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MCAT
Course: MCAT > Unit 9
Lesson 8: Nucleic acids, lipids, and carbohydrates- Nucleic acids, lipids, and carbohydrates questions
- Nucleic acid structure 1
- Antiparallel structure of DNA strands
- Saponification - Base promoted ester hydrolysis
- Lipids - Structure in cell membranes
- Lipids as cofactors and signaling molecules
- Carbohydrates - Naming and classification
- Fischer projections
- Carbohydrates - Epimers, common names
- Carbohydrates - Cyclic structures and anomers
- Carbohydrate - Glycoside formation hydrolysis
- Keto-enol tautomerization (by Sal)
- Disaccharides and polysaccharides
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Keto-enol tautomerization (by Sal)
Keto-enol tautomerization. Created by Sal Khan.
Want to join the conversation?
- What are alpha and beta carbons?(16 votes)
- nice question.
alpha carbons are carbons bonded to functional group.
similarly beta carbons are carbons bonded to alpha carbons.(35 votes)
- Why is the Keto more stable than an enol compound?(11 votes)
- C-O pi bond (present in the keto form) has got greater strength (87 kcal/mol).Whereas the C-C pi bond(in the enol form) has got comparatively lesser strength i.e.60 kcal/mol. Thus,keto form is more stable than enol form. :)(3 votes)
- The hydrogen of the hydronium is positive not the oxygen right?(0 votes)
- The oxygen of the hydronium actually has (if you do the maths) a + charge of formal charge (not real charge). Formal charge is calculated as follows: valence electons - 1/2 shared electrons - non shared electrons. That would be 6- (1/2)*6 - 2 = 1. Sorry for my english, I'm from argentina.(9 votes)
- can it be explained as resonance between the two functional groups as well?(3 votes)
- No. Keto-enol tautomerism is not resonance, because the H atoms change positions. Resonance requires that only electrons change positions.(5 votes)
- Why does the water not take the hydrogen from the other alpha carbon (the left side) instead of the right side?(2 votes)
- It does.But removing the left-hand carbon gives a disubstituted C=C bond. Removal of the other carbon gives a tetrasubstituted C=C. The more highly substituted double bond is more stable. Since all these reactions are equilibria, the more stable product is formed in much larger quantities.(5 votes)
- is constitutional isomer and structural isomer is one and the same ?(2 votes)
- Yes. The Constitutional Isomer is the IUPAC name for what is also called structural isomers. Same thing, two names.(4 votes)
- What exactly are Alpha carbons? If they are carbons bonded to a functional group does that mean each and every functional group apart from carbonyls such as alcohol, carboxylic etc?(2 votes)
- I get the Keto to enol, but how would you relate this to monosaccharide? would the ketone turn to enol? and would it change the monosaccharide?(1 vote)
- what is the acidic property of ketone(2 votes)
- Ketones are Lewis Acids and also Lewis Bases.
If you are talking about the Bronsted definition, ketones have a pKa of ~ 20 and readily form enolates.(2 votes)
- Which form( keto or enol ) will have a better Hydrogen bonding?(2 votes)
- Are there any videos or tutorials on enolates and the mechanism by which they form?(2 votes)
Video transcript
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.