroboros also be a symbol for the law of the conservation of matter, far predating Lavoisier Perhaps only Kekule would have known for sure. 

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.)... 

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