Organic Chemical Reactions: Addition, Substitution, Polymerization & Cracking

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  • 0:02 Addition Reaction
  • 1:58 Substitution Reaction
  • 3:07 Polymerization
  • 4:35 Cracking
  • 5:37 Lesson Summary
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Lesson Transcript
Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Organic molecules can combine, swap atoms, break apart and even make more of themselves. In this lesson, we will explore several organic reactions with examples.

Addition Reaction

Thank you for joining us and welcome to our Saturday night TV lineup. Tonight we have our newest reality show called 'Organic Reactions.' And just a teaser, tonight's show involves a dating portion, a partner swap, a cloning and even a magic show. I can hardly wait, so let's get started.

Now since this show is called 'Organic Reactions', it's worth noting that a reaction is the transformation from one substance to another, and organic refers to a molecule that contains carbon. In this lesson, we'll focus a lot on hydrocarbons, or molecules made up of hydrogen and carbon.

Welcome to the dating portion! Let's introduce our reactants, or the substances that are about to undergo a reaction. Our first reactant is a handsome halogen named chlorine who hopes to find love with the gorgeous organic molecule ethylene.

Chlorine and ethylene
Chlorine and ethylene molecules

Look at this, love at first sight! Wow, they really seem to be hitting it off. It looks like we have made a match. Wait, it looks like they are going behind the curtain for an addition reaction, where two molecules come together to make a larger molecule. While this is occurring, let's take a moment to go over a couple of things about addition reactions.

  • You need a molecule with double or triple bonds. Lucky for chlorine, ethylene fits the bill with her lovely double bond.
  • After the reaction has occurred, no atoms are left over, so it's nice no one is replaced or left behind!

And take a look at this, they have formed one lovely product, or the result of the reaction, named 1,2 dichloroethane, who, like ethylene, is an organic molecule!

What you just witnessed was an addition reaction in an alkene, or a hydrocarbon with a double bond. Ethylene is an alkene. In this addition reaction, the double bonds were broken and replaced with chlorine bonds.

Substitution Reaction

Now we are ready for our date swapping portion. Let's welcome our reactants: yet another chlorine and the organic molecule methane to the stage. Unlike ethylene from our previous segment, methane doesn't have a double bond, so it's called an alkane.

Since this is the date swapping portion, get ready to see a partner switch. If you look closely, it looks like chlorine is breaking up, and one of the chlorines is replacing a hydrogen on the methane. The other chlorine is bonding with the leftover hydrogen.

It looks like we just witnessed a substitution reaction, when two reactants exchange one or more atoms to form two new products. And who are our lovely products? It looks like chloromethane and hydrochloric acid.

Since we've reached the midpoint of the show, let's recap what we've seen so far on 'Organic Reactions': we saw an addition reaction where an alkene named ethylene reacted with chlorine to form 1,2 dichloroethane. Next, we had an alkane named methane who reacted with chlorine in a substitution reaction, where atoms swapped and we ended up with chloromethane and hydrochloric acid.


I understand we have a very special segment now, with another lovely ethylene.

Tonight, ethylene has agreed to show us a polymerization, which is a reaction where a product is produced with repeating units. So, in a sense, ethylene is cloning parts of herself. Well, sort of. Here are some things to know about a polymerization:

  • It works best when the reactant has a double or triple bond. We know from before that ethylene is an alkene with a double bond, so this should run smoothly.
  • The other reactant is often a free radical, or an atom with an electron that isn't paired so it's really unstable.
  • In a polymerization, the free radical attacks the double bond, which opens up the organic molecule (in this case, ethylene), which creates another free radical. Long story short, the free radical creates a chain reaction that causes more and more monomers to attach together.
  • A monomer is the smaller unit and the polymer is just a bunch of monomers linked together. You can think of a monomer as single Lego and a polymer as a bunch of Legos attached together

It looks like ethylene has started, and wow! Look at her go! You'll notice the product here doesn't have the double bonds like ethylene. Those double bonds are broken to create the polymer, which, in this case, is named polyethylene, but you might know it better as plastic!

Polymerization produces products lacking double bonds
Process of polymerization


Thank you, polyethylene! That was fantastic. I'd like to introduce our final guest, Mr. Hydrocarbon. You might remember from earlier in the show that a hydrocarbon is made up of mostly carbon and hydrogen atoms, but did you know that hydrocarbons are found in gasoline and other petroleum products?

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