When drawing Lewis structures, sometimes you will find that there are many
ways to place double bonds and lone pairs about a given framework of
atoms. How does we decide whether one or another placement is correct? The
answer, as it
turns out, is neither and both. The actual arrangement of electrons in a given
molecule is a weighted average of all the valid Lewis structures that can be
drawn for that given atomic connectivity. The "real" molecule, the one that
actually exists in the world, is said to be a resonance hybrid of all its
contributing Lewis structures. Each Lewis structure that contributes to the
resonance hybrid is a resonance structure.
The classical example of resonance is benzene, C6H6. Two good Lewis
structures
for benzene exist that differ only in their placement of double bonds. If
either structure were correct, benzene would consist of alternating long single
bonds and short double bonds. However, it has been determined experimentally
that all six bonds on the ring are identical. The natural interpretation
is that the three double bonds are distributed evenly around the ring, so that
each bond has a bond order of one and a half.
Figure %: The resonance structures of benzene and the resonance
"hybrid".
A double headed arrow is placed between resonance structures to
denote them as such. In addition, sometimes we place all the resonance
contributors within brackets for clarity.
It's important to remember that although the molecule described by
resonance has characteristics of all its resonance contributors, it is
fully neither one. For instance, the color gray might be described as
being a resonance hybrid of white and black. And although gray takes on
characteristics of both black and white, it would be incorrect to
describe gray as being black or white.
The Curved-Arrow Formalism
Sometimes double-headed arrows are used to denote how one resonance structure
can be derived from another via the flow of electrons. This curved-arrow
formalism is a very useful bookkeeping tool that allows us to keep track of
the movement of pairs of electrons during reactions. The arrows are drawn from
the source of the electron motion, which can be a bonded electron pair or a lone
pair, to the destination of the electrons, typically an atom or a place between
two atoms. The figure below illustrates correct and incorrect usages of the
curved-arrow formalism. We will see that it is a very useful tool for
describing reaction mechanisms, the step-by-step processes by which
reactions occur. Such exercises are affectionately referred to as "arrow
pushing".
Figure %: Illustrating correct and incorrect uses of the curved-arrow
formalism.
Possible Misunderstandings Relating to Resonance
Resonance is such an important concept to master early on in your organic
chemistry education that it's worthwhile to clear up two potential
misunderstandings. Resonance and equilibrium, and resonance and isomerism are
often confused.
