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Benzene: Introduction to Aromatic Compounds

Resonance in Benzene

Benzene's remarkable chemical behavior could be explained only with the development of quantum mechanics in the 1920s. Quantum mechanics provide two theories for describing molecular bonds - the resonance theory and the molecular orbital theory.

If the double bonds in benzene were to be localized, such as the Kekulé formula indicates, substitution with bromine would yield two different 1,2-dibromobenzenes. However, two 1,2-dibromobenzenes that display different chemical properties are actually not found. Kekulé reasoned that there is a rapid establishment of equilibrium.

Fig.1
Fig.2

In fact, the Kekulé formulas, which depict two 1,2-dibromobenzenes and differ only in the position of the double bonds, are different Lewis formulas of that same compound. A fundamental postulate of resonance theory states that a compound's chemical and physical properties cannot be derived from a single Lewis structure if the compound's structure may be described by several Lewis structures (resonance strucutres). Rather, the molecule's chemical and physical properties may only be indicated when all available data of all resonance structures are compiled together.

Note:

Resonance structures
Resonance structures do not describe different molecules that are in equilibrium. They are rather used to illustrate the actual structure of molecules, which cannot be visualized precisely by merely one Lewis structure, as a result of a lack in the Lewis structure concept. Therefore, in an illustration the resonance structures of a molecule are combined by double-headed arrows (resonance arrows; ↔), rather than by equilibrium arrows (pairs of single-headed arrows that are directed opposite to each other).

The carbons of benzene are all sp2-hybridized. Thus, each carbon possesses a single-occupied 2p orbital which is perpendicular to the ring plane. The six 2p orbital lobes on each side of the ring overlap in a cyclic pattern. As a result, there is a cyclic π electron cloud on each side of the ring plane.

Fig.3
Fig.4

Today, the π bonds of benzene are usually not illustrated such as in the Kekulé formula, but rather as a circle located inside of a hexagon. The circle indicates the cyclically conjugated double bonds, or the cyclically overlapping 2p orbitals, respectively, which results in the equal length of all carbon-carbon bonds.

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Fig.5

The cyclic 2p orbital overlapping cannot be explained by any other of the benzene structural formulas that have been proposed.

Fig.6
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