Nissa has a masters degree in chemistry and has taught high school science and college level chemistry.
What Are Aromatic Compounds
Take a moment and think of substances that have a strong fragrance. What kind of things come to your mind? Perfume, vanilla or, perhaps, even cinnamon? All of these are fragrant or aromatic. While they certainly smell differently, they do have something in common: these substances are made of aromatic compounds.
So, what are aromatic compounds? Aromatic compounds are substances that consist of one or more rings that contain alternating single and double bonds in its chemical structure. In real life, many aromatic compounds have an odor, however, there are some compounds that are chemically aromatic, but do not have a distinct smell.
For example, benzene is an aromatic compound. The chemical structure of benzene, as illustrated here, contains a hexagon ring with alternating double bonds. Here, you'll see there are two ways to draw benzene. Structure 1 shows all the carbon (C) and hydrogen (H) atoms bonded together, and structure 2 shows a hexagon with alternating double bonds. In structure 2, the edges of the hexagon are the carbon atoms, and the hydrogen atoms are not shown.
Have you ever smelled paint or paint thinners? The strong smell of these products is due to the presence of an aromatic compound called toluene. Similarly, moth balls are very strong smelling because they contain an aromatic compound called naphthalene. Looking at this diagram, you'll notice the chemical structures of the compounds toluene and naphthalene. You'll note that they both contain rings with alternating single and double bonds in their chemical structure.
Delocalized electrons are electrons that are not attached on a fixed atom. In the ring of an aromatic compound, electrons are delocalized, so they are spread out over the ring. Such electrons can also be described as 'floating' around the ring. In benzene, for instance, the two structures show that the double bonds can be moved about the ring because the electrons are delocalized within the ring. This is why alternating double bonds in the ring can also be illustrated as a circle, as in this image.
The ring structure of an aromatic compound must be coplanar, so this means that the atoms involved in the ring are on the same plane. Here, we can see that the carbon (gray) and hydrogen (white) atoms are all on the same plane.
A compound can be classified as an aromatic compound if it has all of these properties and it follows Huckel's Rule.
Earlier, we saw that there are double bonds in the ring that is present in the chemical structure of aromatic compounds. These double bonds contain electrons called 'pi electrons.' The number of pi electrons is very important in determining if the compound follows Huckel's Rule.
In order for a compound to be aromatic, it must satisfy Huckel's Rule. Huckel's Rule states that an aromatic compound must have a certain number of pi electrons. The number of pi electrons must be equal to 4n+2, where n is equal to zero or any positive integer (n = 0, 1, 2…).
To check if a compound follows Huckel's rule, we must first count the number of pi electrons. In our example below, benzene, there are six pi electrons.
Now, we can check if it follows Huckel's Rule:
While benzene and compounds that contain the benzene ring in its chemical structure are known to be aromatic compounds, there are other compounds that look slightly different but are aromatic nonetheless. Let's check out a couple of these other compounds containing rings with alternating double bonds and confirm if they are aromatic based on Huckel's Rule.
Have you ever eaten jarred pickles? They last for a long time in our refrigerators because they have preservatives. Benzoic acid is one common food preservative.
Let's take a close look at the chemical structure of benzoic acid. We will see that there is a double bond between carbon and oxygen outside the ring. In Huckel's Rule, we do not count this double bond and only count the double bonds within the ring, so there are only 6 pi electrons.
Pyridine is a chemical needed to manufacture insecticides and herbicides. Below is the chemical structure of pyridine. Is it aromatic or not?
Let's review. Aromatic compounds contain a ring with alternating single and double bonds in their chemical structure. The electrons in the ring can move about and are spread out around the ring. These electrons are delocalized electrons. While there are many aromatic compounds that have distinct odors, there are also aromatic compounds that have no distinct smell. In order for a compound to be classified as aromatic, it needs to follow the following criteria:
- The chemical structure must contain atoms bonded together to form a ring
- The bonds in the ring should contain alternating double and single bonds
- The ring must be coplanar; atoms in the ring must all be in the same plane
- It must follow Huckel's Rule
Huckel's Rule determines whether a compound is aromatic or not. To check a compound against Huckel's Rule, you must first count its pi electrons. To satisfy Huckel's Rule, the number of pi electrons must be equal to 4n + 2, and n must be a number equal to 0 or any other positive integer.
Aromatic Compounds Key Terms
- Aromatic Compounds: are compounds which may or may not have an odor but stand out because they have a ring with alternating single and double bonds in their chemical structures
- Delocalized Electrons: electrons that have no connections to a fixed atom
- Coplanar: atoms involved in the ring must be on the same plane
- Huckel's Rule: tests whether the compound is aromatic or not
The process of learning about aromatic compounds could enable you to subsequently:
- Recognize aromatic compounds and understand the way in which they combine
- Discuss the chemical structures of benzene, toluene and naphthalene
- Illustrate the work of electrons and atoms in delocalized and coplanar compounds
- State, check for and satisfy Huckel's Rule
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