Resonance hybrid of benzene

The method that commonly is used is to draw a set of structures, each of which represents a reasonable way in which the electrons (usually in p orbitals) could be paired. If more than one such structure can be written, the actual molecule, ion, or radical will have properties corresponding to some hybrid of these structures. A double-headed arrow <—> is written between the structures that we consider to contribute to the hybrid. For example, the two Kekule forms are two possible electron-pairing schemes or valence-bond structures that could contribute to the resonance hybrid of benzene ... 

The resonance hybrid of benzene explains why all C-C bond lengths are the same. Each C-C bond is single in one resonance structure and double in the other, so the actual bond length (1.39 A) is intermediate between a carbon-carbon single bond (1.53 A) and a carbon-carbon double bond (1.34 A). 

The stability of the Wheland intermediate will obviously have a large effect upon the overall rate of this reaction pathway. First, write down the two principal canonical forms of the benzene ring, C6H6, and then write down the resonance hybrid of benzene. 

Dewar structure A representation of the structure of BENZENE in which there is a single bond between two opposite corners of the hexagonal ring and two double bonds at the sides of the ring. The Dewar structures contribute to the resonance hybrid of benzene. 

Kekule structure /kay-koo-lay/ A structure of BENZENE in which there is a hexagonal ring with alternate double and single bonds. Two possible Kekule structures contribute to the RESONANCE hybrid of benzene. The structure is named for the German chemist Friedrich Augus Kekule von Stradonitz (1829-96). 

Dewar structure A proposed structure of benzene, having a hexagonal ring of six carbon atoms with two opposite atoms joined by a long single bond across the ring and with two double C-C bonds, one on each side of the hexagon. Dewar structures contribute to the resonance hybrid of benzene. 

Kekule reasoned that the rapid oscillation of single and double bonds around the ring makes all six carbon atoms, and therefore all six hydrogen atoms, equivalent. Because the two resonance structures are identical, they contribute equally to the resonance hybrid of benzene. The bonds do not rapidly oscillate around the ring. 

In valence bond terms the pyrazine ring may be represented as a resonance hybrid of a number of canonical structures (e.g. 1-4), with charge separated structures such as (3) contributing significantly, as evidenced by the polar character of the C=N bond in a number of reactions. The fusion of one or two benzene rings in quinoxaline (5) and phenazine (6) clearly increases the number of resonance structures which are available to these systems. 

Rule 1 Individual resonance forms are imaginary, not real. The real structure is a composite, or resonance hybrid, of the different forms. Species such as the acetate ion and benzene are no different from any other. They have single, unchanging structures, and they do not switch back and forth between resonance forms. The only difference between these and other substances is in the way they must be represented on paper. 

Some substances, such as acetate ion and benzene, can t be represented by a single line-bond structure and must be considered as a resonance hybrid of two or more structures, neither of which is correct by itself. The only difference between two resonance forms is in the location of their tt and nonbonding electrons. The nuclei remain in the same places in both structures, and the hybridization of the atoms remains the same. 

Benzene is described by valence-bond theory as a resonance hybrid of two equivalent structures. 

Resonance can also occur with many organic molecules, including benzene, QHe, which is known to have a hexagonal ring structure. Benzene can be considered a resonance hybrid of the two forms... 

As pointed out in Chapter 7, the atomic orbital (valence bond) model regards benzene as a resonance hybrid of the two structures... 

Canonical forms of benzene that are calculated to contribute about 22% to the resonance stabilization of benzene. Such resonance structures have no separate physical reality or independent existence. For the case of benzene, the two Kekule structures with alternating double bonds i.e., cyclohexatriene structures) contribute equally and predominantly to the resonance hybrid structure. A dotted circle is often used to indicate the resonance-stabilized bonding of benzene. Nonetheless, the most frequently appearing structures of benzene are the two Kekule structures. See Kekule Structures... 

Dithiolenes are best considered to be a resonance hybrid of the limiting structures (1)—(3). In both bis- and tris-dithiolenes the electron delocalization is not limited to the ligand, but includes the metals to give rise to cyclic delocalization ( aromaticity ). To symbolize this electron delocalization in dithiolenes, they can be represented, in a manner similar to that used for benzene, by formulas containing a ring inside the framework given by the metal, sulfur and carbon atoms. We will use this notation, shown in (4), throughout this chapter. 

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). 

Sometimes more than one satisfactory Lewis structure can be written and there is no reason to select one over another. In such cases a single structural formula is inadequate for a correct representation, and we say that the true structure is a resonance hybrid of the several structures. Common examples of species requiring resonance structures are ozone, 03, carbonate ion, CO " and benzene, C6H6. These... 

Benzene is the parent of the family of aromatic hydrocarbons. Its six carbons lie in a plane at the corners of a regular hexagon and each carbon has one hydrogen attached. Benzene is a resonance hybrid of two contributing Kekule structures ... 

The actual structure of benzene is resonance hybrid of the above two structures and may be represented as under ... 

Benzene is actually a resonance hybrid of the two Kekule structures. This representation implies that the pi electrons are delocalized, with a bond order of 1 between adjacent carbon atoms. The carbon-carbon bond lengths in benzene are shorter than typical single-bond lengths, yet longer than typical double-bond lengths. 

Visualizing benzene as a resonance hybrid of two Kekule structures cannot fully explain the unusual stability of the aromatic ring. As we have seen with other conjugated systems, molecular orbital theory provides the key to understanding aromaticity and predicting which compounds will have the stability of an aromatic system. 

According to the relationship between three-membered rings and 7r-complexes, cyclopropane can be considered as a resonance hybrid of three equivalent methylene-ethene n-complexes Of course, such Ti-complexes do not exist but this is also true in the case of the two cyclohexatriene resonance structures normally used to present benzene. Spin-coupled valence bond calculations of Karadakov and coworkers reveal that there is a small but significant contribution of 3.7% to the electronic structure of 1 resulting from 7r-com-plex structures (see Section V. E). This indicates that the Ti-complex description of is not totally unreasonable and, although seldom used, helps to unravel some of the peculiarities of bonding in 1 ... 

X-ray crystallographic analysis indicated that benzene is a planar, regular hexagon in which all the carbon-carbon bond lengths are 139 pm, intermediate between the single C-C bond in ethane (154 pm) and the C=C bond in ethene (134 pm), and therefore all have some double bond character. Thus the representation of benzene by one Kekule structure is unsatisfactory. The picture of benzene according to valence bond theory is a resonance hybrid of the two Kekule or canonical forms 4 and 9, conventionally shown as in Figure 1.2, and so each carbon-carbon bond apparently has a bond order of 1.5. 

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