10 Resonance

10.1 Resonance.

Resonancerefers to structures that are not easily represented by a single electron dot
structure but that are intermediates between two or more drawn structures.

Resonance is easily misunderstood in part because of the way certain chemistry textbooks
attempt to explain the concept. In science, analogies can provide an aid to understanding,
but analogies should not be taken too literally. It is sometimes best to use analogies to
introduce a topic, but then explain the differences and inevitable complications as further
details on a complicated subject. This is the case for resonance.

Just as entropic principles cannot be applied to individual molecules, it is impossible to say
whether or not any given individual molecule with a resonance structure is literally in one
configuration or another. The actual situation on the molecular scale is that each config-
uration of the molecule contributes a percentage to the possible configurations, resulting
in a ”blend” of the possible structures. Changes in molecular shape occur so rapidly, and
on such a tiny scale, that theactual physical locations of individual electrons can-
not be precisely known(due to Heisenberg’s Uncertainty Principle). The result of all
that complexity is simply this: molecules with resonance structures are treated as mixtures
of their multiple forms, with a greater percentage of probability given to the most stable
configurations.

Thenuclei of the atoms are not movingwhen they are represented by resonance
structure drawings. Rather, the electrons are portrayed as if they were moving instead.
The true situation is that no one can say for certain exactly where any individual electron
is at any specific moment, but rather electron location can be expressed as a probability
only. What a dot structure is actually showing is where electrons almost certainly are
located, therefore resonance structures indicate a split in those same probabilities. Chemists
are absolutely certain where electrons are located when one carbon bonds four hydrogens
(methane), but it is less certain where precisely any given electron is located when six
carbons bond six hydrogens in a ring structrue (benzene). Resonance is an expression of
this uncertainty, and is therefore the average of probable locations.

Resonance structures arestabilizingin molecules because they allow electrons to lengthen
their wavelengths and therebylower their energy. This is the reason that benzene
(C 6 H 6 ) has a lower heat of formation than organic chemists would predict, not accounting
for resonance. Other aromatic molecules have a similar stability, which leads to an overall
entropic preference for aromaticity (a subject that will be covered fully in a later chapter).
Resonance stability plays a major role in organic chemistry due to resonant molecules’
lower energy of formation, so students of organic chemistry should understand this effect
and practice spotting molecules stabilized by resonant forms.