5.10 Resonance

The most common way to represent the benzene ring, as a hexagon with conjugated bonds, is ubiquitous. But in a real molecule of benzene, the single and double bonds are constantly switching places, very very quickly, so that the two states blend together into one. This is known as resonance.

Double-bonded atoms are closer together than single-bonded, so we would expect the benzene ring to have a distorted hexagonal shape, with alternating longer and shorter sides. But that is not the case; the atoms are perfectly equidistant to their nearest neighbors. Resonance makes all the atoms equivalent, as though they share a ring of hybrid one-and-a-half-bonds. Electrons are shared between these atoms, in donut shaped orbitals above and below the plane of the molecule's ring, called pi orbitals because of quantum physics.

(Incidentally, these donut-shaped orbitals are how conjugated systems conduct electricity.)

Resonance differs from tautomerism in that resonance produces one single molecular structure that is a hybrid of states, whereas tautomerism is a free alternation between different distinct molecular structures.

Some other functional groups that display resonance include the carboxylate ion, in which the negative charge is shared equally between the two oxygen atoms, and the strongly basic guanidine H₂NC(NH₂)=NH, whose ion the guanidinium ion has all its nitrogen atoms sharing the positive charge equally.

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