Benzene Kekule formulation

Notice that anthracene cannot be represented by any single Lewis structure m which all three rings correspond to Kekule formulations of benzene but phenanthrene can... 

In general, the most stable resonance structure for a polycyclic aromatic hydrocarbon is the one with the greatest number of rings that correspond to Kekule formulations of benzene. Naphthalene provides a fairly typical exanple ... 

Kekule structure (Section 11.2) Structural formula for an aromatic compound that satisfies the customary rules of bonding and is usually characterized by a pattern of alternating single and double bonds. There are two Kekule formulations for benzene ... 

The knowledge that the bond lengths are equal in the ring in benzene is a point against the Kekule formulation, but a more convincing argument is available from a comparison of the chemistry of benzene with that of 1,3,5-hexatriene, 7 ... 

Nonetheless, the Kekule formulation of benzenes structure was an important step forward and, for very practical reasons, it is still used today. Now we understand its meaning differently, however. 

But there were problems with the Kekule formulation as well as with the four shown in Figure 13.4. Benzene didn t react with halogens or hydrogen halides like any self-respecting polyene, for example. Hydrogenation was much slower than for... 

Alan Rocke claims that the advocacy of the method of hypothesis, which had been familiar to physicists for a generation, began to replace inductivist rhetoric in chemical circles just about the time that Kekule was formulating his benzene theory. But the method of hypothesis was a familiar one to chemical leaders like Berzelius and Dumas, and Dalton hardly avoids the fact of his hypothetical reasoning by his, indeed, inductivist rhetoric.51... 

These three structures, the most stable valence-bond structures that can be formulated for naphthalene, are seen to have about the same energy and to correspond to about the same molecular configuration. It is to be expected then that they will be combined to represent the normal state of the naphthalene molecule, to which they should contribute about equally. Resonance among these three stable structures should stabilize the molecule to a greater extent than does the Kekul resonance in benzene, involving two equivalent structures it is seen from Table 6-2 that the resonance energy of naphthalene, 75 kcal/mole, is indeed much greater than that of benzene. 

With the advent of the computer era, it is now possible to reexamine and rethink the resonance theory at the ab initio level. For example, throughout Pauling and Wheland s books, benzene is supposed to be a hybrid of two Kekule structures, by noting that Dewar and other ionic structures make little contribution to the resonance in benzene. However, classical ab initio VB calculations with all possible 175 resonance structures by Norbeck et al. [51] and Tantardini et al. [3], where strictly atomic orbitals are used to construct VB functions, manifested that the five covalent Kekule and Dewar structures make even less contribution to the ground state of benzene than the other 170 ionic structures. This prompts us to reconsider the mathematical formulations for resonance structures [52]. 

The most basic notion of organic chemistry is probably the quadri-valency of carbon, which was very clearly formulated by K cule in 1858 3>. Olefinic compounds like ethylene suggested that the carbon atom could exhibit the valence three, but these molecules were finally formulated with a double bond, according to Erlenmeyer s proposition 4>. Kekule s benzene formula 5> completed this classic period of valence theory. About 1875, Le Bel 6> and Van t Hoff 7> introduced the theory of steric valency, where the double bonds between carbon atoms were looked at from a new point of view Van t Hoff proposed his famous model, where the tetra-hedra of doubly-bonded carbon atoms were supposed to have an edge in common and those of triply-bonded carbon atoms a face in common. This picture was quite satisfactory for isolated double bonds, but the peculiar properties of conjugated and aromatic systems could be understood only by imagining that different double bonds in a molecule can interact in a way not possible for single bonds. 

This conjectural scenario would relieve Kekule from the suspicion of being disingenuous in his 1865-66 statements about the early origin of his theory. It is also consistent with the hints about aromatics that one finds in his 1858 paper, as well as with his 1859 and 1860 suggestions of two parallel isomeric series. However, accepting it presents us with a new conflict, namely the "dream story" he told in 1890. How could he have had the first idea of the benzene theory in a (day)dream ca. 1862 if he had already formulated the theory in 1858 But 1 believe that it is entirely possible to continue to credit the sincerity of Kekule s reminiscence, allowing a formative role for this personal experience even when that experience did not represent the first formulation of his aromatic theory. 

In a series of papers from 1865 to 1872 [9,11] Kekule proposed and developed the idea of a cyclohexatriene type of structure for benzene [12]. This initial formulation of the structure of benzene assumed fixation of single and double bonds but was soon modified [11]. Kekule is often misrepresented as describing benzene as... 

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