What are Resonance Structures?
Often, the true structure of a molecule as it exists in nature cannot be accurately depicted by only 1 molecular drawing. These molecules can only be represented as a combination, or hybrid, of multiple bond line drawings, which is the key point to understanding what are resonance structures. Each individual drawing that composes the hybrid is called a resonance structure.
To understand this concept, say that someone asks you to describe a mermaid. You could say “a human” but that would only be part of the answer. Likewise, you could also say “a fish” but that too would only be part of the answer. The most accurate depiction of a mermaid would be “part human, part fish.” A mermaid is a hybrid of two structures (a human and a fish), just as molecules often truly exist as a hybrid of multiple resonance structures.
Note that a mermaid is not changing back and forth between a human and a fish. It exists solely as a hybrid of the two, just as molecules do not change back and forth between resonance structures.
Resonance structures then are the various concrete structures that contribute to the overall hybrid, which is how the molecule truly exists in reality. Here is an example of 2 resonance structures.
In reality, the positive charge of this molecule is spread out across carbons (as the hybrid shows) rather than concentrated on any one of the carbons (as the resonance structures show).
Note that resonance structures are connected with a double headed “resonance arrow” and that the structures are surrounded by brackets.
It’s important to understand that the molecule is not interchanging between these resonance structures but rather that the structure of the molecule is a single hybrid of all the resonance structures combined. Fully understanding what are resonance structures takes practice, so we’ll break it down piece by piece.
Drawing curved arrows in organic chemistry
Drawing curved arrows indicates the movement of electrons. When drawing curved arrows in resonance structures, specifically they show how electrons move from one resonance structure to another. The arrow to go from one resonance structure to the next in this example is shown here in red:
The tail of the arrow always starts at an electron-rich source (such as a negative formal charge, double bond, triple bond), and the head is always at a less electron-rich area.
When drawing resonance structures, you must be sure to never exceed the octet rule for elements that follow this rule (C, N, O, F, Cl, Br, I). Below is an example of a resonance structure that would break the octet rule, so it is a resonance structure that cannot be drawn.
There is also never a situation where one should break a single bond when making a resonance structure.
As a final, but very important rule, the overall charge of the molecule never changes from one resonance structure to another. For example, if the starting molecule has a +1 charge, the resonance structure must also have an overall charge of +1.
How do we know when to draw a resonance structure?
To know when we can draw resonance structures, we must first be able to identify areas of abnormal electron density in the molecule. Areas of high electron density generally include double bonds or a lone pair of electrons, while areas of low electron density normally consist of carbocations.
Resonance structures can be drawn when we have an area of high electron density either next to (1) another area of high electron density or (2) an area of low electron density. Both patterns are explored in more detail below.
1. An area of high electron density next to another area of high electron density
In this pattern, 2 areas of high electrons density area are located next to one another. We can therefore move these electrons, forming a resonance structure. Some examples of this pattern are below.
In the resonance structure above, the lone pair of electrons fills into the carbon-carbon bond, forming a double bond. At the exact same time, the left-most carbon-carbon double bond will break, moving its electrons onto a primary carbon. Note that the left-most carbon-carbon bond must break to ensure none of the carbons are breaking the octet rule.
2. An area of high electron density next to an area of low electron density
In this pattern, an area of high electron density is next to an area of low electron density. The area of high electron density donates electrons to the low area of electron density, forming a resonance structure. Take a look at the examples shown below.
In the resonance structure above, the lone pair on the electron-rich nitrogen feeds in to form a carbon-nitrogen double bond. This moves the carbocation from the carbon atom to the nitrogen atom.
In the resonance structure above, the pi electrons of the double bond move one bond over towards the carbocation. Therefore, the area of high electron density (the double bond) is moving towards the area of low electron density (the carbocation). The location of the carbocation changes as the double bond shifts.
So I have all these resonance structures? How do I know which are most important?
Not all resonance structures equally contribute to the hybrid molecule. There are defined criteria to decide how much a resonance structure contributes to the overall hybrid structure. For example, in the 2 resonance structures below, one contributes much more to the hybrid structure than the other.
The actual way the molecule exists (which is a hybrid of the resonance structures) would be weighted more towards the “GREAT” molecule than the “not so great” molecule. Here are the rules to determine how good a resonance structure is:
Rules of Resonance Structures:
- If any atoms don’t have a full octet, the resonance structure is greatly weakened.
- Minimize charges as much as possible.
- If you must have charge, it is better to have the negative charge on an electronegative atom (N, O, S, F, Cl, Br, I). Positive charges prefer to be placed on less electronegative atoms.
1. If any atoms don’t have a full octet, the resonance structure is greatly weakened.
The second structure is much weaker because the carbon with the positive formal charge lacks a full octet. This means that the second structure will contribute much less to the molecular hybrid than the first structure.
2. Minimize charges as much as possible.
The structure on the right isn’t as good because there is a positive and a negative formal charge, while the structure on the right is totally neutral.
3. If you must have charge, it is better to have the negative charge on an electronegative atom (N, O, S, F, Cl, Br, I). On the other hand, positive charges prefer to be placed on less electronegative atoms.
This section covered what are resonance structures and drawing curved arrows organic chemistry. Remember to know these patterns cold as this is how you will identify whether you can draw a resonance structure or not.