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Resonance structures are diagrammatic tools used predominately in Organic Chemistry to symbolize '''resonant bonds''' between Atom s in Molecule s. The electron density of these bonds is spread over the molecule, also known as the Delocalization of Electron s. Resonance contributors for the same molecule all have the same Chemical Formula and same sigma framework, but the pi electrons will be distributed differently among the atoms. Because Lewis Dot Diagram s often cannot represent the true electronic structure of a molecule, resonance structures are often employed to approximate the true electronic structure. Resonance structures of the same molecule are connected with a double-headed arrow. While organic chemists use resonance structures frequently, they are used in inorganic structures, with Nitrate as an example. KEY CHARACTERISTICS The key elements of resonance are:
WHAT RESONANCE IS NOT Significantly, resonance structures do not represent different, isolable structures or compounds. In the case of benzene, for example, there are two important resonance structures - which can be thought of as cyclohexa-1,3,5-trienes. There are other resonance forms possible, but because they are higher in energy than the triene structures (due to charge separation or other effects) they are less important and contribute less to the "real" electronic structure (average hybrid). However, this does not mean there are two different, interconvertable forms of benzene; rather, the true electronic structure of benzene is an average of the two structures. The six carbon-carbon bond lengths are identical when measured, which would be invalid for the cyclic triene. Resonance should also not be confused with a Chemical Equilibrium or Tautomerism which are equilibria between compounds that have different sigma bonding patterns. Hyperconjugation is a special case of resonance. HISTORY The concept of resonance was introduced by Linus Pauling in 1928. He was inspired by the Quantum Mechanical treatment of the H2+ ion in which an electron is located between two hydrogen nuclei. The alternative term mesomerism popular in German and French publications with the same meaning was introduced by Christopher Ingold in 1938 but did not catch on in the English literature. The current concept of Mesomeric Effect has taken on a related but different meaning. The double headed arrow was introduced by the German chemist Arndt (also responsible for the Arndt-Eistert Synthesis ) who preferred the German phrase ''zwischenstufe'' or ''intermediate phase''. Due to confusion with the physical meaning of the word Resonance , after all no elements do actually appear to be resonating it is suggested to abandon the phrase resonance in favor of ''delocalization'' . Resonance energy would become delocalization energy and a resonance structure becomes contributing structure. The double headed arrows would get replaced by commas. EXAMPLES The Ozone molecule is represented by two resonance structures in the top of ''scheme 2''. In reality the two terminal oxygen atoms are equivalent and the hybrid structure is drawn on the right with a charge of -1/2 on both oxygen atoms and partial double bonds. The concept of benzene as a hybrid of two conventional structures (middle ''scheme 2'') was a major breakthrough in chemistry made by Kekule , and the two forms of the ring which together represent the total resonance of the system are called ''Kekule structures''. In the hybrid structure on the right the circle replaces three double bonds. The Allyl Cation (bottom ''scheme 2'') has two resonance forms and in the hybrid structure the positive charge is delocalized over the terminal methylene groups. SEE ALSO REFERENCES |
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