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In this video, we're going to look at the biological redox
reactions of alcohols in phenols.
Over here on the left, we have the ethanol molecule.
So this is our 2-carbon alcohol.
And the carbon that we're most concerned with
is this carbon right here, which has
one bond to this oxygen atom.
And in the liver, ethanol is oxidized to ethanal.
So over here on the right is the ethanal molecule--
a 2-carbon aldehyde.
And once again, we're concerned with that carbon in yellow.
And so one easy way to tell that ethanol was oxidized to ethanal
is to see that, on the left, we have
one bond of that carbon to oxygen.
And over here on the right, we now
have two bonds of that carbon to oxygen.
So an increase in the number of bonds to oxygen is oxidation.
You could also assign oxidation states to this carbon.
And you will see that there's an increase in the oxidation
state of that carbon.
And then, you could also think about electrons.
LEO the lion goes GER-- loss of electrons is oxidation,
gain of electrons is reduction.
And so if I think about these electrons here in magenta,
you can see that those electrons are
lost from the ethanol molecule.
So loss of electrons is oxidation, ethanol is oxidized.
If ethanol is oxidized, something else must be reduced.
That's how redox reactions work.
What's reduced is ***+ over here on the left.
So this is ***+, which stands for "nicotinamide adenine
dinucleotide."
The adenine is hiding in this R portion.
And we have a nitrogenous-based ring
with an amide functional group over here
on the right for the nicotinamide portion
of the molecule.
Plus 1 formal charge on this nitrogen gives us ***+.
This is nicotinamide adenine dinucleotide-- ***+.
And since ethanol is oxidized, ***+ must be reduced.
So reduction means gaining of electrons.
***+ is going to gain those electrons in magenta from
ethanol.
So if we think about a possible mechanism,
if I took these electrons between the oxygen the hydrogen
and moved them in here, that would form our double bond
between the carbon and the oxygen.
But there'd be too many bonds to this carbon right here.
So the electrons in magenta are going to move to this carbon
down here on ***+, to this carbon.
That would push these electrons over here,
and that would push these electrons here off
onto the nitrogen.
So if we showed what happened with the movement of all
of those electrons over here on the right--
this carbon right here at the top already
had a hydrogen bonded to it.
And it gained another hydrogen with two electrons.
The two electrons were the ones in magenta right here.
This hydrogen right here is this hydrogen.
And the electrons in magenta move over there to our ring.
And then, we would also have pi electrons moved over here.
And then, we had a lone pair of electrons
move off onto the nitrogen.
Like that.
And then, we still had some pi electrons over here
on the right.
This molecule is called NADH.
So it's a gained the equivalent of a hydride-- hydrogen
with two electrons.
And so we can see that ***+ gains two electrons.
And gaining electrons is reduction.
So ***+ is reduced to NADH.
Since ***+ is reduced, it allows ethanol to be oxidized.
And so we would refer to ***+ as an oxidizing agent.
It is the oxidizing agent for ethanol,
even though it itself is being reduced.
So that's something that confuses some general chemistry
students sometimes.
All right.
So now over here, we have the NADH molecule.
And this reaction is catalyzed by an enzyme,
and the enzyme is alcohol dehydrogenase.
OK.
So this is catalyzed by the alcohol, dehydrogenase enzyme.
Like that.
And this reaction is reversible.
So if we think about the reverse reaction,
we think about ethanal being reduced to ethanol.
And so if ethanal is reduced to ethanol,
NADH would be oxidized to ***+.
And so let's think about a mechanism
where we could oxidize NADH and reduce the ethanal.
If I took this lone pair of electrons in the nitrogen
and move it back in here, that would push these electrons off
over here.
And now, the electrons-- in magenta on this bond right
here-- would attack this carbon right here.
So the electrons-- in magenta-- we
could think about the electrons as being right here.
And you could think about that as being a hydride--
so a hydrogen with two electrons,
giving it a negative 1 formal charge.
And even though we've seen in some earlier videos
that hydride isn't necessarily the best nucleophile.
You could think about this as being a nucleophilic attack,
if it makes it easier for you, because this carbon right here
would be partially positive.
The negatively charged electrons would attack that carbon.
And in doing so, that would push these pi electrons off
to then grab this proton here.
And that would give you your ethanol molecule,
and that would convert NADH back into ***+.
So you could think about NADH as being oxidized.
It is losing two electrons-- the electrons in magenta.
Loss of electrons is oxidation.
And since NADH is the agent for the reduction of ethanal
to ethanol, you would say that NADH
would be the reducing agent for this example.
And the best way to remember that NADH is the reducing agent
is-- it is the one that has the hydrogen on it.
So it has the hydride, which is capable of being
the agent for the reduction.
So therefore, NADH is the reducing agent.
This ***+, NADH conversion-- and vice versa--
is extremely important in biochemistry.
This happens in numerous biochemical reactions.
And so it's important to understand
what's happening with those electrons on these molecules.
Let's look at another biochemical example of redox.
And here, we have on the left, phenyol.
Right?
So this is our phenol molecule.
And once again, we're most concerned about this carbon,
the one that's attached to this oxygen.
And there are many ways to oxidize phenols.
So if we oxidize phenol with something like the Jones
Reagent-- with sodium dichromate, sulfuric acid,
and water-- would be capable of oxidizing phenol
to this molecule over here on the right, which
we call "benzoquinone."
This right here is a benzoquinone molecule.
And just real fast, you could see that this carbon right now
has two bonds of carbon to oxygen so it has been oxidized.
So phenol can be oxidized to benzoquinone
using numerous organic reagents.
Once you make benzoquinone, you could reduce that
to this molecule over here on the right,
which is called "hydroquinone."
There are several, again, organic reagents
that can reduce benzoquinone to hydroquinone.
Let me change that spelling there.
And then, from hydroquinone, you could oxidize hydroquinone back
to benzoquinone pretty easily.
And so once again, in organic chemistry,
there are lots of reagents that can do these redox conversions.
And in the body, you're usually talking about the ***+,
NADH system.
So we've just studied that.
And if we look here at this molecule,
you can see it's a quinone.
Right?
So you can see the benzoquinone portion of this molecule.
And this is called "ubiquione." "Ubi" referring to the fact
that this is ubiquitous.
This compound is found everywhere.
It's found in all the cells in nature.
And the other name for this would be "coenzyme Q."
This is a very important part of the electron transport chain.
And if we look at ubiquinone-- going
to this molecule over here on the right--
you can see this is like a hydroquinone analog here.
So this is ubiquinol.
These carbons are being reduced from this chemical reaction
that I've drawn here.
So ubiquinone is being reduced to ubiquinol.
If ubiquinone is being reduced, something else
must be oxidized.
All right.
So the NADH is being oxidized to ***+.
The *** is the one that has this hydride on here, which
can serve as the reducing agent.
So here NADH is acting as the reducing agent-- the agent
for the reduction of ubiquinone to the ubiquinol molecule
over here on the right.
And so this is just an oversimplification
of part of the electron transport chain
where you're transporting electrons, which eventually
leads to oxidative phosphorylation and also
ATP synthesis, which, of course, gives us energy.
This isn't meant to be an exhaustive detail of those
biochemical processes, but it's just to show you how you can
analyze biochemistry using a simple knowledge of organic
chemistry and the importance of ***+ NADH in biological
systems.