Some organic molecules are not accurately represented by a single drawing of their chemical structure. The ‘true’ structure may in fact be the average of two or more different drawings of this structure. This is due to a property of some functional groups known as resonance. Some functional groups are more accurately represented by a set of resonance structures. These are different representations of the same chemical structure, which differ only based on the locations of electrons – the positions of atoms do not differ between resonance structures. Therefore, resonance structures will usually involve the movement of double/triple bonds and/or lone pairs of electrons.
Resonance is often important in molecules where a lone pair on one atom is separated from a multiple (double or triple) bond by one single bond, or when two (or more) multiple (double or triple) bonds are separated by a single bond. Consider the following simple structures:
In the first structure (which is an enol), the two lone pairs on the oxygen and the carbon-carbon double bond are separated by only one single bond. In the second structure (an α,β-unsaturated aldehyde), the carbon-carbon double bond is separated from the carbon-oxygen double bond by only one single bond. These kinds of arrangements of lone pairs and multiple bonds often have major impacts on the reactivities of different functional groups.
Consider the carboxylate functional group (that is, the conjugate base of a carboxylic acid). Here is a generic carboxylate group, shown with and without lone pairs:
Notice that we have an arrangement where the lone pairs on the negatively charged oxygen are one single bond away from the carbon-oxygen double bond. For the carboxylate group, we can draw a resonance structure group, in which the negative charge is on the top oxygen, and the double bond goes to the bottom oxygen:
These structures differ only based on the locations of electrons. However, neither structure alone fully represents a carboxylate group. In reality, the structure of a carboxylate is an average of the two different resonance structures. One way of imagining the ‘real’ structure is that there is half of a double bond going between the central carbon and each oxygen, and that each oxygen has a partial negative charge:
The relationship between resonance structures can be drawn using mechanistic arrows (that is, arrows that show the movement of electrons). Here’s the mechanism showing how we can go between the two carboxylate resonance structures:
First, notice the arrow in the middle; this double-headed arrow is used to indicate that the two chemical structures are resonance structures. It does not mean that the first structure is turning into the second structure. Instead, it shows that these structures differ only in where the electrons are drawn.
Mechanistic arrows (like the red arrows above) are covered in much more detail in later tutorials on reactions and mechanisms. For now, all you need to know is that each red arrow represents the movement of two electrons. The arrow moving from the negative charge on the oxygen to the carbon-oxygen single bond shows the movement of a lone pair from the oxygen to make the double bond between the carbon and oxygen. The upper red arrow shows that the carbon oxygen double bond is breaking, and that the two electrons in the double bond are moving onto the oxygen, giving it a negative charge. Here’s another representation of the same mechanism, showing the lone pairs are shown: