AROMATIC COMPOUND

The term 'aromatic' is used by chemists for substances with distinct odors. In this lesson, we will learn all about aromatic compounds, the features of such compounds' chemical structures, and Huckel's Rule.

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.

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.

Chemical structure of the aromatic compound benzene

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.

Examples of the aromatic compounds toluene and naphthalene

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.

Benzene has delocalized electrons in the ring.

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.

Benzene has a coplanar ring.

A compound can be classified as an aromatic compound if it has all of these properties and it follows Huckel's Rule.

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.

Counting pi electrons in benzene

Now, we can check if it follows Huckel's Rule:

Benzene is aromatic; it follows Huckels Rule.

Determining Aromaticity

At some point you’ll be asked to answer the following question: “is this molecule aromatic?”

I really like making tables. It’s a great way to systematize things. So when I’m trying to figure out if a molecule is aromatic or not, I’ll build a table like this. It helps make decisions much easier.

Aromatic molecules have the following characteristics:

1. They are cyclic.
2. They are conjugated all around the ring. The easiest way to  put it is this: every atom in the ring must be able to participate in resonance. So atoms that have pi-bonds, carbocations, or lone pairs are OK; but atoms with four single bonds are out. 
3. the molecule must have (4n+2) Pi electrons. This is just a different way of expressing the series (2,6,10,14, 18, etc. )**. Huckel’s Rule states that all aromatic molecules must satisfy this condition. Now, Pi electrons can either come from double bonds, or from lone pairs that can align themselves with the pi system. [What does 4n+2 mean?]
4. The molecule must be flat. This is generally assumed to be true for most molecules that meet the previous 3 conditions, except in cases where steric hindrance prevents it. An example is 

Note how pyridine (the third example) has a lone pair but it doesn’t “count” for purposes of aromaticity! More on that next time.

Thanks for reading! James

PS Yet another way of expressing (4n+2) I’ve heard is “odd numbers of electron pairs” i.e. 1,3,5,7… electron pairs representing 2, 6, 10, 14 pi electrons.