Overview of Free Radical Chain Reactions

Hi and welcome to this video looking at free radical chain reactions. Free radical chain reactions involve three steps that are continually happening. The first step, initiation, produces a pair of free radicals by homolytically breaking a covalent bond. This means that one electron goes with each atom, forming a radical, which is a highly reactive species with an unpaired electron. Propagation uses the free radicals in a reaction with a more stable molecule to produce a new molecule and a free radical and finally termination uses up the free radicals in a reaction which produces stable molecules. For a chain reaction to occur the concentration of radicals has to be kept low enough to avoid the third step the termination but high enough that you still have propagation occurring. We're going to look at each of the steps in turn. The overall reaction that we're looking at is a substitution reaction. This is the reaction that we're going to concentrate on here, the reaction between chlorine and methane. Now, previously you would have said there was no reaction between chlorine and methane. However, now, using a free radical chain reaction, we can substitute one of the hydrogen atoms in the methane for a chlorine atom to produce methyl chloride and hydrogen chloride. The first step in this reaction is the initiation step. And this occurs when UV light breaks the chlorine-chlorine bond. So if we draw out chlorine using a dot and cross diagram, you can see that we have this bond here between the chlorine atoms. If we break this bond using UV light, Then we get two chlorine radicals where each chlorine has an unpaired electron. This makes radicals extremely reactive and means that they can go on and participate in the chain reaction. We can also write this part here as 2Cl dot, the dot meaning that we have a radical. So there are a few different ways that you can write this out. We can have the chlorine molecule with UV light splitting up to give us two chlorine radicals, or you can show it using arrows. to show the direction of movement of the electrons. Now this is what we would look at in advanced higher. So a single headed arrow shows the movement of one electron. So remember this line represents these two electrons that we had here. One electron goes to this chlorine and one electron goes to this chlorine to leave us with two chlorine radicals. Once you've produced a radical, then this can go on to react with stable molecules in propagation reactions. So there are two propagation reactions that are going to happen in our substitution reaction. The first one is our chlorine radical can react with our methane. to steal this hydrogen to form hydrogen chloride, which is one of the products that we were looking at. So the chlorine radical's electron will form a bond with one of the electrons from this bond, and the other electron will go with the methyl part to form a methyl radical. So here we've formed HCl. and a CH3 radical. The other propagation step is where this CH3 radical can react with a chlorine molecule, Cl2, and we have the same movement of electrons. This produces... chloromethane which was one of our products and another chlorine radical. As you can see we have one radical going into the reaction and one radical coming out. This means that these radicals that are produced can then carry on the reaction and cause the chain reaction. We've also produced two products here so we have HCl and CH3Cl. These were the products that we were looking at in the full reaction. The final step is termination. This is where you have two radicals come together to produce a stable molecule. So there are three termination steps for this reaction. We can have chlorine plus chlorine to give you a Cl2 molecule. You could have a methyl radical join with a chlorine radical to produce methyl chloride. which is one of our products, or you could have two methyl radicals join to produce ethane. So you get a very complex mixture of products formed when you have a free radical chain reaction and any of these products could also go on further to be involved in other free radical chain reactions. Draw out the steps in the chain reaction between ethane and bromine. In a reaction between ethane and bromine, the first thing we want to think about is what is our overall reaction. So the overall reaction is one of substitution, where you're taking your ethane molecule plus Br2 and you're substituting. one of the H's for a Br so you'll end up with C2 H5 Br plus HBr. So that's the overall reaction that we're looking at. So the first steps would be in the initiation. And this is always the same, this is where you have your halogen with UV light splitting up to give you two halogen radicals. For the propagation step, the first one involves the bromine radical plus... ethane. The bromine radical takes one of the H's to give you HBr and leave behind C2H5 as a radical. And then the second propagation step takes this new ethyl radical and reacts with a bromine molecule to produce your second product. and a bromine radical which can carry on in a propagation. Your final three termination steps. Your first one can be two Br radicals coming together. To form bromine, you could have your ethyl radical and a bromine radical. to produce bromoethane. And then finally, you could have two ethyl radicals joining together to give you butane. Let's have a look at a summary of the three steps that we've looked at. The first step uses UV light, which is found in sunlight, to split up the halogen bond to produce radicals. This is one of the reasons that chlorofluorocarbons, or CFCs, were a big problem. If they got up into the upper atmosphere, they were able to be split up using the sunlight to produce halogen radicals, which could then react with ozone, which started to break down the ozone layer. Propagation steps, where you have a radical to produce another radical from a molecule. And then finally, termination steps, you have two radicals reacting together to produce a stable molecule. Thank you for watching. I hope that you found this video on free radical chain reactions helpful. Remember to subscribe or follow me on Twitter at Miss Adams Chem for regular updates on new videos.

This means that one electron goes with each atom, forming a radical, which is a highly reactive species with an unpaired electron. Propagation uses the free radicals in a reaction with a more stable molecule to produce a new molecule and a free radical and finally termination uses up the free radicals in a reaction which produces stable molecules. For a chain reaction to occur the concentration of radicals has to be kept low enough to avoid the third step the termination but high enough that you still have propagation occurring. We're going to look at each of the steps in turn.

The overall reaction that we're looking at is a substitution reaction. This is the reaction that we're going to concentrate on here, the reaction between chlorine and methane. Now, previously you would have said there was no reaction between chlorine and methane. However, now, using a free radical chain reaction, we can substitute one of the hydrogen atoms in the methane for a chlorine atom to produce methyl chloride and hydrogen chloride.

The first step in this reaction is the initiation step. And this occurs when UV light breaks the chlorine-chlorine bond. So if we draw out chlorine using a dot and cross diagram, you can see that we have this bond here between the chlorine atoms.

If we break this bond using UV light, Then we get two chlorine radicals where each chlorine has an unpaired electron. This makes radicals extremely reactive and means that they can go on and participate in the chain reaction. We can also write this part here as 2Cl dot, the dot meaning that we have a radical. So there are a few different ways that you can write this out.

We can have the chlorine molecule with UV light splitting up to give us two chlorine radicals, or you can show it using arrows. to show the direction of movement of the electrons. Now this is what we would look at in advanced higher. So a single headed arrow shows the movement of one electron. So remember this line represents these two electrons that we had here.

One electron goes to this chlorine and one electron goes to this chlorine to leave us with two chlorine radicals. Once you've produced a radical, then this can go on to react with stable molecules in propagation reactions. So there are two propagation reactions that are going to happen in our substitution reaction. The first one is our chlorine radical can react with our methane. to steal this hydrogen to form hydrogen chloride, which is one of the products that we were looking at.

So the chlorine radical's electron will form a bond with one of the electrons from this bond, and the other electron will go with the methyl part to form a methyl radical. So here we've formed HCl. and a CH3 radical. The other propagation step is where this CH3 radical can react with a chlorine molecule, Cl2, and we have the same movement of electrons.

This produces... chloromethane which was one of our products and another chlorine radical. As you can see we have one radical going into the reaction and one radical coming out. This means that these radicals that are produced can then carry on the reaction and cause the chain reaction.

We've also produced two products here so we have HCl and CH3Cl. These were the products that we were looking at in the full reaction. The final step is termination. This is where you have two radicals come together to produce a stable molecule.

So there are three termination steps for this reaction. We can have chlorine plus chlorine to give you a Cl2 molecule. You could have a methyl radical join with a chlorine radical to produce methyl chloride.

which is one of our products, or you could have two methyl radicals join to produce ethane. So you get a very complex mixture of products formed when you have a free radical chain reaction and any of these products could also go on further to be involved in other free radical chain reactions. Draw out the steps in the chain reaction between ethane and bromine.

In a reaction between ethane and bromine, the first thing we want to think about is what is our overall reaction. So the overall reaction is one of substitution, where you're taking your ethane molecule plus Br2 and you're substituting. one of the H's for a Br so you'll end up with C2 H5 Br plus HBr. So that's the overall reaction that we're looking at. So the first steps would be in the initiation.

And this is always the same, this is where you have your halogen with UV light splitting up to give you two halogen radicals. For the propagation step, the first one involves the bromine radical plus... ethane.

The bromine radical takes one of the H's to give you HBr and leave behind C2H5 as a radical. And then the second propagation step takes this new ethyl radical and reacts with a bromine molecule to produce your second product. and a bromine radical which can carry on in a propagation.

Your final three termination steps. Your first one can be two Br radicals coming together. To form bromine, you could have your ethyl radical and a bromine radical. to produce bromoethane.

And then finally, you could have two ethyl radicals joining together to give you butane. Let's have a look at a summary of the three steps that we've looked at. The first step uses UV light, which is found in sunlight, to split up the halogen bond to produce radicals.

This is one of the reasons that chlorofluorocarbons, or CFCs, were a big problem. If they got up into the upper atmosphere, they were able to be split up using the sunlight to produce halogen radicals, which could then react with ozone, which started to break down the ozone layer. Propagation steps, where you have a radical to produce another radical from a molecule. And then finally, termination steps, you have two radicals reacting together to produce a stable molecule.

Thank you for watching. I hope that you found this video on free radical chain reactions helpful. Remember to subscribe or follow me on Twitter at Miss Adams Chem for regular updates on new videos.

Transcript for:Overview of Free Radical Chain Reactions

Transcript for:
Overview of Free Radical Chain Reactions