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, C 6 H 6 . 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.
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.
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".
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.
Do not confuse the double-headed arrows that denote resonance with the two single-headed arrows that denote equilibrium. The molecules on the ends of equilibrium arrows are distinct molecules that can undergo a chemical transformation to become the other. Resonance is not an equilibrium in that the resonance structures don't actually exist on their own. A resonance hybrid is not the average of resonance contributors that fluctuate from one to the other.
In the next chapter we will discuss isomers, molecules that have the same molecular formula but are different in some way. It is important to note that resonance structures are not isomers of one another, simply because they don't actually exist. Another common question is of the following nature: The molecule propene can be written in two separate orientations, but both represent the same molecule; in other words, the structures are identical. Now consider the resonance structures of the allyl cation. Aren't these identical too? And if so how can we consider them distinct resonance structures? Once again, the answer is that the resonance forms are only representations of the true allyl cation; the resonance forms themselves don't exist. The positioning of electrons on resonance structures is all-important. Seemingly similar resonance forms are often necessary in order convey the true nature of some molecule.
In our earlier definition of resonance we said that the resonance hybrid is a weighted average of its resonance forms. In fact, not all resonance structures contribute equally. As an extension of the color analogy, imagine that we try to describe a very dark shade of gray as a hybrid of white and black. Clearly, dark gray resembles black much more than it resembles white. Such dominant resonance forms are said to be major resonance structures, whereas the less dominant forms are minor resonance structures. For instance, both resonance forms of the allyl cation contribute equally (hence both are major). However, the acetaldehyde enolate is dominated by the major form on the left:
The ability to roughly assess the stability of organic molecules is an important skill, and one that you will acquire as you increase your intuition of organic chemistry. For the enolate example, we can use a simple electronegativity argument to justify why the left form is more stable. Because oxygen is more electronegative than carbon, the negative charge is stabilized more by residing on oxygen. Thus, in the resonance hybrid, we would expect oxygen to bear a larger partial negative charge than carbon. We would also expect the C-C bond to have greater double-bond character than the C-O bond.
We have seen that resonance structures can be used to describe molecules whose electrons are not fixed on specific atoms or bonds but are spread out over several atoms or bonds. This phenomenon is called electron delocalization. Delocalization is an energetically favorable process: by distributing charge over a greater volume, the net energy of the molecule is lowered. The result is that a resonance hybrid is more stable than any of its contributing resonance structures because of delocalization in the resonance hybrid. Hence, resonance hybrids are said to be resonance stabilized. An important guideline in organic chemistry is that a molecule becomes more stable whenever more resonance structures can be drawn for it. Additional resonance structures, even those relatively high in energy, always stabilize; they never destabilize. Of course, the more stable the additional resonance form is, the more stabilization the resonance hybrid receives.