Resonance
10.4 What resonance is not.
Significantly, resonance structures do not represent different, isolatable structures or com-
pounds. 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, intercon-
vertable 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^19 or tautomerism^20 which are equilibria between compounds that
have different sigma bonding patterns. Hyperconjugation^21 is a special case of resonance.
10.5 History
The concept of resonance was introduced by Linus Pauling^22 in 1928. He was inspired by
the quantum mechanical^23 treatment of the H 2 +ion in which an electron is located between
two hydrogen nuclei. The alternative termmesomerismpopular in German and French
publications with the same meaning was introduced by Christopher Ingold^24 in 1938 but
did not catch on in the English literature. The current concept of Mesomeric effect^25 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^26 ) who preferred
the German phrasezwischenstufeorintermediate phase.
Due to confusion with the physical meaning of the wordresonance^27 , as no elements do
actually appear to be resonating, it is suggested to abandon the termresonancein favor of
delocalization
UNKNOWN TEMPLATE Ref
Kerber
. Resonance energy would become delocalization energy and aresonance structurebecomes
contributing structure. The double headed arrows would get replaced by commas.
19 https://en.wikipedia.org/wiki/Chemical%20equilibrium
20 https://en.wikipedia.org/wiki/Tautomerism
21 https://en.wikipedia.org/wiki/Hyperconjugation
22 https://en.wikipedia.org/wiki/Linus%20Pauling
23 https://en.wikipedia.org/wiki/Quantum%20mechanics
24 https://en.wikipedia.org/wiki/Christopher%20Ingold
25 https://en.wikipedia.org/wiki/Mesomeric%20effect
26 https://en.wikipedia.org/wiki/Arndt-Eistert%20synthesis
27 https://en.wikipedia.org/wiki/Resonance