A Resonance Picture of Bonding in Benzene

Twentieth-centnry theories of bonding in benzene provide a rather clear pictnre of aromaticity. WeTl start with a resonance description of benzene. 

The two Keknld strnctnres for benzene have the same arrangement of atoms, but differ in the placement of electrons. Thns they are resonance forms, and neither one by itself correctly describes the bonding in the actnal molecule. As a hybrid of the two Kekule structures, benzene is often represented by a hexagon containing an inscribed circle. 

The circle-in-a-hexagon symbol was first suggested by the British chemist Sir Robert Robinson to represent what he called the aromatic sextet —the six delocalized TT electrons of the three double bonds. Robinson s symbol is a convenient time-saving shorthand device, but Kekul6-type formulas are better for counting and keeping track of electrons, especially in chemical reactions. 

FIGURE 11.1 Bond distances and bond angles of benzene. 

PROBLEM 11.1 Write structural formulas for toluene (CgHsCHa) and for benzoic acid (C6H5CO2H) (a) as resonance hybrids of two Kekule forms and (b) with the Robinson symbol. 

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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. 

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. 

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... 

Because all six carbon atoms and all six p orbitals in benzene are equivalent, it s impossible lo define three localized tt bonds in which a given p orbital overlaps only one neighboring p orbital. Rather, each p orbital overlaps equally well with both neighboring p orbitals, leading to a picture of benzene in which the six -tt electrons are completely delocalized around the ring. In resonance terms (Sections 2.4 and 2.5), benzene is a hybrid of two equivalent forms. Neither form... 

Qualitatively, the resonance picture is often used to describe the structure of molecules, but quantitative valence-bond calculations become much more difficult as the structures become more complicated (e.g., naphthalene, pyridine, etc.). Therefore the molecular-orbital method is used much more often for the solution of wave equations.5 If we look at benzene by this method (qualitatively), we see that each carbon atom, being connected to three other atoms, uses sp1 orbitals to form a bonds, so that all 12 atoms are in one plane. Each carbon has a p orbital (containing one electron) remaining and each of these can overlap equally with the two adjacent p orbitals. This overlap of six orbitals (see Figure 2.1) produces six new orbitals, three of which (shown) are bonding. These three (called it orbitals) all occupy approximately the same space.6 One of the three is of lower energy than... 

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. 

The valence bond approach is especially useful in organic chemistry where so many molecules are built of tetrahedral C atoms, sp hybridised. The concept of hybrids is intuitively very satisfying because they fit visually with our perceived picture of the shape of a molecule with its directed bonds between pairs of atoms. Unfortimately, the VB approach is not satisfactory for species like C03 , NOj, and benzene because the VB picture does not reflect the known chemical structure. A new concept of resonance hybrids must be introduced, and C03 must now be represented by a combination of three Lewis-octet structures. Worse still, the VB approach cannot easily give a satisfactory bonding picture for either of the important molecules O2 or CO. 

Benzene, and most aromatic compounds, usually undergo aromatic substitution reactions and not the addition reactions of ordinary alkenes. Phenanthiene (Structure 5), however, is different When treated with bromine, it simply adds the bromine to the 9,10 double bond to yield 9,10-dibromophenanthrene. This unusual reactivity of phen-anthrene can easily be understood from the valence bond viewpoint. In terms of valence bond pictures, we can desaibe the structure of the skeleton of this molecule to a first approximation with the complete set of five Kekule forms (resonance forms), as shown in Structure 5 ... 

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