Kekule and the Structure of Benzene

The classification of hydrocarbons as aliphatic or aromatic took place in the 1860s when it was already apparent that there was something special about benzene, toluene, and their derivatives. Their molecular formulas (benzene is CgHg, toluene is CyHg) indicate that, like alkenes and alkynes, they are unsaturated and should undergo addition reactions. Under conditions in which bromine, for example, reacts rapidly with alkenes and 

In 1861, Johann Josef Loschmidt, who was later to become a professor at the University of Vienna, privately published a book containing a structural formula for benzene similar to that which Kekule would propose five years later. Loschmidt s book reached few readers, and his ideas were not well known. 

Furthermore, only one monobromination product of benzene was ever obtained, which suggests that all the hydrogen atoms of benzene are equivalent. Substitution of one hydrogen by bromine gives the same product as substitution of any of the other hydrogens. 

Chemists came to regard the six carbon atoms of benzene as a fundamental structural unit. Reactions could be carried out that altered its substituents, but the integrity of the benzene unit remained undistmbed. There must be something special about benzene that makes it inert to many of the reagents that add to alkenes and alkynes. 

In 1866, only a few years after publishing his ideas concerning what we now recognize as the structmal theory of organic chemistry, August Kekul6 applied it to the structure of benzene. He based his reasoning on three premises  

Faraday is better known in chemistry for his laws of electrolysis and in physics for proposing the relationship between electric and magnetic fields and for demonstrating the principle of electromagnetic induction. 

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Regarding the carbon-hydrogen compounds, Kekule [16] published the theory of the connectivity of C atoms with four bonding units each and the structure of benzene molecules One can now assume that several carbon... 

On March 11, 1890, on the occasion of the twenty-fifth anniversary of his announcement of his benzene theory, Kekule gave a speech in Berlin in which he revealed that both his structural theories and the structure of benzene were revealed to him in dreams. Scholars have tended to dismiss his account of his own creative processes and have placed more stock in the early training that Kekule received in architecture as the key to his inspiration. SEE ALSO Berzelius, Jons Jakob Liebig, Justus von Organic Chemistry Pauling, Linus. 

Another classic example of resonance is the benzene molecule. The localized resonance forms are termed Kekule forms (after Friedrich August Kekule, who first deduced the structure of benzene) and have alternating single and double bonds between carbon atoms. The actual benzene molecule is a resonance hybrid of the contributing resonance forms as the bond lengths are equal (single and double bonds have different lengths). 

The wavefunction in Eq. (49) could be the wavefunction for a simple molecule, allowing for three bond dissociating, or it could be one of the structures of benzene. The wavefunction for benzene, containing two Kekule structures for the 7i-system, is pictorially given in Fig. 3 and in formula by ... 

The Mills-Nixon hypothesis had, as its foundation, certain differences in the chemical behaviour of indan (3) and tetralin (4) from which a localization of the aromatic 7r-bonds was predicted to occur in the direction depicted by la rather lb. The original experimental evidence upon which the effect was based was shown to be erroneous, but calculations at various levels of theory indicated that aromatic bond localization should exist and become more pronounced as the size of the annelated ring decreases In essence one can recognize that the structure of benzene has a symmetry such that both Kekule structures must contribute equally. With Q tetralin (4) (and the lower homologues) no such symmetry requirement exists and ring annelation could induce bond length alternation within the arene nucleus. As the strain imposed by the fused ring increases, the Mills-Nixon effect should increase. The hypothesis has been the subject of considerable discussion and the controversy is far from settled. 

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. 

The development of stereochemical ideas entered a new stage in 1858 when August Kekule (Fig 6) introduced the idea of the valence bond and the pictorial representation of molecules as atoms connected by valence bonds. His main thesis was that the carbon atom is tetravalent, and that a carbon atom can form valence bonds with other carbon atoms to form open chains and that sometimes the carbon chains can be dosed to form rings (9). This led directly to his proposal for the structure of benzene. On the occasion of celebrations held in his honor, Kekule in 1890 delivered a speech before the German Chemical Sodety describing the origin of his idea of the liiUung of atoms (9). 

The years between 1860 and 1880 were characterized by a strong dispute about the concept of -> molecular structure, arising from the studies on substances showing optical isomerism and the studies of Kekule (1861-1867) on the structure of benzene. The concept of the molecule as a three-dimensional body was first proposed by Butlerov (1861-1865), Wislicenus (1869-1873), Van t Hoff (1874-1875) and Le Bel (1874). The publication in French of the revised edition of La chimie dans I espace by Van t Hoff in 1875 is considered a milestone in the three-dimensional conception of the chemical structures. 

In the course of the study of the structure of benzene, several structural formulas have been proposed for it. Well known structures are Kekule s cyclohexatriene, Ladenburg s prismane, Thiele s partial bond structure, and Dewar s bicyclohexadiene. Later, Huckel proposed benzvalene as another structure of benzene. Since Kekule s structure was gradually accepted as the structure of benzene, other structures are only of historical importance. 

The debate over the structure and bonding in benzene raged for at least 30 years. In 1865, Friedrich Kekule (1829-1896) suggested that the structure of benzene was intermediate betw een two structures [part (b) of the preceding structures] that we now call resonance structures. We often represent benzene as... 

In 1859 Kekule started to use graphical representations of organic molecules, in part to emphasize the tetravalent nature of carbon atoms and their ability to form chains. He then turned his attention to the structure of benzene (CsHg), a compound with unusual properties that could not be explained by any theories of the day. 

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