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Butene: Structural Formula, Boiling Point & Isomers Video

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  • 0:04 Butene Isomers
  • 0:50 Constitutional vs.…
  • 1:50 Why Different Boiling Points?
  • 3:02 Lesson Summary
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Lesson Transcript
Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson we will learn about the isomers of butene and how they are related to each other. We will also learn how the structures affect the boiling point.

Butene Isomers

Butene has a variety of uses, from the fuel in your car to the grocery bags you carry home! The chemical formula for butene is: C4 H8, which means it's made up of four carbon atoms and eight hydrogen atoms. The '-ene' part of the name refers to an alkene, so we know that butene's structure must include a carbon double bond.

There are several different isomers, or molecular structures, that this compound can form:

  • alpha-butylene (but-1-ene)
  • cis-beta-butylene - ((2Z)-but-2-ene)
  • trans-beta-butylene - ((2E)-but-2-ene)
  • isobutylene (2-methylprop-1-ene)

Though they all share the same formula, their structures differ, as you can see in this diagram:


Structures of the isomers of butene
isomers


You don't have to memorize what each of these structures look like, but you should take a quick second to pause and get a general sense of what they look like before you move on with this lesson.

Constitutional vs. Cis-Trans Isomers

The relationships between each of these isomers are mostly constitutional, meaning they have the same molecular formula but different bond connections (order). Meaning to transform them, you would have to just move around the hydrogen and carbons; in other words, the atoms and functional groups.

The exception is between cis-beta-butylene and trans-beta-butylene. Let's take a second to get into this.

Have you ever heard that trans fats are bad for you but unsaturated fats are good for you? Did you know that the only difference between these two fats is that one has a trans bond and the other has a cis bond? This slight differentiation can make a big difference in the function of the molecule.

With cis-beta-butylene and trans-beta-butylene, the atoms are in the same order, but the polarities are different. The cis isomer is polar, with both CH3 groups on the same side. This makes it really bulky and difficult to stack.

The trans isomer is non-polar, and the bulky CH3 groups alternate, giving more space in the molecule. This relationship is called cis-trans isomerism.

cis isomers are polar, while trans isomers are not
cis-trans

Why Differing Boiling Points?

Even though each of these isomers of butene are made up from the same materials, they each have different physical properties. Let's look at the boiling point of each:

  • Cis-beta-butylene: 3.7 degrees C
  • Trans-beta-butylene: 0.8 degrees C
  • Isobutylene: -6.9 degrees C
  • Alpha-butylene: -6.3 degrees C

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