Markovnikov's Rule and Carbocations Markovnikov's Rule and Carbocations. Figuring out which addition reaction is more likely.
Markovnikov's Rule and Carbocations
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- In the last video, we saw a potential mechanism
- where if we reacted hydrogen bromide
- with this alkene right over here,
- that we could essentially add.
- We had the addition of this halide
- to what started as an alkene,
- and then it ended up as 2-bromo-pentane, as an alkane.
- But when we did that, we made 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's 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 this is the bromine.
- One, two, three, four, five, six, seven valence electrons.
- You have the hydrogen.
- I'll do it in the same color.
- The hydrogen has its electron right there.
- This is partially positive
- and this is partially negative.
- The hydrogen might want to swipe one of these electrons away.
- Let's do it from this guy right here.
- So 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 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?
- So we have a carbon bonded to two hydrogens,
- and it only has a single bond to the other carbon,
- which is bonded to the original hydrogen right over there.
- Let me write my hydrogens a little bit--
- actually, 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
- it 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.
- Let me draw.
- So the blue electron that was here
- has now gone over to the orange hydrogen.
- Let me draw it a little bit neater than that.
- It 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 seven valence electrons:
- one, two, three, four, five, six, seven.
- And then it nabbed an extra electron from the hydrogen,
- so now it will have a negative charge.
- It is a negative ion.
- It 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 now has a positive charge.
- He's a carbocation.
- 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 electron,
- so the hydrogen bonded to the other carbon.
- And so you can imagine, from here,
- something very similar happens
- as 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 bromide ion is obviously negative,
- so maybe he'll want to swipe his electron away.
- So this electron then goes to the carbocation
- and then it will form a bond.
- This green will go to the carbocation
- and then this purple one still stays with the bromine.
- And so they'll have a bond.
- They're paired up, you can imagine it.
- So then we're left with,
- we have a carbon.
- We have our original hydrogens.
- We have this carbon, that hydrogen,
- the rest of the chain: CH2, CH3.
- And then you have this hydrogen right here that it bonded to.
- That was 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 possible mechanism.
- This one we ended up with 2-bromopentane, right?
- Because it's on the number two carbon.
- Here we have 1-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 it,
- because on a first cut,
- these both seemed like reasonable mechanisms.
- But if you did it experimentally,
- you would see that this is the one that you'd really observe.
- 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.
- And there's a couple of ways to think about it.
- When Markovnikov thought it up, or he observed it,
- it seemed to work.
- They weren't 100% sure about why it worked.
- We can think a little bit about why it worked.
- So Markovnikov's rule,
- a couple of ways you can think about it.
- You can think of it as the thing that already has more hydrogens
- is more likely to get more hydrogens,
- so that's what happened here.
- This thing had more hydrogens on it
- than the right carbon right here.
- This right carbon had a hydrogen,
- but it had some other alkyl group attached to it.
- And so the thing that had more hydrogens
- 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 here had no groups.
- He ended up with the bromine.
- So the thing that has more hydrogens
- ends up with more hydrogens.
- The thing that has more groups ends up with more groups.
- So I guess you kind of go more in the direction
- that you are going in.
- But that still is just a rule, so why does that make sense?
- It starts to make sense when you think about
- that in both mechanisms,
- we had to have a carbocation.
- We talked about it in the last video.
- We had a carbocation right over there.
- This is the left carbon being a carbocation.
- This is the right carbon being a carbocation.
- And Markovnikov's rule all comes from
- which carbocation is more stable,
- which one has a lower energy level.
- It turns out that the carbocation
- that is a bonded to more electron-rich molecules or atoms
- is going to be more stable.
- You can imagine it has more things that,
- look, it's positive, but it has more carbons around it
- so it can share some of those electrons.
- 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, when you're only bonded to one other carbon,
- you're called a primary carbon.
- And if you're carbocation,
- this is a primary carbocation right here.
- This guy is bonded to two carbons,
- so he would be called a secondary carbon.
- Since it's a carbocation, it's a secondary carbocation,
- so this right here is secondary.
- So a secondary carbocation 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 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 one.
- So Markovnikov's rule all is a byproduct of the fact
- that this carbocation is more stable
- than this one over here.
- That's because it's secondary versus primary.
- Because it's secondary,
- it can 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 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 than the 1-bromo-pentane.
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