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.