Petroleum Defined

When you think of chemistry, images of test tubes containing mysterious boiling liquids may come to mind. Or a lab coat wearing scientist with crazy hair and thick goggles who adds two chemicals together, causing a loud, booming reaction may pop into your head. Of all of the images ‘chemistry’ conjures up, petroleum probably isn’t one of them. But ‘petroleum’ actually is a big part of chemistry and without the brilliant petroleum chemists out there figuring out how to get usable materials out of crude oil (or petroleum that has yet to be refined), our world would be a lot different.

Before we delve into the world of petroleum in chemistry, let’s get some vocabulary under our belt. The word petroleum translates into ‘rock oil’ in Latin, and that’s a pretty fitting name because it’s a thick, mixture of hydrocarbons found under the earth’s surface that can be broken down, or refined, into other stuff (we’ll get there shortly). Petroleum is the result of dead plants and animals that were buried and exposed to immense pressures and temperatures for over 300 million years. We get petroleum out of the ground by digging a well and pumping it out.

A pump jack can be used to obtain petroleum onshore in regions where the petroleum is plentiful

Oh, and a hydrocarbon is a substance that contains the elements carbon and hydrogen. Specifically, a hydrocarbon has a ‘backbone’ made up of carbon and hydrogen atoms are attached to that backbone.

The black balls represent carbon and the grey balls represent hydrogen. This hydrocarbon is named octane since it has eight carbon atoms

Now that you’re a little more familiar with petroleum, let’s take a look at petroleum in chemistry, starting with industrial fractions.

Industrial Fractions

Petroleum, as previously mentioned, can be broken down into usable materials. In fact, you may not realize it, but a large percentage of the items you depend upon probably come from petroleum. For example, the fertilizers for your lawn, some of the ingredients utilized when your shirt is dry-cleaned, the Vaseline in your chap stick, and even the asphalt on the highway all come from petroleum. And that’s not to mention the fuel for your car and home! See, life certainly would be challenging without petroleum. All of these items are industrial fractions, or the parts of petroleum that get broken down based on their chemical makeup.

Breaking up the petroleum into its fractions can be a complicated process, but it usually begins with distillation, or heating up the crude oil and separating it based on the different boiling points of the fractions.

Very simple distillation. The crude oil is the impure liquid that gets separated based on the boiling points of the substances within it

Before we move on, let’s take a moment to look at some of the fractions that come from petroleum in the table, which is sorted by items with the lowest boiling point to items with the highest boiling point.

Name Uses
Petroleum Gas Liquified petroleum gas is used to heat homes, some is used in fertilizers
Gasoline Fuel for cars, dry cleaner ingredient
Kerosene Fuel for lanterns
Fuel oil Fuel for buses, trucks, generators
Lubricating oil Lubrication for certain types of machines
Paraffin wax Ingredient in Vaseline, candles, certain makeup, and match sticks
Asphalt Material in roads


While distillation is utilized, cracking is also often used to break petroleum into its fractions as well. Like the name implies, a large chained hydrocarbon is ‘cracked’ or broken up, into smaller chains. Large hydrocarbons with a lot of carbon and hydrogen atoms can be broken down into smaller hydrocarbons with the use of pressure, heat and/or a catalyst, or a substance that makes reactions occur faster. This process is often used to make gasoline and diesel fuel from larger hydrocarbons.

Cracking. Note that a large alkane, or type of hydrocarbon, is broken down into smaller alkanes

Octane Number and Anti-Knocking Compounds

Since gasoline is probably one of the more important hydrocarbons in your life, let’s take a moment to explore it in further detail, starting with the octane number. This number measures how well fuel avoids ‘knocking’ in engines. Wait, what does all of this mean? Excellent question. Normally, air and fuel are compressed in the engine and then a spark plug ignites this mixture. Occasionally, the fuel can ignite before the spark plug does its thing, which results in a knocking sound, among other problems. Fuels with a high octane number avoid igniting before they’re supposed to.

So, what exactly are these ‘anti-knocking’ compounds found in fuel? Let’s delve a little deeper. Gasoline contains numerous hydrocarbons, such as octane. You might remember from the image earlier that octane has eight carbon atoms and is a chain. Another hydrocarbon found in gasoline is called iso-octane.

Remember octane from before? Note the chain structure

Like octane, iso-octane is made up of eight carbon atoms, however, iso-octane is branched. The chained hydrocarbons, like octane, are more likely to cause knocking in engines, whereas the branched hydrocarbons, like iso-octane, resist the pre-ignition problem that results in knocking.

Iso-octane has a branched structure