Representation of the benzene ring

Other symbols as shown below are representations of the benzene ring. 

I is a particularly useful representation of the benzene ring, since it emphasizes the equivalence of the various carbon-carbon bonds. The presence of the circle distinguishes the benzene ring from the cyclohexane ring, which is often represented today by a plain hexagon. 

Figure 5. Reproductions of the three earliest representations of the benzene ring by (a) Laurent (1854), (b) Loschmidt (1861), and (c) Kekuld (1866).

Structure. The representation of the benzene molecule has evolved from the Kekule ring formula (1) to the more electronically accurate (2), which iadicates all carbon—carbon bonds are identical. 

O Use a molecular model set to build a model of the benzene ring. Examine your model. Does your model give an accurate representation of benzene s bonding system Explain your answer. 

Figure 9-11 Representations of the bonding in the benzene molecule, CgHg. (a) Lewis formulas of the two valence bond resonance structures, (b) The six p orbitals of the benzene ring, shown overlapping to form the (hypothetical) double bonds of the two resonance forms of valence bond theory, (c) In the MO description the six electrons in the pi-bonded region are dehcalized, meaning they occupy an extended pi-bonding region above and below the plane of the six C atoms.

In the molecular orbital explanation, which we shall describe in much more depth in Chapter 14, we begin by recognizing that the carbon atoms of the benzene ring are s]f hybridized. Therefore, each carbon has a p orbital that has one lobe above the plane of the ring and one lobe below, as shown on the next page in the schematic and calculated p orbital representations. 

Figure 1.11 Thermal desorption spectra of a 4-nitrobenzene (4-NB) layer grafted on an H-terminated (111) Si surface. Inset schematic representation of a monolayer of 4-NB grafted on an H-terminated (111) Si surface. Small (C H, mass 26) and big (C=CHN02, mass 71) fragments of the benzene ring are depicted by thin and thick continuous lines, respectively, while NO (mass 30), NO (mass 46), and (NO) (mass 60) are illustrated by dashed, dotted, and dash-dotted lines, respectively. Adapted and reprinted with permission from Ref. [197]. Copyright (2005) by Elsevier Publishing Group.

There have been several representations of the bonding in benzyne. The one most generally used pictures benzyne as being similar to benzene but with an additional weak bond in the plane of the ring, formed by overlap of the two sp orbitals. Comparison of the NMR characteristies with MO ealeulations indieate that the eonjugation is maintained and that benzyne is a strained but aromatie moleeule. ... 

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. 

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