Alkenes: Definition, Properties & Examples

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  • 0:00 What Are Alkenes?
  • 1:38 Properties of Alkenes
  • 2:38 Uses of Alkenes -…
  • 3:37 Uses of Alkenes -…
  • 4:43 Lesson Summary
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Lesson Transcript
Instructor: Nissa Garcia

Nissa has a masters degree in chemistry and has taught high school science and college level chemistry.

Alkenes are organic compounds that are recognized by the double bonds that are present in their chemical structure. You can find alkenes in things such as rubber and plastic products that we use every day.

What Are Alkenes?

Alkenes, also known as olefins, are organic compounds that consist of carbon and hydrogen atoms with one or more carbon-carbon double bonds in their chemical structure. Alkenes are unsaturated hydrocarbons. They are hydrocarbons because they are made of only carbon and hydrogen atoms, and they are unsaturated because they have one or more double bonds in their chemical structure.

Let's take a look at the general chemical structure of an alkene (RC=CR'). We can see two carbon atoms that are bonded together with a double bond, and they can be illustrated in two ways. The first way is one where the carbon atoms are shown as the letter C with lines indicating the bonds. The other way is to represent the carbon atoms with a bend in the line. These carbon atoms have R and R' side groups attached to them. These side groups can be any group of carbon and hydrogen atoms that are bonded together.

Let's compare the chemical structures of two alkenes, isobutylene and 1-pentene. We can see that they are alkenes because they have carbon-carbon double bonds. Besides that, notice their names; do you see anything in common? They both end with -ene. Alkenes are conventionally named with the suffix -ene.

These two examples are chemical structures of alkenes that look like carbon chains that can have branches. Now, let's imagine attaching these carbon atoms to each other and closing them to form a ring. Here, we have another type of alkene called a cycloalkene, which has rings of carbon atoms bonded together, and within the rings, it contains one or more double bonds.

Properties of Alkenes

What does an alkene look like? Are they solids, liquids, or gases? The answer is: all of the above. Earlier, we talked about alkenes as chains of carbon and hydrogen atoms bonded together. The physical state (solid, liquid, or gas) of alkenes is dependent on the number of their carbon atoms.

Simple alkenes, like ethene and propene, that have two to four carbon atoms generally exist as gases.

Alkenes with five to sixteen carbon atoms are generally liquids. Alkenes having seventeen or more carbon atoms are generally waxy solids, like paraffin wax, which is used to make candles.

While the physical state of alkenes varies depending on the number of carbon atoms, what alkenes have in common is that they are both insoluble in and less dense than water. However, they are, in general, soluble in organic solvents, so they will dissolve if you try mixing an alkene with an organic solvent like acetone or benzene.

Uses of Alkenes --Plastics and Rubber

Alkenes are very important chemical compounds that have found their way into our homes and are part of our everyday lives. Let's take a look at some common uses of alkenes.

In our homes, we most likely have plastic products like bottles and storage containers. Seemingly simple alkenes, for instance ethene and propene, are essential for the manufacture of polymers like polystyrene and polyvinyl chloride (PVC) that are needed to make plastic products.

An alkene, 1,3-butadiene, is also responsible for the manufacture of plastic products like our toilet seats, which is something that we use every day. Besides plastic products, 1,3-butadiene is also used to manufacture rubber products like golf club heads and bumper bars. Do you play golf? Do you ever wonder about the good shock absorption when you hit a golf ball with your club? This is because of the rubber that is on the golf club head. Bumper bars found on our cars or trucks are very helpful in preventing physical damage to the car.

Uses Of Alkenes--Chemical Production

Besides manufacturing plastic and rubber products, alkenes are used to make other chemicals like gasoline and detergent additives. For instance, the alkene isobutene is used to make methyl tertiary-butyl ether (MTBE), a gasoline additive that improves a vehicle's octane rating, which refers to the compression that a gasoline can take before it ignites. Having a higher octane rating is better for gasoline because if it ignites, it may damage the engine. So, by having MTBE as a gasoline additive, the octane rating of the gasoline improves, and there is a lower risk of engine damage.

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