Aliphatic Hydrocarbons: Definition & Properties

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  • 0:02 Aliphatic Hydrocarbon
  • 3:34 Properties
  • 5:50 Lesson Summary
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Lesson Transcript
Instructor: Elizabeth (Nikki) Wyman

Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.

Learn about aliphatic hydrocarbons and discover where you can find these types of compounds in everyday life. Understand the definition and properties of aliphatic hydrocarbons.

Definition of Aliphatic Hydrocarbon

Check out some of these symbols:

Flammable symbols
signs with flammable symbols

Have you ever seen them on anything before? If you haven't seen them, where might you expect to see them?

Maybe in a flammables container in the chemistry lab. Maybe on the highway, or on the train tracks. Maybe when you're getting ready to grill, or maybe when you've been camping with a portable stove.

What kinds of compounds are these symbols warning us about? What makes them potentially dangerous?

Some of the most common, flammable and volatile compounds are aliphatic hydrocarbons. Part of this class of compounds is probably somewhat familiar to you. Does the term hydrocarbon ring any bells?

The term hydrocarbon means that the compound is made of--you guessed it--hydrogen and carbon atoms bonded together through the sharing of electrons. Carbon-based compounds are often referred to as organic compounds.

The term aliphatic stems from a Greek word meaning fat or oil. It's thought that some of the first aliphatic compounds discovered came from fats or oil. In our case, aliphatic refers to hydrocarbon compounds that are straight chained, branched or non-aromatic.

Put these two terms together and you have the definition for an aliphatic hydrocarbon - a carbon-based compound that is straight, branched or non-aromatic. An aliphatic compound may be a straight chain made of carbon and hydrogen bonds, like butane, or it may be branched, like isobutane.

structure of butane and isobutane

Notice that in a straight chain, the carbon atoms are never bonded to more than two carbons at a time. Look at butane on the left. How many carbons is the end carbon bonded to? How many carbons are the middle carbons bonded to? In a branched compound, there may be three or four carbons bonded to a single carbon. In isobutane, the outside carbons are only bonded to one carbon each, while the central carbon is bonded to three carbons.

An aliphatic compound may also be non-aromatic, like cyclobutane. A non-aromatic hydrocarbon is cyclic and not of an especially stable nature, or alicyclic. These compounds include cyclopropane, methylcyclohexane, and heptalene.

Hydrocarbons that are cyclic and especially stable are called aromatic hydrocarbons. There's a lot to say about aromatic hydrocarbons, so we've created another video just for them! Check it out if you want more information.

Aliphatic hydrocarbons may contain elements other than hydrogen and carbon. It's not uncommon to have oxygen, nitrogen, sulfur or chlorine atoms present.

An aliphatic hydrocarbon can be either saturated or unsaturated. When saturated, a hydrocarbon contains only single bonds and contains the maximum number of hydrogens. Unsaturated hydrocarbons are the opposite. They may have one or more double bonds and less than the maximum number of hydrogens.

When a hydrocarbon has only single bonds, it is known as an alkane. When there is at least one double bond, then it is known as an alkene. A hydrocarbon that contains at least one triple bond is known as an alkyne. Aliphatic hydrocarbons may be alkanes, alkenes or alkynes.

Properties of Aliphatic Compounds

I know we should probably wait until last to talk about the most exciting property of aliphatic hydrocarbons, but let's go with the best first.

Aliphatic hydrocarbons are generally quite flammable. They react with oxygen in the atmosphere to produce carbon dioxide, water, and energy in what are known as combustion reactions. All you need to get them going is a little spark.

We burn methane gas (CH4), a very simple aliphatic hydrocarbon, in our homes, fireplaces and even in our vehicles! Propane is a popular gas for grilling, powering small lamps, water heaters - you name it. These flammable gases must be properly labeled and stored in pressured vessels so they don't escape and cause trouble. Methane smells like dead things, which helps us identify when there is a leak.

Liquid aliphatic hydrocarbons, like butane, hexanes, and octane, have really high vapor pressure. This means that molecules can easily escape the liquid phase and enter the gas phase, leading to high-density collection of gas particles right over the surface of the liquid. This gas is easy to light, so careful! A fire started over a flammable liquid is one you need to have an extinguisher for. Even solid aliphatic hydrocarbons can ignite. Waxes and fats can sustain a flame bright enough to read by.

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