Written by tutor Amelia P.
Part I: Resonance
The delocalization of electrons can be complicated, confusing and confounding! How do those little dots move about the molecule on my paper and what does all of it mean? On paper, we only estimate the location of electrons. In reality they occupy areas of space around atoms called orbitals. Further complicating things, electrons are often shared between atoms. In order to clarify the process of electron delocalization and the many effects that it can have we will start with a molecule that has received a lot of media buzz in recent years, ozone. Ozone is O3, three molecules of oxygen bonded together. Let’s take a look at the Lewis Structure of ozone.
At this point we notice that the green oxygen (in the center) has one double bond and one single bond. Each bond represents a shared pair of electrons. The pink oxygen is sharing 2 electrons with the green oxygen while the green oxygen shares 4 electrons with the blue. The lone pairs associated with each oxygen atom are there to give each oxygen atom a full octet. Count the electrons in the lone pairs and the electrons shared by each oxygen atom. You will see that each atom has 8 electrons. If you’re particularly observant you might be asking- isn’t there another way for each atom to have 8 electrons? Why, yes there is! What if we had drawn the structure this way:
This is also an accurate depiction of ozone! The relationship between these two images is that they are resonance structures. All that changed from one to the other is the location of the electrons! Let’s take a closer look at the movement of the electrons.
To keep track of the movement of electrons notice in the second structure that the pi-bond formed between the pink oxygen and the green oxygen is pink to indicate that it was formed from the electrons that were previously on the pink oxygen. Also note that the “new’ lone pair on the blue oxygen is drawn in black to indicate that it came from the pi-bond between the green and blue oxygen molecules. So, how does ozone exist in nature? In fact, ozone exists “in between” its two resonance forms. We can consider each bond to be a “1&1/2 bond.” It is neither a single bond nor a double bond, instead, it has characteristics of both.
Note the use of curved arrows in the illustration of the resonance structure. Curved arrows always indicate the movement of electrons (not atoms). Electrons always move away from more electronegative atoms and towards more positive atoms. By being aware of and following the rules of curved arrows drawing resonance structures can be very straightforward. For practice, draw the resonance structures of NO3 (Hint: this compound has 3 resonance structures!)
Part II: An Example of The effects of Electron Delocalization
The specific location of electrons can have far reaching effects. For example, certain reactions require a specific electron configuration around certain atoms. In a benzyne reaction, for example, both resonance structures of the substituted benzene must be considered to determine the products. I will spare you the gory details (ie- the mechanism) of this reaction but we will see the effect that electron delocalization has on the reaction, regardless. Observe the following example:
In the first illustration we see that our starting material is metafluoro-toluene. Both structures are valid representations of metafluoro-toluene. We will now observe the product(s) of each resonance structure upon treatment with NaH and H3O+.
Note that there are three unique products from this reaction. The second and fourth product illustrated are the same. Without knowledge of electron delocalization and the know-how to react BOTH resonance structures of metafluoro-toluene one would incorrectly resolve that there are 2 products of this reaction. Hopefully this example helped to illustrate the far-reaching implications of electron delocalization.
Part III: Aromaticity
Aromatic compounds are extremely important in chemistry. Aromatic compounds satisfy a specific set of criteria. They are conjugated (consist of double bonds alternated with single bonds), planar (all atoms are sp2 hybridized), have a continuous pi system, and satisfy the Huckel 4n+2 rule (4n+2= # π electrons where n=positive integer). When this specific set of criteria is met the compound is extremely stable. Aromatic compounds may contain anions, cations and any heteroatom.
The ability of electrons to delocalize throughout an aromatic molecule is what makes the molecule exceptionally stable. Specifically the second criterion, continuous pi system, relies on the concept that electrons can delocalize/resonate throughout the molecule. Review the following compounds and determine if they are aromatic:
All three compounds are aromatic as they satisfy the criteria outlined above. As an example, the first one is analyzed below.
A resonance structure illustrates another form of a molecule as it exists in nature.
It is important to consider electron delocalization when writing a reaction mechanism.
Electrons do not move from or change after a bond is formed.
An aromatic compound must meet the following criteria:
I. Continuous pi system
III: Huckel Rule
IV: Tetrahedral Carbon molecules
VI: Contain only carbon and hydrogen
Which of the following is a valid resonance structure for carbon dioxide?