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Current time:0:00Total duration:9:09

Markovnikov's rule and carbocations

Video transcript

in the last video we saw a potential mechanism where if we reacted hydrogen bromide with this alkene right over here then we can essentially add we had the addition of this of this halide to but it was started as an alkene and then it ended up it ended up as to bromo pentane as an alkane but when we did that we made a I guess you could call it a somewhat arbitrary decision or I didn't explain why we made the decision we said look this hydrogen is going to be partially positive because this guy is so electronegative and maybe when it's partially positive it'll be attracted maybe it'll just bump in just the right way into one of these carbons it'll maybe swipe its electron we somewhat arbitrarily in the last video decided that it would swipe this guy's electron but you could just as easily imagine a world where it swipes an electron from this guy so let's draw a mechanism for that and just think about which one is more likely to actually happen so what happens so once again this guy let me draw all of his valence electrons so there's the bromine one two three four five six seven valence electrons you have the hydrogen I'll do it the same color the hydrogen has its electron right there this is partially positive and this is partially negative the hydrogen might want to swap one might want to swipe one of these electrons away let's do it from this guy right here let's do it from let's do it from this guy so he has he has this electron right over here so the other side of that bond it goes to the hydrogen when the hydrogen goes near it or maybe it's attracted to it and when it goes to the hydrogen then the hydrogen lets go of the electron that the bromine wanted all along because it's so electronegative so then that electron goes to the bromine so after we do that what will it look like what will be the next step in our reaction and it will look fundamentally different than this right over here so now what happens now what happens so we have a carbon bonded to two hydrogen's two hydrogen's and only has a single bond to the other carbon which is bonded to the original hydrogen over there the rest let me write my hydrogen's a little bit so let me write this whole thing a little bit neater so you have your carbon bonded to a hydrogen and another hydrogen and now it only has a single bond to this carbon right here which is bonded to a hydrogen and then the rest of the chain let me just draw the rest of the chain right here and now this electron went to the hydrogen the other electron that was paired with is still with this carbon so now this carbon is now bonded to that hydrogen over there so this blue electron is now with the hydrogen so let me draw so the blue electron that was here is now gone over to the orange hydrogen has now gone over let me draw a little bit neater than that has now gone over to this hydrogen right over there and then the hydrogen lost its electron to the bromine so the bromine originally had 7 valence electrons 1 2 3 4 5 6 7 and then it nabbed an extra electron from the hydrogen so now it will have a negative charge it is a negative ion is bromide it's a bromide anion I guess you could call it and since this guy lost an electron he had four valence electrons lost one to the hydrogen he is now he now has a positive charge he's a carbo cation he's a carbo cation and then from here so notice the difference before this guy lost the electron and so the hydrogen bonded to this carbon in this situation this guy lost the electrons to the hydrogen bonded to the other carbon and so you can imagine from here something very similar happens is what happened in the first video but now it happens to this carbon right over there so let's do that this carbon is positive the bromb the bromide ion is obviously negative so maybe he'll want to swipe his electron away so this electron then goes to the carbo cation and then it will form a bond it will form a bond this green one will go to the carbon cation and then this purple one still stays with the bromine and so they'll have a bond there paired up you can imagine it so then we're left with we have a carbon we have our original hydrogen's we have this carbon that hydrogen the rest of the chain the rest of the chain ch2 ch3 and then you have this hydrogen right here that it bonded to that was kind of our first step and now the bromine has bonded to this carbon right over here the bromine has bonded over to that carbon right over there and we're done this is another this is another possible mechanism this one we ended up with two bromo pentane right because it's on the number two carbon here we have one bromo pentane one two three four five still five carbons it's just the bromine is attached to the one carbon here attached to the two carbon here so we now need to think about because you know a lot of first cut these both seem like reasonable mechanisms but if you did it experimentally if you do it experimental you would see that this is the one that you would really observe I haven't actually I actually haven't done this exact experiment so I don't know the proportions but you're going to observe this one disproportionately the great majority of the products that you see are going to be this one not that one and so the question is well you know they both seem like reasonable things to do up here why is this one so much more likely to happen than that one it all comes from something called markovnikov's rule markovnikov's rule maher qov Nick Hobbes Nick Hobbes rule and there's a couple of ways to think about it and when markovnikov thought it up or he observed it it seemed to work it they weren't 100% sure about why it worked we can't even think a little bit about why it worked so markovnikov's rule a couple ways you could think about it you could think of it as the thing that already has more hydrogen's is more likely to get more hydrogen's so that's what happened here this thing had more hydrogen's on it than this then the right carbon right here this right carbon had a hydrogen's but it had some other I guess you'd call it an alkyl group attached to it and so the thing that had more hydrogen's ended up with the hydrogen and then the thing that had more groups this character right here had more groups right he had one group over here this carbon over he had no groups he ended up with the bromine so the thing that has more hydrogen with more hydrogen's the things that ends up that has more groups ends up with more groups so I guess you kind of go more in the direction that you were going in but that still is just you know a rule so you know why does that make sense it all makes sense or it starts to make sense when you think about that in both mechanisms we had to have a carbo-cation right we talked about in the last video carbo carbo cation we had a carbo cation right over there this is the left carbon being a carbo cation this is the right carbon being a carbo cation and markovnikov's rule is all all comes from which carbo cation is more stable which one has a lower energy level it turns out that the carbo cation that is bonded to more I guess electron rich electron rich molecules or atoms is going to be more stable if you can imagine that it has more things that look it's positive but it has more carbons around it so it can share some of those electrons you know the electron clouds will help it out a little bit to be a little bit more stable this one right here is only bonded to one other carbon so not as much sharing this is bonded to two so in general so when you're only bonded to one other when you're only bonded to one other carbon you're called a primary carbon primary carbon and if your carbo cation this is a primary carbo cation right here this guy is bonded to two carbons so he would be called a secondary carbon since it's a carbo cation and it's a secondary carbo cation so this right here is secondary secondary so a secondary carbo cation is more stable than a primary and actually a tertiary if you had another carbon group here or something else that had a lot of electrons around it that would be even more stable so the three bonded to three things more stable than two things and when I say two things two things other than hydrogen and then more stable than 1 so the markovnikov's rule all is a byproduct of the fact that this carbo cation is more stable than this one over here and that's because it's secondary versus primary because the secondary can kind of borrow electrons from some of its friends it has more neighbors to borrow electrons than this one and since this is more stable this is more likely to happen this this is a more likely intermediate to have this is a less likely outcome to have in general and that's why you're more likely to get to this left product the 2-bromo pentane then the one bromo pentane