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- Let's explore another mechanism that we can have
- with the ketone.
- And actually, an aldehyde can undergo a very similar or
- actually the same type of reaction.
- So let's say that I had a ketone that looked like this.
- Let me draw my carbonyl group, just like that, and then it is
- bonded to a carbon that is bonded to
- two other CH3 groups.
- And just to make it clear, there's three hydrogens off of
- this carbon there implicitly.
- But I'm going to draw the fourth bond here, which is to
- a hydrogen, because this hydrogen is going to be
- important for this reaction.
- Now, we know that the oxygen has two
- lone pairs of electrons.
- Let me draw it up here.
- And let's just imagine it's floating around in some water,
- and we know that in water there is some
- concentration of hydronium.
- And let's say that one of the hydroniums is right over here.
- Hydronium is just positively charged, so
- this is right here.
- Let me do it in a different color.
- This is what water looks like.
- And if water gives away an electron to a proton,
- it looks like this.
- It is hydronium, and then it only has one
- lone pair of electrons.
- It gave away one of the other electrons in its other lone
- pair to a proton.
- So you can imagine a reality, where it's like, hey, I could
- grab that proton from this hydronium, and then this will
- turn back into water, and in that situation, the mechanism
- would look like this.
- Let me do it in a different color.
- This blue electron gets given to this proton, if they just
- bump into each other just right, and then the hydrogen's
- electron gets taken back by what will
- become a water molecule.
- So if that happens, what do our molecules now look like?
- So now, what was a ketone looks a little bit different
- than a ketone.
- It looks like this.
- I changed it to a slightly lighter color of green, so it
- looks like that.
- We have our lone pair over here, but we no longer have
- this lone pair.
- At this end, we still have this magenta electron, but now
- it is in a covalent bond with the blue electron, which was
- now given to the hydrogen proton.
- Let me scroll up a little bit.
- It was given to this hydrogen proton up here.
- And then this hydronium molecule, it took back an
- electron, and now it is just neutral water.
- It took back that magenta electron, so now it has two
- lone pairs again, so it is just neutral water.
- Since this oxygen up here in the carbonyl group gave away
- an electron, it now has a positive charge.
- But this is actually resonance stabilized.
- You could maybe see that this would be in resonance, or
- another resonance form of this would be-- if this guy's
- positive, so he wants to gain an electron, so maybe he takes
- an electron from this carbon, the carbon in the carbonyl
- group right over there.
- So if you takes that electron, then the other resonance form
- would look like this.
- Let me doing it in the same colors.
- You have now only a single bond with this oxygen up here.
- This carbon down here is still bonded to the same carbons,
- and then this carbon over here, we could call this an
- alpha carbon.
- This is an alpha carbon to the carbonyl group.
- It still has a hydrogen on it right over there.
- And this oxygen, since it gained this magenta electron,
- now it has two lone pairs.
- It has this pair over there, and then it gained this
- electron and this electron, so it has another lone pair.
- And, of course, it has the bond to the hydrogen.
- Since it gained an electron, it is now neutral.
- This carbon lost an electron, so now it is positive.
- So now this carbon right over here is positive, and these
- two are two different resonance forms, so they help
- stabilize each other.
- And the reality is actually someplace in between.
- I could actually draw it in brackets to show that these
- are two resonance structures.
- Now, you can imagine, just as likely-- and actually, I
- shouldn't just draw this as a one-way arrow, because this
- guy could take a hydrogen from this hydronium, or a water
- could take a hydrogen from this guy, so this actually
- could go in both directions.
- So let me make that clear.
- This could go in both directions.
- You could say that they're in equilibrium with each other.
- You're just as likely to go in that direction as you really,
- for the most part, are to go on the other direction.
- But you can now imagine, this has now turned from a carbonyl
- group, this has now an OH group, this has now turned
- into an alcohol, although we have this carbocation here,
- that this does not like being positive.
- And so you could imagine where this electron right here on
- this hydrogen nucleus might want to go really bad to this
- carbocation, and it just needs something to nab the proton
- off for it to go there.
- And the perfect candidate for that would
- just be a water molecule.
- We have this water floating around, so let me draw another
- water molecule, just like this.
- It has two lone pairs.
- It can act as a weak base.
- It can give one of its electrons to
- this hydrogen proton.
- If it does that at the exact same time, bumps into it in
- the exact same way, this electron can then go to the
- carbocation.
- And if that happened, you could go in either direction.
- This reaction is just as likely to happen as the
- reverse reaction, so we could put this in equilibrium.
- But if that were to happen, then what started off as our
- ketone now looks like this.
- We have a bond to an OH group just like this, and over
- here-- actually, let me draw the rest of it.
- We had our molecule that looked like that, but now,
- this electron gets giving back to this carbocation.
- We now have a double bond here between what was a carbonyl
- carbon and our alpha carbon.
- So now we have this double bond right over here.
- That hydrogen has been taken by the water,
- and now that is hydronium.
- So let me draw the water or the hydronium.
- So that water, it had that one lone pair, and then the other
- lone pair got broken up, because it gave one of the
- electrons to this hydrogen right over here, and it went
- back to being hydronium.
- So what happened here?
- We started with a ketone, and they sometimes will call this
- the keto form of the molecule, and then we ended up with
- something called the enol form.
- An enol comes from the fact that it is an alkene that is
- also an alcohol.
- You could even call it an alkenol.
- It has a double bond, and on one of the carbons that has a
- double bond, it has an OH group.
- And the whole reason I show you this mechanism is, one,
- just to show you a mechanism that could happen with an
- aldehyde or a ketone.
- This was a ketone, but if this was a hydrogen right here,
- this would have been occurring with an aldehyde.
- But even more, this is a pretty common mechanism that
- you'll see in organic chemistry classes, and
- actually has a lot of functions
- in biology, in general.
- And these two molecules, this ketone and this enol form,
- these are called tautomers.
- And the keto form is actually the much more stable form.
- In a solution, you won't see much of the enol form, but
- these can occur.
- It can spontaneously through equilibrium get to
- the actual enol form.
- And so you could imagine, these are tautomers, so this
- mechanism is actually called a tautomerization, and these are
- the keto and enol forms of the tautomers.