Classification of Organic Compounds

Instructor: Sarah Pierce

Sarah has a doctorate in chemistry, and 12 years of experience teaching high school chemistry & biology, as well as college level chemistry.

This lesson discusses the classification of organic compounds including hydrocarbons, open chain molecules, closed chain compounds, homocyclic compounds, heterocyclic compounds, aromatic molecules, alicyclic compounds, and homologous series.

What is an Organic Compound?

Imagine you are getting ready for a date. You pick out your new mauve shirt and drive to your date's house. After dinner, you are sipping a glass of wine. Their fragrant cologne tickles your nose. Suddenly a skunk walks through your back yard, completely ruining the mood. What do all of these things have in common? Believe it or not, they're all organic molecules, from the color of your shirt to the smell of the skunk.

Organic compounds are molecules that are composed mostly of carbon and hydrogen atoms. If the molecule just contains carbon and hydrogen atoms, it's called a hydrocarbon. Organic molecules can also contain other atoms such as oxygen, nitrogen, sulfur, and halogens. The gas you put in your car on the way to the date? That contains organic compounds!

Organic Molecule Classification

Organic molecules can be classified a number of ways. They can be put into categories based on how they are connected or by the other atoms present like oxygen, nitrogen, sulfur, or the halogens. The organic molecule that is responsible for the skunk smell has a sulfur atom in it, while the ethanol in your wine has an oxygen atom in it. Let's investigate some other categories of organic molecules.

Open Chain Versus Closed Chain

Molecules can either be an open chain or closed chain.

  • open chain - A molecule that doesn't form into a ring. It can either be a straight chain molecule or a branched chain (i.e. one that goes off in different directions).
  • closed chain - A molecule that does form a ring. This is also known as a cyclic compound.

Comparison of open chain and closed chain organic molecules
open versus closed

The gas in your car has both the open chain and closed chain molecules.

Homocyclic Versus Heterocyclic

Cyclic compounds are pretty important because they have some really unique properties. The dye of your purple shirt... That's a cyclic molecule! Cyclic compounds can be divided into two categories:

  • homocyclic - composed of just carbon and hydrogen atoms
  • heterocyclic - contains an atom or atoms that are different than carbon and hydrogen

Mauvine is a heterocyclic organic molecule that dyes clothes purple

Aromatic Versus Alicyclic

Homocyclic and heterocyclic compounds can also be grouped into two other categories: aromatic and alicyclic.

Mauvine, the purple dye, is an aromatic compound. Most often, an aromatic molecule is a cyclic organic compound with an odd number of double bonds. Sometimes this is shown as a circle inside a molecule. Benzene is the most common example of an aromatic molecule. Benzene is in your gasoline, as well as the Kevlar of bullet proof vests. All other cyclic compounds that are not aromatic are called alicyclic.

Benzene is an example of an aromatic molecule

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