welcome back class so I want to apologize for my light shirt I know that's going to make it a little more difficult to see what I'm writing on the board it's not impossible to see but just a little bit more strenuous on the eyes and I usually remember to not wear a light shirt when I record these videos but today I'm I forgot so my apologies so the first reaction that we want to talk about today is the chlorination of methane so here's our methane here's our chlorine and you can see that we have our methyl fluorine right here as our product now when we look at this we have how do I know it's a radical mechanism because it's going to tell you it's going to often have something like light whenever you see light above the arrow here it's it's definitely going to be a radical mechanism another thing if you want this methyl chlorine right here you have to use the methane in excess and you're going to limit the amount of chlorine that you have because if it was the other way around if you had your methane and you had excess chlorine all right plus excess chlorine what you're going to do is get poly uh chlorination so you'll get something like this you'll you'll replace all four hydrogens with a chlorine atom whereas when you do it this way you see that you only get replace one hydrogen with the chlorine unless explicitly stated in the problem you're typically going to have the mindset of this reaction right here you're only going to add one chlorine but if you have excess chlorine note that it's going to replace all of the hydrogens but whenever we're going to talk about this reaction or this condition we have to explicitly tell us or write it down and say what's going on okay but when it's just written like this okay forget the product side when it's just written like that you're just going to assume that is an excess that is limiting and that will promote only one chlorination event so what we want to be able to do is look at this and how does the mechanism work and remember there's going to be three steps there's the initiation step and then the propagation step and the termination step the propagation step well I'll stop there let's go back to the initiation step so what's going to happen is maybe I should label it we'll label it for one for the initiation step so we are going to have our chlorine chlorine fluorine bonds rather large a large Bond and when we treat it with light it's going to turn into a radical so that chlorine chlorine bond so we already have the lone pairs it's going to the light is going to bust that chlorine chlorine bond to give us two radicals right and oftentimes it's just easier to show that this is a radical by not drawing in the lone pairs if you just go like that then I know we have some radicals and we're going to have two of those radicals so that is our initiation step so in step two where the propagation step we're going to have our methane and I'm going to draw it just a little bit differently you see how I I still have four hydrogens I just drew three over here and then just one extended and what's going to happen is we're going to have our chlorine radical here and I'll draw the radical on that side this time and what's going to happen is it's going to react in such a way that it's going to form another radical and so we're going to have this carbon hydrogen bond now react in this manner like that so when we follow those arrows I see this radical is reacting with one electron from the carbon hydrogen to give me HCL hydrochloric acid but what do I have left over I have my H3 boom like that now I have a carbon radical now I can take that carbon radical and do a abstraction here so I can take my carbon radical right there and then I can treat it with another chlorine molecule now when we do the initiation step the light does not convert all the chlorine in the reaction into radicals it converts a very small fraction of the chlorine into the chlorine radicals so we still have an abundance of chlorine present and so what's going to happen now is we are going to do that halogen abstraction step where we're going to have this come this come like that and then look at this what do we get we now have made our product oh wait made a mistake that is not a hydrogen the arrows are showing it has to be a chlorine but what else did I generate I generated another chlorine radical so now this can keep going and it can keep reacting with another methane molecule and when you do this reaction you'll generate more radicals so the reaction propagates itself it keeps doing what it's going to do until all the radicals are gone it's then for the reaction to stop or the termination step we have to get no more radicals and so there's three ways that this can happen you can have two chlorine radicals get real close to each other and that will turn back into chlorine so that will stop the reaction because you can see two radicals reacting and now in the product there's no more uh radicals that's one possibility you could in fact have what we've seen already this is also a termination step when you form the product okay so this step right here is um this step right there well let's get it well it's not a termination we're getting the same product you see how this and this is the product there but it did it in a different way it form our product but no radicals were created so that's a termination step so yes you can get some of your product from a termination step this and this are the exact same thing I just have the hydrogens here swapped in different orientations but that that's okay now another radical uh step or a termination step that you could Envision happening and I'm going to put it over here you could have a methyl radical get really close to another radical methyl radical and when those two species get close together you could uh have them react shown with our arrows there and then what would we generate we would generate ethane so that would kind of be like a side product that we're not too interested in because our whole goal was to generate this compound right there but that's how the reaction can stop from these one two three termination steps and eventually you're going to use up all your radicals and the reaction will stop okay so if we take a look at if we had excess chlorine what we have now is a process called a chain reaction and so if that chlorine was in excess what's going to happen is you're going to then replace another hydrogen with chlorine so now we're at ch2cl2 and then that's going to react again if you have excess chlorine cl3 and then eventually you will get all of the chlorine to replace all the hydrogens all right so this right here when it just keeps going and going and going we call that a chain reaction all right and so the mechanism from chain reaction to chain or this process right here is going to be basically the same thing that you see right here it's just now the this starting molecule right here looks a little bit different all right so what would the propagation step or yeah well with the propagation step be for this guy right here I'll start us off and then you guys can figure it out on your own this would be a good step here or a process here so if I have our c h three I could draw it like this let's draw like H2 carbon H like that and then what's right there there's my chlorine so this molecule is this molecule there so what would happen here we could have a chlorine radical right here and we would just do the same mechanism here that's not how far I want to go we could go like this foreign radical just like that and then you could take this species right there and do this same step right there right there okay perfect so let's let's take a look at some other things now I'm going to clean this board now let's take at let's take a look at radical initiators radical initiators are going to initiate the radical reaction right now what we've seen so far is when we take a look at chlorine and if we shine some light on it so we shine some light we're going to get a radical process going and then we would get our two radicals like we've seen already right okay but this chlorine chlorine bond is strong in comparison to other bonds let's say if we take a look at peroxide like that if we shine some light on this guy we will have a radical process here a homolytic cleavage and we could get some a radical like this now what's the difference here between these two they're both doing the same thing well please also remember that you could also initiate a radical mechanism by adding heat so if we look at it in terms of heat this chlorine chlorine bond is going to be stronger than this oxygen oxygen bond in this peroxide so the energy required to break the chlorine bond is 200 43 kilojoules per mole but to break the peroxide it's a whole it's a whole lot less 159 kilojoules per mole so what that does for us is like this reaction in order to get the chlorine to turn into radical it has to be temperatures greater than 100 degrees C but to do this reaction it's going to be around 80 degrees C so there's a difference in temperature so we don't have to use such harsh conditions if you look at it in terms of light for chlorine you're going to have to use UV light and for the peroxide you you can use lower energy light to get the radical to get going so we also have acyl peroxides where it looks something like this like that almost looks like an anhydride but it's not it's an acyl peroxide and so we could form China with some light or some heat same concept here we'll generate our radicals but this auction oxygen bond is even less strong than this oxygen oxygen Bond so it even jumps down or decreases the amount of heat to 121 kilojoules per mole so a little bit easier to work with so it's just about making these initiating steps a little bit easier so that's that's pretty cool so you could Envision here and I just noticed that I chopped off all of this right here as I'm looking at the computer screen now so I bet you can already figure out what it is if you listen to what I said these this is just talking about light and heat so that's what all these symbols here so the symbols for light is what planks constant times new so that symbol right there is what is being cut off over here so my apologies now you can have radical initiators but then you can also have radical inhibitors pivot tours all right you got to step us to the side a little bit so you can see that that so a very common radical inhibitor is Elemental oxygen so O2 when we take a look at O2 in its Lewis structure we can see that it looks like this but you could also represent that uh oxygen molecule or that Lewis structure like this we could take one electron put it there and then one electron put it there and then we could see something that looks like this and now that auction looks like a die radical and that di radical right there is going to consume any radical that's in the reaction so if you're trying to do a radical reaction you have to remove all the elemental oxygen from the reaction because that's going to terminate the reaction another inhibitor that's common commonly used is the hydroquinone so let's draw what that is another radical inhibitor we'll we'll put it right here so Hydro that's a q been on like that okay and the way that molecule looks maybe I can get it down here let's see if I can I think I can like that so we have a Benzene ring with two alcohols attached to it so that's hydroquinone just know that that is a radical inhibitor right well by looking at the time of the video I think this is going to be a good time to stop we're going to end this video now