Molecule Kekule structures

The two Kekule structures for benzene have the same arrangement of atoms but differ m the placement of electrons Thus they are resonance forms and neither one by Itself correctly describes the bonding m the actual molecule As a hybrid of the two Kekule structures benzene is often represented by a hexagon containing an inscribed circle... 

A single Kekule structure does not completely descnbe the actual bonding in the molecule Ketal (Section 17 8) An acetal denved from a ketone Keto-enol tautomerism (Section 18 4) Process by which an aldehyde or a ketone and its enol equilibrate... 

A single Kekule structure does not completely describe the actual bonding in the molecule. 

Kekule structure (Section 1.4) A method of representing molecules in wrhich a line between atoms indicates a bond. 

Resonance hybrid (Section 2.4) A molecule, such as benzene, that can t be represented adequately by a single Kekule structure but must instead be considered as an average of two or more resonance structures. The resonance structures themselves differ only in the positions of their electrons, not their nuclei. 

We can use the concept of resonance to explain these characteristics of the benzene molecule. There are two Kekule structures with exactly the same energy they differ only in the positions of the double bonds. As a result of resonance... 

The convenience and usefulness of the concept of resonance in the discussion of chemical problems are so great as to make the disadvantage of the element of arbitrariness of little significance. Also, it must not be forgotten that the element of arbitrariness occurs in essentially the same way in the simple structure theory of organic chemistry as in the theory of resonance — there is the same use of idealized, hypothetical structural elements. In the resonance discussion of the benzene molecule the two Kekule structures have to be described as hypothetical it is not possible to synthesize molecules with one or the other of the two Kekule structures. In the same way, however, the concept of the carbon-carbon single bond is an idealization. The benzene molecule has its own structure, which cannot be exactly composed of structural elements from other molecules. The propane molecule also has its own structure, which cannot be composed of structural elements from other molecules — it is not possible to isolate a portion of the propane molecule, involving parts of two carbon atoms... 

The conclusions we draw regarding the structure of the normal benzene molecule are the following. The principal contributions to the structure are made by the two Kekule structures, resonance between them stabilizing the molecule... 

So each bond has a bond order of 1.5, which is consistent with the observed bond length. These two resonance structures are often called Kekule structures because they were first proposed in 1865 by Kekule, who imagined that the molecule converted very rapidly from one form to the other. This, however, is not the case the molecule never has either of the Kekule structures but only a single structure, which is intermediate between these two hypothetical structures and is approximately represented as follows ... 

Ozonolysis was once used to locate the position of a double bond (or bonds) in unsaturated compounds of unknown structure—largely because of the ease of characterisation of the carbonyl products— but has now been superseded by physical methods, e.g. n.m.r. spectroscopy, which are easier and quicker. Benzene forms a triozonide which decomposes to yield three molecules of glyoxal, OHC—CHO the sole reaction of benzene that suggests it may contain three real double bonds in a Kekule structure Alkynes also undergo ozonolysis, but at a much slower rate than alkenes. 

Hydrocarbons containing one or more triple bonds in addition to double bonds have been excluded from the tile, as have been radicals (e.g. the allyl radical C3H5 ) and aromatic molecules, i.e. molecules for which more than one unexcited resonance structure (Kekule structure) can be written. Consequently, hydrocarbons such as phenyl-substituted polyenes, or annulenes — bridged or unbridged—have not been included. 

A rather special case are unsaturated, cyclic hydrocarbons undergoing second-order double-bond localization14, e.g. cyclobutadiene or pentalene. Although equivalent pairs of Kekule structures can be written for these molecules, they assume a structure with alternant single and double bonds, corresponding to only one of these structures. These molecules will be dealt with later, in a separate section. 

Owing to the 7tCc-ocrc CT interactions, the usual equivalence between the two Kekule structures of benzene is broken and the ring distorts strongly to D3h symmetry, with pronounced alternation (by 0.02 A) of C=C bond lengths. Complexes of this type are evidently closely related to the metallocene sandwich compounds discussed elsewhere in this book (Section 4.9.5), with the benzene molecule described as a tridentate ligand in the language of metal coordination chemistry. 

Fig. 28. Localized 7T-MO s in catacondensed molecules for which the LMO s correspond to Kekul structures,... 

Molecular orbital calculations snggest that the jr electrons in naphthalene are delocalized over the two rings and this results in substantial stabilization. These molecules are planar, and all p orbitals are suitably aligned for overlap to form n bonding molecular orbitals. Although we can draw Kekule structures for these compounds, it is strictly incorrect to use the circle in hexagon notation since the circle represents six jr electrons. Naphthalene has 10 carbons, and therefore 10 jr electrons, and anthracene has 14 jr electrons. The circle notation suggests 12 or... 

In principle, 23 regioisomers of the dihydro fullerene CggH2 are possible. The formal addition of an A-B molecule, for example H2, to the externally Cgg sphere could proceed in three ways [30] (1) Addition to one double bond of the low-energy Kekule structure (Figure 5.3), which would leave all the other bonds unchanged ([6,6] double bonds and [5,6] single bonds) (2) conjugate addition of two atoms, which requires... 

If we consider the Kekule structure of benzene, it is evident that the two proposed structures differ only in the positions of the electrons. Therefore, instead of being two separate molecules in equilibrium, they are indeed two resonance contributors to a picture of the real molecule of benzene. 

There are many organic compounds stabilized by resonance. As a first example we may cite benzene, in which there is resonance between the two Kekule structures. In this case, the resonance energy could be calculated to give good agreement with the experimental data on the heat of formation of the molecule. Perhaps even more remarkable is the compound KC5H5, which is formed when potassium reacts with ryr/opentadiene. 

Not all even AHs can be represented by Kekule structures. The w-quino-dimethane 471 can only be represented by diradical structures, and molecules... 

It is often asked whether or not the constituent structures of a resonating system, such as the KekulS structures for the benzene molecule ... 

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