Ever wondered where the term "aromatic" comes from? In the early days of organic chemistry, many compounds were grouped together based off of their "aromatic" fragrances. For instance, benzaldehyde has a sweet smell and was isolated from cherries while methyl salicylate (methyl-2-hydroxy-benzoate) has a wintergreen scent. Another common fragrant compound is vanillin (4-hydroxy-3-methoxy-benzaldehyde), the major component of the vanilla flavor. All of these compounds are considered to be aromatic compounds. But benzene? Benzene does not have a particularly pleasant smell like the others so why is it aromatic?

It turns out that the term aromatic has much more to do with chemical behavior rather than scent. So what kinds of things are aromatic? Can we define them and have some rules that will help us determine aromaticity? Fortunately, the answer is yes! There are some rules that can be followed to help determine if something is aromatic and the best part is that you don't have to smell it!

In order for a compound to be defined as an aromatic species it must meet all of the following criteria: 1. The molecule must be cyclic. 2. Every atom must be sp2 hybridized (all must have a p-orbital). 3. The system must have 4n + 2 pi electrons. 4. The system in question must be flat. Let's look at benzene first, then go through some other compounds.

What happens if a compound meets all of the criteria except for the 4n + 2 pi electrons rule (also known as Huckel's rule)? If this happens, we describe a compound as being anti aromatic. These compounds are unstable and highly reactive. In general, these compounds have only 4n pi electrons but meet all of the other conditions for aromaticity. Let's look at an example below.

Need more help? Watch the video below for even more examples and answers to common questions! Good luck!