 In this video we're going to be looking at how we write some equations, we construct models to represent reactions of saturated hydrocarbons when substituted with halogens. So a couple of differences. Firstly, we're going to use some models to help us do this. We have to make sure that we can write the equations and we also want to look at our saturated hydrocarbons in a substitution reaction. So a couple of differences between substitution reactions and the addition reactions we looked at in the previous video. You remember when we looked at addition reactions, we went from two reactants to one product and that's what we get for addition. When we look at a substitution reaction, however, what we are looking at in our substitution is two reactants going to two products. So this is a difference already. One of the simplest ways to do this is to look at the substitution reaction when we have a halogen reacting with a hydrocarbon, in this case a saturated hydrocarbon. So we're going to use ethane. We're going to add it to chlorine and we're going to see what we get. So here is ethane. Ethane has two carbons and six hydrogens and this particular molecule is an alkane because it has a single bond between the two carbons. If I was to add chlorine, a chlorine molecule Cl2 to my ethane, it's not going to add across a double bond. So I'm not going to be able to push both of these chlorine atoms into my ethane in the same way that I did with ethane. With ethane, the double bond breaks and both of the two chlorine atoms will add across the double bond. So I'll end up with just one product. In this case though, we have a substitution reaction. So what has to happen in order for me to get a chlorine into this structure is the chlorine must substitute for a hydrogen. Now unlike addition reactions, when that happens, we end up with two products. So we've started with two reactants and we've ended up with two products. So the two products are the halogenated alkane and the hydrogen halide. So let's have a look in this case. We would have chloroethane and hydrogen chloride. So let's write that equation down. Our product is going to be chloroethane, but we're also going to have hydrogen chloride, incidentally not hydrochloric acid, as a second product. So this is a little bit messy. Let's just have a quick look at the structural formula. H H H H H H H plus Cl2 is going to give us C C Cl H H H H H plus HCl. If we contract those a little bit further, you can see we've got C 2 H 6 plus Cl2 gives C 2 H 5 Cl plus HCl. So here are a range of different equations that we can write to represent the same process. Remember the keys to the substitution reactions are that we don't have a single product, but we have two products. Both of the carbonate, the chlorine atoms are not going into the organic structure. Only one of them is. And in order to do so, it has to push out a hydrogen in order to do that. We can have multiple steps in our substitution reactions. So we can substitute more hydrogens to increase this to a di-chloro or a trichloroethane. And again, we need to look at rules like Markovnikov's rule, which I very briefly mentioned in the addition reaction video, for where we already have one halogen present. So if we look at this molecule here, and we would then to add another chlorine molecule to it, what we would find is that the substitution would occur on the carbon that already has the chlorine, which is this one. So we would find that, say for example, this particular hydrogen would be the one that would be substituted with a chlorine in order to give us another molecule of hydrogen chloride. And so we would have a 1-1 di-chloroethane. This is another one of these areas where you want to have a look at a range of different types of examples in order to get some of these things set in your mind. So thanks for watching.