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Current time:0:00Total duration:13:29

Video transcript

in the video on e 2 reactions I showed you how a strong base and all this happens simultaneously a strong base can nab a hydrogen off of this carbon right here it's just nabbing the proton itself it's not grabbing the hydrogen and the electron and then that electron goes to this carbon right over there and then that allows that carbon to give away the electron that was forming a bond with the chlorine to go with the chlorine this all happens at once the chloride got eliminated now one thing that might pop out in your brain is why did I pick this hydrogen why did I pick the hydrogen right over here why couldn't I have picked why couldn't I have picked that hydrogen over there and I'm going to introduce you a little bit of terminology and then I'll introduce you to a rule and then I'll tell you a little bit about why people think this rule works so in general the carbon that has the functional group on it that's the alpha carbon so let me label it this carbon right here is the alpha carbon and then in order to have an e2 reaction in this case when we did the video on e 2 reactions but actually the rule will hold as well for e 1 reactions but in order to have the e2 reaction the hydrogen has to get swiped off of a beta carbon and a beta carbon is just a carbon that's one away from the Alpha carbon so this is a beta carbon and this is also a beta carbon and so it's completely reasonable one would think that well it could swipe it from there or it could swipe it from there and let's think about this reaction let's just draw it out so you could visualize it a little bit better so here we swiped it from this beta carbon let me redraw the reaction where we're swiping it from the other the other beta carbon so on I want both reactions on the screen at the same time so let me draw let me draw our methoxide so we have our oxygen bonded to a ch3 to a methyl group the oxygen has 7 valence electrons 1 2 3 4 5 6 7 I'll do the seventh one the one that will be the one that will bond with the hydrogen I'll do it Green and instead of attacking that hydrogen or taking that proton I should say because it's not taking the electron with it it does it to this one so it what it does is it takes this proton or it bonds with that hydrogen proton and then the hydrogen protons or that hydrogen's electron can then be taken by this molecule so then it goes to the alpha carbon to form a double bond between this beta carbon and the alpha carbon and so now this alpha carbon got that electron doesn't need the electron it has it doesn't need the electron that's bonded with the chloro group anymore and so that goes to the chlorine to form chloride it was already chlorine was already way more electronegative it was already hogging it now it gets to go there and now when all is said and done our products look like this we still have so we still have the methanol just like we had in the original reaction because this grabbed this hydrogen let me draw it so you have your o ch3 and let me draw all of this so you have that pair right over there you have this pair right over here and then you have this purple electron and now it's bonded with this green electron which is ah now on the hydrogen so it is now bonded with this green electron that has been given to the hydrogen oh and not to make sure we don't forget it with this oxygen over here it's seven valence electrons so it had a negative charge neutral oxygen would have six so this had a negative charge but now that it gave its electron to this hydrogen it now has a neutral charge and is now methanol exactly what we saw in when we first learned about II 1 reactions we also know that the claw the chloro group it took that electron it's now a chloride anion so let me draw that and that's exactly the same as what we won in the first video on e 1 reactions so it's now a chloride anion it has grabbed this orange electron it has grabbed the orange electron let me do it in orange it has grabbed this orange electron so it now has a negative charge you can imagine that the negative charge has been transferred from the methoxide to the chloride anion and now what's different this time is that the double bond is now between the 1 and the 2 carbon and not between the 2 and the 3 carbons so now it's going to look like this so now the result this product if the reaction went this way would look like this we have this carbon right here it is bonded to two hydrogen's it is bonded to two hydrogen's now it has a double bond with this carbon it has a double bond and I'll do the second bond I'll do the pi bond of the double bond in purple right over there I'll assume that that's the PI bond that's the new double bond formed and now this carbon this carbon let me do which was the alpha carbon is right over here this was the alpha carbon it's bonded to one hydrogen and then let me draw everything else so then you have a carbon carbon this guy is bonded to three hydrogen's I could have just written a ch3 if I wanted this guy is bonded to two hydrogen's and we're done and so instead of as we saw in the first video on e 2 reactions instead of forming butte 2 een we now have it still one two three four carbons so still butte but the double bond is on the one carbon right we'd start 1 2 3 4 so we could call this butte 1e nor 1 butene at either way so let's call this beaut beaut 1 een so the question is which is more likely to happen do both happen does you know or does one happen disproportionately and the answer is is that yes what happens disproportionately this one this is the dominant product if you were to perform this reaction this is what and you were analyzed in your beaker wherever you're performing the reaction what you see most of the majority product the great majority is going to be the Butte to lean not the Butte 1e maybe you see very very little of this and the question is why or how would you even be able to determine that or how could you have predicted that and to predict it there's something called site sells site says rule and I'm sure I'm mispronouncing it but let me write it down so site sells Zaitsev Zaitsev rule and it's kind of analogous to markovnikov's rule but for elimination reactions if you think about the addition reactions that we did many videos ago are the opposite of the elimination reactions the addition reactions we're adding the chloro group and the elimination we're taking it off and so the site subs rule is kind of analogous to markovnikov's rule now first I'll just tell you the rule then we can think a little bit about why it works and you know the jury is not out on this people are they think they know why it works but they're not 100% sure so Zhai steps rule says the carbon that is going to lose the hydrogen is the one that has fewer hydrogen's so let me write it down over here carbon carbon more likely more likely to lose hydrogen is I should say the hydrogen proton because it keeps the electrons still is the one with fewer fewer hydrogen's fewer hydrogen's so if you were to look at this reaction right here we have our alpha carbon either this beta carbon or this beta carbon could lose its hydrogen this one has three hydrogen's on it this one only has two so zayats halves rule tells us that this that this is the hydrogen or actually the proton that is more likely to be reacted with the base you could almost view this it is the more acidic proton it is it is is a lower hanging fruit for this strong base to capture now a more interesting question that's a pretty easy rule to follow and if they both have the same number then you'd see equal products depending in on on which side it gets now the question is the question is is why is this happening and here something called hyperconjugation comes into effect hyperconjugation I'm not going to go into details in it into the quantum mechanics of it hyper hyper conjugation and hyper conjugation is the notion is the notion that the fact so we said that the one with the fewer hydrogen's is the one that's less likely to lose or the one that few with fewer hydrogen's is the one more likely to lose the hydrogen proton but the one with fewer hydrogen's is also bonded to more carbons this guy is bonded to one carbon this outside of the alpha carbon is actually bonded to the Alpha and this carbon this guy right here is only bonded to the Alpha carbon and hyperconjugation is the notion is the notion that not the beta carbon but the carbons one over from that help stabilize the double bond that eventually forms I almost think of it you have you know you have more electrons over here because carbon carbons have more electrons to offer than hydrogen's so you kind of have at the end of the day this this guy is more likely able to donate electrons to form on from the right-hand side to make a double bond then from the left-hand side I won't go to the detail of hyperconjugation but it's all based on the notion that the more stable double bond will be formed if we have other carbons near the double bond and another way to think about it is to look at the products so we saw or Zaitsev rule tells us that butte two een is a more likely product than butte 1 een and if you look at butte 1 een if you look at butte wanting we could rewrite it like this we could rewrite it like this we could draw the we could draw the double bond like this this carbon is what was this carbon is what was the alpha carbon we could draw a carbon right here and then it is bonded to a hydrogen it's bonded to this hydrogen and then it's bonded to just a chain of it's just a chain of carbons and we'll just draw we'll just write our that and then this guy is just bonded to two hydrogen's this guy is bonded to two hydrogen's so this is butte this is well this isn't necessary Bude wanting I just put an R here but this is how it could be represented now the butte to wean the butte to wean if we wanted to draw it like this would look like this if we wanted to call we could call this right here we could call this our prime it's just a chain of carbons and then we could call this and then we could call this our prime prime it's another it's it's not even a chain it's just one carbon but if we call it that then the butte to e let me draw it down here where i have more real estate the two in would look like this you have your carbon-carbon double bond carbon carbon double bond the left hand carbon is bonded to a hydrogen that hydrogen right there and to our prime and to our prime and the right hand carbon is bonded to a hydrogen and our prime prime so it's bonded to a hydrogen and our prime prime all I did here let me see if I can fit it all on the same screen as I just redrew this and I just abstracted away the chain as it goes away from the double bond and I did that so that we can look at it this way we can just have the double bond kind of as our focus of attention and think about what's going on around it over here for the beaut 1e and we say we already said this is the lesser product so this is the greater product this is the greater or the dominant product and the lesser product we only have if we go off of the double pair double bond we only have one we only have one alkyl group right there that are right here over here we have two and we say that this is more substituted and when we say substitute it you're imagining that you're substituting hydrogen's with carbon chains with alkyl groups so this one right here is more more substituted and hyperconjugation so the idea would have it is that these these carbon chains that are near the double bond help stabilize it they're able to kind of sum of their Sigma Sigma electrons and Sigma orbitals are there to somehow to somehow help stabilize the PI orbitals and now this is getting into quantum mechanics and all of that so it's a little bit and it's you know the world is 100% clear whether that's definitely the mechanism although people have run the experiment and they know that the more substituted the more substituted product is what you're going to see more of as opposed to the less substituted and then that all comes from sight cells rule so hopefully you at least get Sciences rule the hyper conjugation that's kind of a deeper concept and it's you know the jury is not even out on it that's a belief of why site saves rule works but the rule itself is pretty straightforward if you're trying to pick between two beta carbons the one that's going to lose the hydrogen is the one that already has fewer hydrogen's or the one that's bonded to more carbons and this is true I drew it we did everything we focused on right now was in an e2 reaction but it would it's just as true in an e1 reaction