Kekule benzene formula

Such oscillation of the double bonds of the benzene ring is one of the assumptions in connection with the Kekule benzene formula as referred to when we were discussing the constitution of benzene (p. 474). 

The basic pattern common to all aromatics is the benzene ring as illustrated in Kekule s formula ... 

Structure. The representation of the benzene molecule has evolved from the Kekule ring formula (1) to the more electronically accurate (2), which iadicates all carbon—carbon bonds are identical. 

Nor can there be any question of real tautomerism in the case of phenol. In its chemical properties phenol resembles the aliphatic enols in all respects. We need only recall the agreement in the acid character, the production of colour with ferric chloride, and the reactions with halogens, nitrous acid, and aromatic diazo-compounds (coupling), caused by the activity of the double bond and proceeding in the same way in phenols and aliphatic enols. The enol nature of phenol provides valuable support for the conception of the constitution of benzene as expressed in the Kekule-Thiele formula, since it is an expression of the tendency of the ring to maintain the aromatic state of lowest energy. In this connexion the hypothetical keto-form of phenol (A)—not yet obtained—would be of interest in comparison with... 

Kekule 1865 Benzene formula, aromaticity as structural feature... 

Compare Loschmidt s representation of benzene (left) in 1861,4 years before Kekule s formula, and the modem representation (right)... 

In the French version Arm, Chim. (3), 53, 489 (1858)) Coupcr adds the following cyclic formula for cyanuric acid, seven years before Kekule proposed the cyclic benzene formula ... 

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. 

The simple circle-inscribed hexagon on the right has become a popular alternative to the classical Kekule structure and is probably the benzene formula most widely used. 

The only serious challenge to Kekule s formula came from the prism formula (II) [14], but this was soon shown to be incompatible with the known facts concerning the isomerism of substituted benzene derivatives. 

Kekule s benzene formula was published in 1865 (see p. 555) and alternative formulae soon began to appear. Dewar and Wislicenus proposed a diagonal formula I. Claus proposed other diagonal formulae II and III, in which each carbon atom is linked by single bonds with three others. He preferred formula II ... 

Ladenburg criticised Kekule s formula on the ground that two different ortho-compounds should be obtainable. He showed that only three disubstitution products and only one pentachlorobenzene exist. In an elaborate experimental investigation he proved that all six hydrogen atoms in benzene are equivalent. In his first paper in 1869 he proved that all the carbon atoms are joined by single linkages, giving three formulae (see p. 803). 

Ladenburg admitted that Baeyer s experiments provided evidence against the prism formula, but still thought that it could apply in some cases. Baeyer later used Kekule s formula for phloroglucinol, saying that he was convinced that the behaviour of benzene in different compounds corresponds sometimes... 

The preparation of a triozonide of benzene by Harries supported Kekule s formula with three double bonds. It is only recently that what is believed to be an acceptable representation of the structure of benzene has been achieved it involves the formulae which are discussed in this place (see p. 966). 

Kekule extended the static bond concept by the oscillation hypothesis, according to which the two benzene formulae constantly interchange with each other. 

Kekule < " cyclohexatnene benzene formula struetural basis of aromaticity ... 

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 ... 

The way how August Kekule discovered the structural formula of benzene is one of the legends of the history of chemistry [1, 2]. In 1865/6, Kekule proposed for benzene the hexagonal formula 1 shown in Fig. 11.1, according to which the carbon atoms would be three-valent (or, in more modem terminology three-coordinate). In order to make the carbon atoms four-valent, in 1872 Kekule inserted three double bonds into his benzene formula, which can be done in two distinct ways— formulas 2 and 3 shown in Fig. 11.1. Nowadays, these are referred to as the Kekule structures or the Kekule structural formulas of benzene. 

F. A. Kekule Kekule s Symbols Blomstrand s Symbols Wuitz s Formulae Loschmidt s Formulae Graphic Formulae Kekule s Benzene Formula The Benzene Hexagon Affinity Diagram (Bergman)... 

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