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Markovnikov's Rule and Carbocations

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    In the last video, we saw a potential mechanism
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    where if we reacted hydrogen bromide
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    with this alkene right over here,
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    that we could essentially add.
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    We had the addition of this halide
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    to what started as an alkene,
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    and then it ended up as 2-bromo-pentane, as an alkane.
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    But when we did that, we made a somewhat arbitrary decision,
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    or I didn't explain why we made the decision.
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    We said, look, this hydrogen is going to be partially positive,
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    because this guy's so electronegative,
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    and maybe when it's partially positive, it'll be attracted.
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    Maybe it'll just bump in just the right way into
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    one of these carbons.
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    It'll maybe swipe its electron.
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    We somewhat arbitrarily in the last video decided
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    that it would swipe this guy's electron.
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    But you could just as easily imagine a world
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    where it swipes an electron from this guy.
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    So let's draw a mechanism for that
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    and just think about
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    which one is more likely to actually happen.
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    So what happens?
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    So once again, this guy--
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    let me draw all of his valence electrons,
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    so this is the bromine.
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    One, two, three, four, five, six, seven valence electrons.
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    You have the hydrogen.
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    I'll do it in the same color.
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    The hydrogen has its electron right there.
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    This is partially positive
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    and this is partially negative.
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    The hydrogen might want to swipe one of these electrons away.
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    Let's do it from this guy right here.
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    So he has this electron right over here,
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    so the other side of that bond,
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    it goes to the hydrogen when the hydrogen goes near it,
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    or maybe it's attracted to it.
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    And when it goes to hydrogen,
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    then the hydrogen lets go of the electron
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    that the bromine wanted all along,
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    because it's so electronegative.
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    So then that electron goes to the bromine.
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    So after we do that, what will it look like?
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    What will be the next step in our reaction?
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    And it will look fundamentally different
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    than this right over here.
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    So now what happens?
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    So we have a carbon bonded to two hydrogens,
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    and it only has a single bond to the other carbon,
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    which is bonded to the original hydrogen right over there.
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    Let me write my hydrogens a little bit--
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    actually, let me write this whole thing a little bit neater.
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    So you have your carbon bonded to a hydrogen
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    and another hydrogen,
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    and now it only has a single bond to this carbon right here,
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    which is bonded to a hydrogen and then the rest of the chain.
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    Let me just draw the rest of the chain right here.
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    And now this electron went to the hydrogen.
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    The other electron that
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    it was paired with is still with this carbon,
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    so now this carbon is now bonded to
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    that hydrogen over there.
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    So this blue electron is now with the hydrogen.
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    Let me draw.
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    So the blue electron that was here
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    has now gone over to the orange hydrogen.
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    Let me draw it a little bit neater than that.
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    It has now gone over to this hydrogen right over there.
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    And then the hydrogen lost its electron to the bromine.
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    So the bromine originally had seven valence electrons:
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    one, two, three, four, five, six, seven.
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    And then it nabbed an extra electron from the hydrogen,
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    so now it will have a negative charge.
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    It is a negative ion.
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    It is bromide.
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    It's a bromide anion, I guess you could call it.
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    And since this guy lost an electron,
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    he had four valence electrons, lost one to the hydrogen,
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    he now has a positive charge.
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    He's a carbocation.
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    So notice the difference.
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    Before, this guy lost the electron,
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    and so the hydrogen bonded to this carbon.
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    In this situation, this guy lost the electron,
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    so the hydrogen bonded to the other carbon.
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    And so you can imagine, from here,
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    something very similar happens
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    as what happened in the first video,
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    but now it happens to this carbon right over there.
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    So let's do that.
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    This carbon is positive.
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    The bromide ion is obviously negative,
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    so maybe he'll want to swipe his electron away.
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    So this electron then goes to the carbocation
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    and then it will form a bond.
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    This green will go to the carbocation
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    and then this purple one still stays with the bromine.
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    And so they'll have a bond.
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    They're paired up, you can imagine it.
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    So then we're left with,
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    we have a carbon.
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    We have our original hydrogens.
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    We have this carbon, that hydrogen,
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    the rest of the chain: CH2, CH3.
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    And then you have this hydrogen right here that it bonded to.
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    That was our first step.
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    And now the bromine has bonded to this carbon right over here.
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    The bromine has bonded over to that carbon right over there.
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    And we're done!
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    This is another possible mechanism.
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    This one we ended up with 2-bromopentane, right?
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    Because it's on the number two carbon.
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    Here we have 1-bromo-pentane.
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    One, two, three, four, five.
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    Still five carbons.
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    It's just the bromine is attached to the one carbon here,
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    attached to the two carbon here.
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    So we now need to think about it,
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    because on a first cut,
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    these both seemed like reasonable mechanisms.
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    But if you did it experimentally,
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    you would see that this is the one that you'd really observe.
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    I actually haven't done this exact experiment,
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    so I don't know the proportions.
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    But you're going to observe this one disproportionately.
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    The great majority of the products
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    that you see are going to be this one, not that one.
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    And so the question is, well, you know,
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    they both seem like reasonable things to do up here.
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    Why is this one so much more likely to happen than that one?
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    It all comes from something called Markovnikov's rule.
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    And there's a couple of ways to think about it.
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    When Markovnikov thought it up, or he observed it,
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    it seemed to work.
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    They weren't 100% sure about why it worked.
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    We can think a little bit about why it worked.
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    So Markovnikov's rule,
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    a couple of ways you can think about it.
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    You can think of it as the thing that already has more hydrogens
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    is more likely to get more hydrogens,
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    so that's what happened here.
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    This thing had more hydrogens on it
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    than the right carbon right here.
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    This right carbon had a hydrogen,
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    but it had some other alkyl group attached to it.
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    And so the thing that had more hydrogens
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    ended up with the hydrogen,
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    and then the thing that had more groups,
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    this character right here had more groups, right?
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    He had one group over here.
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    This carbon over here had no groups.
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    He ended up with the bromine.
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    So the thing that has more hydrogens
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    ends up with more hydrogens.
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    The thing that has more groups ends up with more groups.
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    So I guess you kind of go more in the direction
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    that you are going in.
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    But that still is just a rule, so why does that make sense?
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    It starts to make sense when you think about
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    that in both mechanisms,
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    we had to have a carbocation.
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    We talked about it in the last video.
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    We had a carbocation right over there.
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    This is the left carbon being a carbocation.
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    This is the right carbon being a carbocation.
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    And Markovnikov's rule all comes from
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    which carbocation is more stable,
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    which one has a lower energy level.
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    It turns out that the carbocation
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    that is a bonded to more electron-rich molecules or atoms
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    is going to be more stable.
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    You can imagine it has more things that,
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    look, it's positive, but it has more carbons around it
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    so it can share some of those electrons.
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    The electron clouds will help it out a little bit
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    to be a little bit more stable.
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    This one right here is only bonded to one other carbon,
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    so not as much sharing.
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    This is bonded to two.
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    So, in general, when you're only bonded to one other carbon,
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    you're called a primary carbon.
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    And if you're carbocation,
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    this is a primary carbocation right here.
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    This guy is bonded to two carbons,
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    so he would be called a secondary carbon.
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    Since it's a carbocation, it's a secondary carbocation,
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    so this right here is secondary.
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    So a secondary carbocation is more stable than a primary.
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    And actually a tertiary,
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    if you had another carbon group here or something else
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    that had a lot of electrons around it,
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    that would be even more stable.
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    So bonded to three things, more stable than two things.
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    And when I say two things, two things other than hydrogen,
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    and then, more stable than one.
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    So Markovnikov's rule all is a byproduct of the fact
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    that this carbocation is more stable
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    than this one over here.
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    That's because it's secondary versus primary.
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    Because it's secondary,
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    it can borrow electrons from some of its friends.
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    It has more neighbors to borrow electrons than this one.
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    And since this is more stable,
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    this is more likely to happen.
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    This is a more likely intermediate to have.
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    This is a less likely outcome to have in general.
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    And that's why you're more likely
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    to get to this left product,
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    the 2-bromo-pentane than the 1-bromo-pentane.
Title:
Markovnikov's Rule and Carbocations
Description:

Markovnikov's Rule and Carbocations. Figuring out which addition reaction is more likely.

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Video Language:
English
Duration:
09:09

English subtitles

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