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Current time:0:00Total duration:13:45

Video transcript

let's think about what type of reaction we might be able to get going if we had some methane so if we had some methane and some molecular chlorine and some molecular chlorine so if we just let this be and we didn't heat it up or put any UV light into this reaction pretty much nothing will happen both of these molecules are reasonably happy being the way they are but if we were to add heat into if we were to start making all the atoms and molecules vibrate more and bump into each other more or we were to add energy in the form of UV light what we could start doing is breaking some of this these chlorine chlorine bonds out of all of the bonds here those are the weakest that would be the most susceptible to breakage so let's say we were to add some heat what would happen so let's see let me draw the valence electrons of each of these chlorines this chlorine has one two three four five six seven valence electrons and this chlorine over here has one two three four five six seven valence electron to break what's going to happen we haven't drawn an arrow like this just yet but what's going to happen is that each of these chlorines this bond just going to break each of these chlorines are just going to take their part of the bond so this guy on the Left he's just going to take his electron and notice I draw it with this half-hour oh it looks like a fishhook it's just half an arrowhead this means that this electron is just going to go back to this chlorine and this other magenta electron it's going to go back to the right chlorine so we can draw it like we could draw it like that my if I you know if it was up to me I would have drawn it more like this I would have drawn it more like this to show that that electron just goes back to the chlorine but the convention shows that you could show that half of the bond is going back to the entire atom is going back to the entire atom now after this is happens what will everything look like well we're still going to have our methane here it hasn't really reacted so we still have our methane let me draw it a little bit so we still have our methane here and all that's happened because we've put energy into the system we've been able to break this bond the molecular chlorine has broken up into two chlorine atoms so we have the one on the left over here we have the one on the left over here and then we have the one on the right and let me draw the lefts valence electrons it has one two three four five six seven I just flipped it over so that the lone electron is on the left-hand side right here and then you have the guy on the right he has one two three four five six seven valence electrons now now that each of these guys have an unpaired electron they're actually very very very reactive and we actually call any molecule that has an unpaired electron and is very reactive a free radical so both of these guys now are free radicals and actually the whole topic of this video is free radical reactions both of these guys are free radicals and you've probably heard the word free radical before in the context of nutrition that you don't want free radicals running around and it's the exact same idea it's not necessarily chlorine that they're talking about but they're talking about molecules that have unpaired electrons they'll react with some of the stuff that your cells machinery maybe even with your DNA maybe cause mutations that might lead to things like cancer so that's why people think you shouldn't have free radicals in your body but as soon as we form these free radicals and this step right here where we put energy in the system to break this bond we call this the initiation step so let me put this we used energy here this was endothermic we use energy this right here is the initiation step initiation initiation step and what we're going to see in general with free radical reactions is you need some energy to get started but once it started once it gets started it kind of starts this chain reaction and it was one free radical reacts with something else it creates another free radical and that keeps propagating until really everything has reacted and that's why these can be so dangerous or so bad for biological systems so I've told you that they react a lot so how will they react now well this guy wants to form a pair with someone else and maybe if he swipes by this methane in just the right way with just enough energy what will happen is he could take the hydrogen off of the carbon and not just the proton the entire hydrogen he will form a bond with the hydrogen using the hydrogen's electrons so they'll get together and they'll form a bond the hydrogen will contribute one electron notice I'm drawing the half arrow again and so the hydrogen isn't giving away the electron to someone else that would be a full arrow the hydrogen is just contributing its electron to half of a bond and then the carbon the carbon would do the same under that in blue so the carbon this valence electron right here could be contributed to half of a bond and then they will bond and this bond over here will break and so the carbon over here on the Left this carbon over here will take back will take back its electron so what does it look like what does everything look like after that's done so our methane now it's no longer methane it is now if you think about it so we have three hydrogen's it took its electron back it is now a free radical it now has an unpaired reactive or electron the hydrogen and this chlorine have bonded the hydrogen and the chlorine have bonded so let's me draw the chlorine it has this electron right over here it has the other six valence electrons 1 2 3 4 5 6 and we have the hydrogen we have the hydrogen with its pink electron that it's contributing to the bond and so we we have them bonded now this chlorine is no longer free radical although this one out here is still a free radical let me copy and paste it so let's just cut hanging around copy and paste and now notice we had one free radical react but it could formed another free radical that's what we call this a propagation step we call this a propagation step so this right here is a propagation propagation propagation step when one free radical rate reacts it created another free radical now what's that free radical likely to do you might be tempted say it's going to just react with that other chlorine but think about it these klore these molecules there there's a gazillion of them in this solution so the odds this guy is going to react exactly what that other free radical is actually very low especially early on in the reaction where most of the molecules where most of the molecules are still either methane or molecular chlorine so this guy is much more likely to bump into another molecular chlorine than he is to bump into one of these original free radicals that formed so if he bumps into another molecular chlorine in just the right way so let me draw another molecular chlorine so that's another molecular chlorine and each of these one two three four five six seven one two three four five six seven there is a bond here if they bump in just the right way this chlorine this chlorine electron might get contributed and this free unpaired electron will be contributed can get contributed and then this this the ch3 I guess we could call it this free radical this carbon free radical or this methyl free radical will then form a bond with this chlorine and what's what everything we're going to look like after that well after that happens this is now bonded to a chlorine it's now chloromethane let me draw it so it's carbon hydrogen hydrogen hydrogen now it's bonded to a chlorine let me draw the electrons so we can keep track of everything we have that magenta electron right over there and then we have the chlorine with its 1 2 3 4 5 6 7 of aelion's electrons they are now bonded this is chloro methane and now you have another free radical because this guy and I should have drawn it there this guy that bond was broken so he gets back his electron so he's sitting over here he is now a free radical so this is another this is another propagation step so this is another propagation propagation propagation step and we still have that original free radical guy sitting out over here so we keep forming more and more free radicals as this happens now eventually eventually we're going to start running out of methane we're going to start running running we're going to running out of the molecular chlorines so they're going to be less likely to react and you're actually going to have more free radicals around so once the concentration of free radicals gets high enough then you might start to see them reacting with each other so when the concentration of free radicals get high enough you might see you might instead of this step happening this happened a long time until most of most of the free radicals or most of the non free radicals disappear but once we have a soup of mainly free radicals you'll see things like this you'll see you'll see the ethyl free or the methyl free radical so let me draw it like this you'll see him maybe reacting with another methyl free radical with another methyl free radical with another methyl free radical where they both contribute an electron to form a bond and then once the bond forms you have ethane I could just write a CH 3 H 3 C so you might have something like this and so this type of a step we're two faradic free radicals kind of cancel each out each other out this is a termination step because it's starting to lower the concentration of free radicals in the solution but this is only once the concentration of free radicals becomes really high you might also see some of the chlorines cancel out with each other again so chlorine free radical and another chlorine free radical I'll only draw the unpaired electron they can bond with each other and form molecular chlorine again that again is a termination step or you could see something like the methyl free radical let me just just for shorthand I'll write it like this a ch3 C the methyl free radical and a chlorine free radical might also just straight-up react and form chloromethane and form a CH 3 C CL so this will all happen once the the concentration of free radicals gets really high now another thing that might happen once this reaction proceeds we have a lot of the propagation steps is that you might have a situation where you already have a chloro you already have a chloromethane you already have a chloromethane so it looks like this you already have a chloromethane and once you have enough of these then becomes more likely that some free radical chlorine that some free radical chlorine might be able to react with this thing so it might it might actually add another chlorine to this molecule in the way it would do it this chlorine over here I'm just drawing the free electron pairs it would form a bond it would form a bond with this hydrogen right over there they'd both contribute their electrons and then the carbon would take back its electron the carbon would take back its electron notice all the half arrows and then we'd be left with you'd be left with the hydrogen and the chlorine would have bonded and now this guy is going to be a free radical but he's going to be a chlorinated free radical so it's going to look like this let me see el he has a free electron over there hydrogen hydrogen and then he might be able to react with another he might be able to react with another chlorine molecule he contributes an electron maybe this guy contributes an electron this guy I want to draw a full arrow he contributes an electron to a bond and then this guy takes his electron back and becomes a free radical and becomes a free radical and then we're left with what we're left with a we're left with a doubly chlorinated chlorinated methane so then we have CL CL and then a hydrogen and a hydrogen and this could actually keep happening you might keep as the concentration of these get higher then it becomes more likely that this can react with another chlorine and of course this chlorine over here becomes another free radical but the general idea here that I wanted to show you is that once a free radical reaction starts and this one this the first step requires some energy to break this chlorine chlorine bond but once it happens these guys are highly reactive will start reacting with other things and as they react with other things it causes more and more free radical so it starts this chain reaction and actually all in all this required energy to occur this step right here this propagation step it requires a little bit of energy but it's almost neutral it required energy to break this bond but it creates energy when this bond is formed it still requires a little net energy and then things like this start to become exothermic and especially once you start getting to the termination steps once you start getting to the termination steps you start releasing a lot of energy so it's actually all in all this reaction is actually going to release energy but it needed some energy to get started