Benzene resonance hybrid

Know the meaning of Kekule structure, benzene resonance hybrid, resonance or stabilization energy. 

The accepted configuration of naphthalene, ie, two fused benzene rings sharing two common carbon atoms in the ortho position, was estabUshed in 1869 and was based on its oxidation product, phthaUc acid (1). Based on its fused-ring configuration, naphthalene is the first member in a class of aromatic compounds with condensed nuclei. Naphthalene is a resonance hybrid ... 

Pyrrole has a planar, pentagonal (C2 ) stmcture and is aromatic in that it has a sextet of electrons. It is isoelectronic with the cyclopentadienyl anion. The TT-electrons are delocalized throughout the ring system, thus pyrrole is best characterized as a resonance hybrid, with contributing stmctures (1 5). These stmctures explain its lack of basicity (which is less than that of pyridine), its unexpectedly high acidity, and its pronounced aromatic character. The resonance energy which has been estimated at about 100 kj/mol (23.9 kcal/mol) is intermediate between that of furan and thiophene, or about two-thirds that of benzene (5). 

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. 

B Benzene is a resonance hybrid whose structure is intermediate between two line-bond structures. 

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

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. 

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... 

Benzene, C6H(l, is another molecule best described as a resonance hybrid. It consists of a planar hexagonal ring of six carbon atoms, each one having a hydrogen atom attached to it. One Lewis structure that contributes to the resonance hybrid is shown in (11) it is called a Kekulc structure. The structure is normally written as a line structure (see Section C), a simple hexagon with alternating single and double lines (12). 

It is often asked whether or not the constituent structures of a resonating system, such as the Kekul4 structures for the benzene molecule, are to be considered as having reality. There is one sense in which this question may be answered in the affirmative but the answer is definitely negative if the usual chemical significance is attributed to the structures. A substance showing resonance between two or more valence-bond structures does not contain molecules with the configurations and properties usually associated with these structures. The constituent structures of the resonance hybrid do not have reality in this sense. 

Which of two formulas for benzene is correct The answer is neither. The two forms are called resonance structures. The term resonance is a bit misleading because it implies that the two forms are oscillating back and forth. In reality, the carbon-carbon bond lengths in a resonating structure such as benzene are all the same. Resonant structures have only one form, a resonance hybrid somewhere between the two possibilities. 

When two or more benzene rings are fused together to give naphthalene, anthracene, etc., X-ray diffraction studies show that some localization of double bonds occurs (22-24) this affects the chemical reactivities of different regions in the molecule. The experimentally measured bond lengths in PAHs are those that would be expected from a consideration of the various types of resonance hybrids (25) that are possible. 

A common language unifies a community of practitioners, at the same time that it sets them apart from other communities. As the chemist Mayer wrote Lewis about Slater s lecture at the Baltimore meeting of the American Physical Society, the talk of tetrahedral axes and coupling sounded so familiar that "I felt much at home " 15 In this regard, the invention of the phrase "resonance hybrid" should be remarked as a stroke of genius. No term could better have expressed the union of the natural philosophy tradition ("resonance") and the natural history tradition ("hybrid") in the theory of the electronic constitution of benzene. Both physicists and chemists felt at home. 

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... 

Canonical forms of benzene with alternating double bonds Le., cyclohexatriene ), structures which contribute equally and predominantly to benzene s resonance hybrid structure. 

Benzene, CgHg, is a common industrial solvent. The benzene molecule is based on a ring of covalently bonded Ccirbon atoms. Draw two acceptable Lewis structures for benzene. Based on the structures, describe a likely resonance hybrid structure for benzene. 

You can see resonance structures for benzene in the following figure. Adjacent carbons in the ring are held together with either single or double covalent bonds, depending on the resonance structure. So in the resonance hybrid structure, each Ccirbon-carbon bond is identical and is like a one-and-a-half bond rather than a single or double bond. 

The resonance theory accounts for the much greater stability of benzene (resonance energy) when compared with the hypothetical 1,3,5-cyclohexa-triene. It also explains why there is only one 1,2-dibromobenzene rather than two. Therefore, the structure of benzene is not really a 1,3,5-cyclohex-atriene, but a hybrid structure as shown above. 

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 ... 

To be reasonable, all structures in a set representing a resonance hybrid must have exactly the same locations of the atoms in space. For example, formula 7 does not represent a valid member of the set of valence-bond structures of benzene, because the atoms of 7 have different positions from those of benzene (e.g., 7 is not planar) ... 

In using the resonance method, we assume that all the resonance structures contributing to a given resonance hybrid have exactly the same spatial arrangements of the nuclei but different pairing schemes for the electrons. Therefore 11, 12, and 13 are not to be confused with bicyclo[2.2.0]-2,5-hexadiene, 15, because 15 is a known (albeit not very stable) molecule with different atom positions and therefore vastly different bond angles and bond lengths from benzene ... 

Clearly, it is inconvenient and tedious to write the structures of the contributing forms to show the structure of a resonance hybrid. A shorthand notation is therefore desirable. Frequently, dashed rather than full lines are used where the bonding electrons are expected to be delocalized over several atoms. For benzene, 16a or 16b is quite appropriate ... 

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. 

Resonance hybrid. A molecular structure that is a hybrid of two structures that differ in the locations of some of the electrons. For example, the benzene ring can be drawn in two ways, with double bonds in different positions. The actual structure of benzene is a blending of these two equivalent structures. 

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). 

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