Benzene bond line structure

FIGURE 14.22 The bond line structures of two common aromatic hydrocarbons, benzene and toiuene. 

A line structure (introduced in Section C) represents a chain of carbon atoms as a zigzag line. The end of each short line in the zigzag represents a carbon atom. Because carbon nearly always has a valence of 4 in organic compounds, we do not need to show the C—H bonds. We just fill in the correct number of hydrogen atoms mentally, as we see for methylbutane (6), isoprene (7), and propyne (8). As explained in Section 2.7, a benzene ring is represented by a circle inside a hexagon, and we need to remember that one hydrogen atom is attached to each carbon atom. 

Recall that resonance does not describe the motion of electrons, but rather, resonance is the way that chemists deal with the inadequacy of bond-line drawings. Specifically, each drawing alone is inadequate to describe the structure of benzene. The problem is that each C—C bond is neither a single bond nor a double bond, nor is it vibrating back and forth between these two states. Instead, each C—C bond has a bond order of 1.5, exactly midway between a single bond and a double bond. To avoid drawing resonance structures, benzene is often drawn like this ... 

As we have seen above, two formula types are conunonly used to depict benzene rings. The traditional bond-line representation allows easier depiction of mechanisms involving the tt electrons, as we shall need to do in upcoming chapters, whereas the circle in the hexagon notation better suggests the structure and properties of benzene rings. 

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. 

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. 

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

In these line-angle formulas it is understood that there is a carbon atom at each vertex of the hexagon hydrogen atoms are not shown. This model is consistent with many of the properties of benzene. The molecule is a planar hexagon with bond angles of 120°. The hybridization of each carbon is sp2. However, this structure is misleading in one respect Chemically, benzene does not behave as if double bonds were present... 

Both of these structures satisfy the formal valence rules for carbon, but each has a serious fault. Each structure shows three of the carbon-carbon bonds as double bonds, and three are shown as single bonds. There is a wealth of experimental evidence to indicate that this is not true. Any one of the six carbon-carbon bonds in benzene is. the same as any other. Apparently the fourth bond of each carbon atom is shared equally with each adjacent carbon. This makes it difficult to represent the bonding in benzene by our usual line drawings. Benzene seems to be best represented as the superposition or average of the two structures. For simplicity, chemists use either one of the structures shown in (30) usually expressed in a shorthand form (SI) omitting the hydrogen atoms ... 

Reviews on 1 H-imidazoles (1), have mentioned only isolated examples of the iso compounds 2 and 3, although a later treatise has covered the latter two classes in more detail. The present review seeks to cover all known 2//-imidazoles, whereas the following one will deal with the 4H compounds. Chemical Abstracts have been covered up to and including issue 4 of Volume 97, using a comprehensive CAS on line substructure search a few additional references are included from the more common international journals. The search excluded structures with exocyclic double bonds, and hence those fused to a benzene ring. 

Figure Bll.l. (a) Bonding interactions and structure of benzene-NO+ complex with Cev symmetry. b) Bonding interactions and structure of benzene-NO+ complex with Cs symmetry, (c) Interaction diagram for the Cs complex. The dashed lines correspond to possible assignments for the two lowest transitions.

A shorthand notation of hybrid structures frequently is used in which the delocalized 7r-bonding is shown as a broken line. For benzene, an inscribed circle also is used to indicate continuous tt bonding ... 

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 ... 

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