Benzene resonance representation

Chemists sometimes represent the two benzene resonance forms by using a circle to indicate the equivalence of the carbon-carbon bonds. This hind of representation has to be used carefully, however, because it doesn t indicate the number of tt electrons in the ring. (How many electrons does a circle represent ) In this book, benzene and other aromatic compounds will be represented by a single line-bond structure. We ll be able to keep count of tt electrons this way but must be aware of the limitations of the drawings. 

The Resonance Representation The resonance picture of benzene is a natural extension of Kekule s hypothesis. In a Kekule structure, the C — C single bonds would be longer than the double bonds. Spectroscopic methods have shown that the benzene ring is planar and all the bonds are the same length (1.397 A). Because the ring is planar and the carbon nuclei are positioned at equal distances, the two Kekule structures must differ only in the positioning of the pi electrons. 

Resonance representations must take account of the pattern estabhshed for benzene and the relevant heterocycle. Contributors in which both aromatic rings are disrupted make a very much smaller contribution and are shown in parentheses. 

These two structures of benzene are the Kekule s structures, named after August Kekule, who first proposed them in I865. As resonance structures, they do not exist in equilibrium, nor do they exist independently rather, they are used to represent the actual structure of benzene Another representation of benzene is shown below. 

The Resonance Representation The resonance picture of benzene is a natural extension of Kekuld s hypothesis. In a Kekuld structure, the C—C single bonds would be longer than the double bonds. Spectroscopic methods have shown that the benzene... 

A methoxy substituent on a benzene ring has an effect that is opposite that of a nitro substituent, so the benzyl anion is a model for the MOs of anisole. Excitation removes an electron from and places it in ij/s, which produces a charge difference suggested by the resonance representation shown in Figure 12.52. These simple models help us understand the photochemically induced solvolysis in which m-nitrophenyl trityl ether (103) does not react in the dark but does readily undergo photosolvolysis, as shown in Figure 12.53. ° ... 

Structurally benzene is the simplest stable compound having aromatic character, but a satisfactory graphical representation of its formula proved to be a perplexing problem for chemists. Kekule is usually credited with description of two resonating structures which. 

Benzene was probably the fust compound in chemical history where the valence bond concept proved to be insufficient. Localizing the nr-systems, one comes up with two equivalent but different representations. The true bonding in benzene was described as resulting from a resonance between these two representations (Figure 2-46). 

Benzene has already been mentioned as a prime example of the inadequacy of a connection table description, as it cannot adequately be represented by a single valence bond structure. Consequently, whenever some property of an arbitrary molecule is accessed which is influenced by conjugation, the other possible resonance structures have to be at least generated and weighted. Attempts have already been made to derive adequate representations of r-electron systems [84, 85]. 

The resonance interaction of chlorine with the benzene ring can be represented as shown in 13 or 14, and both of these representations have been used in the literature to save space. However, we shall not use the curved-arrow method of 13 since arrows will be used in this book to express the actual movement of electrons in reactions. We will use representations like 14 or else write out the canonical forms. The convention used in dashed-line formulas like 14 is that bonds that are present in all canonical forms are drawn as solid lines, while bonds that are not present in all forms are drawn as dashed lines. In most resonance, a bonds are not involved, and only the n or unshared electrons are put in, in different ways. This means that if we write one canonical form for a molecule, we can then write the others by merely moving n and unshared electrons. 

Whenever there are two alternative Lewi.s structures, one alone will be an inaccurate representation of the molecular itructure. A more accurate picture will be obtained by the superposition of. the two structures into a new model, which lor benzene indicated by 3. The. superposition of two or more Lewis structures into a composite picture is called resonance. 

Figure 2.2, in which a constants are plotted against log klk0, for the bromina-tion of monosubstituted benzenes, shows an example of the usefulness of these new parameters. As can be seen from Structures 12 and 13—which are representations of the intermediates in the ortho and para bromination of anisole—substituents electron-donating by resonance ortho or para to the entering bromine can stabilize the positive charge in the intermediate and therefore also in the transition state by through-resonance. 

Scheme 3. Representations of. T-Delocalization in Benzene (a) Hiickel s t-MO s. (b) Wheland— Pauling s Resonance Hybrid, (c) Symbolic Representation of T-Delocalization by a Dotted Circle3...

The three classical Kekule structures (already alluded to in section III.E) of naphthalene are shown in Scheme 36a. Two of them are designated as Ki and K2 and represent the annulenic resonance along the perimeter of the naphthalene, while the third one, Kc, has a double bond in the center and transforms as the totally symmetric irreducible representation, Ag of the Dzh group. The Ki and K2 structures are mutually interchangeable by the i, C2, and ov symmetry operations of the point group, much as in the case of benzene. An in-phase combination transforms, therefore, as Ag, whereas an out-of-phase one transforms as B2u. These symmetry adapted wave func-... 

Valence bond theory is somewhat out of favour at present a number of the spectroscopic and magnetic properties of transition-metal complexes are not simply explained by the model. Similarly, there are a number of compounds (with benzene as an organic archetype) which cannot be adequately portrayed by a single two-centre two-electron bonding representation. Valence bond theory explains these compounds in terms of resonance between various forms. This is the origin of the tautomeric forms so frequently encountered in organic chemistry texts. The structures of some common ligands which are represented by a number of resonance forms are shown in Fig. 1-11. 

For cyclobutadiene, keeping the K0 term as necessary to describe the background exerted by a skeleton, the average of the two Kekule structures is K0-Ja-Jb. Correspondingly, for benzene, confined also to Kekule structures, the non-resonant part can be conventionally defined as K0-3Ja/2-3Jb/2. In this way we estimate a resonance stabilization for benzene of — 8.56 kcal/mol, comparable with the CASVB calculation [22] giving — 7.4 kcal/mol. Here one should note that with respect to the adopted definition and the method of estimation, the resonance energy is disputed in a very large interval (— 5 to — 95 kcal/mol) [23]. The representation in... 

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... 

Figure 1.13 Representation of the aromatic benzene molecule with two resonance structures (left) and, more accurately, as a hexagon with a circle in it (right). Unless shown by symbols of other atoms, it is understood that a C atom is at each corner and that 1 H atom is bonded to each C atom.

Benzene has two major resonance structures that contribute equally to the resonance hybrid. These are sometimes called Kekule structures because they were originally postulated by Kekule in 1866. You may also encounter benzene written with a circle inside the six-membered ring rather than the three double bonds. This representation is meant to show that the bonds in benzene are neither double nor single. However, the circle structure makes it difficult to count electrons. This text uses a single Kekule structure to represent benzene or its derivatives. You must recognize that this does not represent the true structure and picture the other resonance structure or call upon the MO model presented in Section 16.3 when needed. 

Thus the central circle in the resonance hybrid representation actually captures the physical reality of the bonding in benzene. 

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. 

The resonance-delocalized picture explains most of the structural properties of benzene and its derivatives—the benzenoid aromatic compounds. Because the pi bonds are delocalized over the ring, we often inscribe a circle in the hexagon rather than draw three localized double bonds. This representation helps us remember there are no localized single or double bonds, and it prevents us from trying to draw supposedly different isomers that differ only in the placement of double bonds in the ring. We often use Kekule structures in drawing reaction mechanisms, however, to show the movement of individual pairs of electrons. 

Using this resonance picture, we can draw a more realistic representation of benzene (Figure 16-1). Benzene is a ring of six sp2 hybrid carbon atoms, each bonded to one hydrogen atom. All the carbon-carbon bonds are the same length, and all the bond angles are exactly 120°. Each sp2 carbon atom has an unhybridized p orbital perpendicular to the plane of the ring, and six electrons occupy this circle of p orbitals. 

The concept of "resonant rings" follows from Clar s representations of benzenoids [48] in which one identifies "pi-sextets", that coincide with six pi-electrons of various individual benzene rings in polycyclic-benzenoids- If a pi-sextet is represented by an inscribed circle we can Immediately see that, for example, rings 1 and 4 in the benzenoid considered are "resonant", i-e-, both rings can simultaneously be "sextet" rings- However, rings 2 and 4 are not resonant, because we cannot... 

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