Alkene reactions
Addition of Water (Acid-Catalyzed) Mechanism Addition of Water (Acid-Catalyzed) Mechanism
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- In a typical organic chemistry problem set or exam, they'll
- give you some type of reaction.
- In this case, we have-- what is this molecule?
- We have one, two, three, four, five carbons.
- It is an alkene.
- It starts on the number two carbon if we start numbering
- from here, and the number two carbon also
- has a methyl group.
- So this is 2-methyl-pent-2-ene, usually
- naming it-- we don't have to name it when we worry about
- mechanisms. But they'll give you a molecule like
- 2-methyl-pent-2-ene.
- They'll draw it out like this and they'll draw a reaction.
- They're saying in the presence of water and catalyzed by
- hydronium ions, we end up with this thing over here.
- And then they'll give you a bunch of white space below it,
- and they say how did this happen?
- Give us the mechanism.
- And that's what we're going to do in this video.
- So let's start with this molecule right there, the
- 2-methyl-pent-2-ene.
- So let me draw it right here.
- So we have double-bond carbon.
- I have a hydrogen.
- I have two carbons right over here, and then I have a
- carbon, H3C, and then-- and we put H3 before the C just to
- make it clear the carbon is what's bonded
- to the other carbon.
- That's why we don't write CH3, but that wouldn't be
- completely incorrect, and then you have HC3 over here.
- And then we're going to have some
- hydronium molecules around.
- So let me draw a hydronium molecule.
- So it is an oxygen in the middle and then
- we have three hydrogens.
- One, two, and three.
- And oxygen also will have one unbonded pair of electrons,
- and it has a positive charge here.
- And the reason why this has a positive charge, if you think
- about regular water, regular water looks like this: six
- valence electrons.
- Let me do the valence electrons in another color.
- One, two, three, four, five, and six.
- The other half of these bonds comes
- from hydrogen's electrons.
- Now, in the case of a hydronium molecule you have a
- proton sitting out here.
- It has absolutely no electrons, and to form the
- bond, one of oxygen's unpaired valence electrons go to the
- hydrogen, and that's what forms the bonds.
- You could imagine that that electron is right over there.
- That's why it has a positive charge.
- Now, oxygen wants that electron back.
- It doesn't like having this positive charge, and we've
- seen in the last videos that sometimes the carbons, one of
- the carbons in the double bond, is pretty predisposed to
- at least temporarily giving up an electron.
- And we saw that the one that's most likely to give up an
- electron is the one that's bonded to the most carbons.
- And in this case, this guy right here is bonded to the
- most carbons.
- He's bonded to two carbons.
- This guy's only bonded to one.
- He is a secondary, or if he loses his electron, it will be
- a secondary carbocation as opposed to if this guy loses
- his electron, it will be a primary carbocation.
- This one will be more stable.
- So what we can do is, we'll say, look, this electron
- that's at this end of this bond right here, it goes to
- the hydrogen right over there.
- And then the electron that this hydrogen was taking from
- the oxygen will go back to the oxygen, and it all kind of
- happens simultaneously.
- This electron is attracted, I guess you could say, to the
- partial positive charge, because maybe this electron
- spends more time over there.
- And then this electron goes back to the
- oxygen to make water.
- So what happens after this end of the reaction or this part
- of the reaction happens?
- So after that happens, we will-- let me clear out this
- water, actually.
- That's not essential to our mechanism, to
- our reaction mechanism.
- So after that happens, what will everything look like?
- Well, we'll have our basic molecule.
- So we have a H3C, we have a carbon, we have another H3C.
- It is now a single bond to the other carbon, which is bonded
- to a hydrogen and two other carbons, CH2-CH3.
- Now, this guy has lost his electron.
- That electron went to the hydrogen.
- So this guy now is a positive carbocation.
- Carbocation by definition is positive so maybe I'm being a
- little redundant there.
- So it's positive.
- And then this bond-- let me do this in orange.
- This bond is now between this carbon and the hydrogen.
- The electron went to that hydrogen to form a bond.
- So this bond is now with this hydrogen over here and then
- this thing that was a hydronium molecule will now be
- just water.
- So I'll draw it like this.
- It got its electron back.
- Let me draw its two electron pairs, just like that.
- And then it has two hydrogens.
- It is H2O.
- And you can imagine that maybe that was one of the electrons
- that was on oxygen end, and then this was the electron
- that it had given essentially to the hydrogen, but it's now
- giving back, and now they're both back with the oxygen.
- That's the one it always had on its end, and this is the
- one that it just got back from the hydrogen.
- Now, what do you think will happen next?
- Well, when you can just look at the final products of this
- reaction you say, well, at some point, I have to have an
- OH attached right over here, and that's usually a good way
- to figure out what's going to happen next.
- So somehow, it looks like this might attach to
- that positive charge.
- It was attached to a positive charge before.
- It was attracted to that hydrogen proton, so why can't
- it do it with the carbocation?
- And then once it does that, maybe we lose
- one of these hydrogens.
- And actually this reaction-- I shouldn't draw these strict
- arrows here, because every step of this reaction is
- really-- there's some form of equilibrium.
- It doesn't only go in one direction.
- Things can go in both directions here.
- So let me clear this.
- And let me show that these aren't just one-way reactions.
- They can go in both directions.
- But if we are trying to show a mechanism, we're assuming that
- we're going in the direction that we're
- drawing this out in.
- So in the next step, let's making that oxygen attack that
- carbocation.
- Or another way, I guess a better way, the carbocation is
- attacking the oxygen.
- It's going to take back an electron.
- So let's do that.
- Let's make it-- lets draw it taking back electron or an
- electron being given back to this carbocation.
- So then we'll have, let's say, that orange electron right
- there-- let me do it in a different color.
- Let's say we have this.
- That orange one is attracted to the
- carbocation and forms a bond.
- Now, what's going to happen?
- Let me draw it as an equilibrium reaction.
- But we're going to go in this direction just to see how we
- can get that final end product.
- We have H3C.
- We have the carbon that was a carbocation.
- We have H3C.
- Now, we have the single bond to a hydrogen to two carbons
- right over here.
- We have this business, that bond that was formed in the
- early part of our mechanism.
- Let me draw that.
- And then we have that blue hydrogen there that helps us
- keep track where things came from.
- And now this has attached.
- We have gotten that orange electron to the carbocation.
- It will form a bond.
- It's going to form a bond.
- And what's that bond going to look like?
- Let me draw this oxygen over here like that.
- It has that electron pair still all to itself.
- It has the two hydrogens.
- It had that green electron, but now it's not just paired
- up with this orange electron.
- That orange electron went.
- It left the oxygen and went to that carbocation.
- So now the orange electron is right over there and
- it formed this bond.
- Now carbon has four valence electrons.
- This carbon has one, two, three, four valence electons.
- This oxygen just lost an electron again and so it now
- has a positive charge again.
- Now, what do you think will happen next?
- We're very close to the final product.
- The thing we have here looks almost identical to the thing
- that we have to end up at, except we have a whole
- hydronium group, I guess you could say, or
- actually a whole water.
- We have H2O attached here.
- They only have an OH.
- And this oxygen right here, it's a positive charge.
- It wants to get one of its electrons back.
- It could take it back from here or it could take it back
- from one of these hydrogens.
- And if it takes it back from one of these hydrogens, then
- we will be at our final end product.
- We will have a neutrally charged OH group right there.
- So how would that happen?
- Well, we're in an aqueous solution.
- There is water all around us.
- There are molecules of water, so let me draw one of those
- molecules of water that's just floating around, and maybe one
- of its electrons right here.
- Let me color code it.
- That makes it a little bit more fun.
- Maybe one of it's electrons are attracted to the partial
- positive end of this molecule or this hydrogen.
- So I'll do it in green.
- It goes to this hydrogen.
- This hydrogen no longer needs this electron on this end of
- the bond, and so it goes back to the oxygen like that.
- And then what is going to happen?
- What do we have left at that end, or, in general, after
- that part of the reaction happens?
- We have all of this stuff over here.
- Maybe I should just copy-- well, let me just redraw it.
- So we have H3C, a carbon-carbon single bond.
- I'm going to draw everything in yellow that we started off
- with in yellow.
- CH2-CH3, H3C, and then we have that first hydrogen that got
- attached on this end.
- We figured out it was going to be this end more likely
- because of Markovnikov's rule.
- One way to remember is this carbocation was more stable.
- The other way is the side that has more hydrogens will get
- more hydrogens.
- The side that has more functional groups will get
- more functional groups.
- But now we have all of this business.
- We have this magenta bond here, and we have an oxygen in
- the middle.
- It is still bonded to that hydrogen over there.
- This pair of electrons, I'm just going to do it on the
- side, just so we can draw it how we can
- conventionally draw it.
- And now we had-- and let me make it very
- clear with the colors.
- We had this electron on one end of a bond, so it's still
- there on that end of the bond.
- And then we have this electron that was with the hydrogen,
- but now it's gone back to the oxygen.
- So now we have two pairs again.
- We have six valence electrons around the oxygen: one, two,
- three, four, five, six.
- It is neutral.
- And now one of those water molecules has become a
- hydronium molecule.
- So now that water looks like this.
- That thing that was a water molecule looks like this.
- It has its lone pair.
- And then it has one end, that electron right there, but this
- blue electron went and joined the hydrogen.
- Let me draw it little closer to the hydrogen right there.
- And then it has that bond.
- So when all is said and done, we were able to get to the
- molecule that the mechanism, or that the reaction
- described, and it was actually the most
- reasonable path to follow.
- We followed Markovnikov's rule.
- We figured out that this was the most stable carbocation,
- and then we just added the water.
- This is an addition reaction of water in the presence of an
- acid, in the presence of hydronium.
- Hopefully, you found that vaguely useful.
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