Leah here from leah4sci.com and in this video we'll begin our discussion of Oxidation and
Reduction reactions of Organic Compounds as you'll see in Organic Chemistry.
Think back to when you first learned about Redox in General Chemistry. You’ve probably
seen one of these mnemonics. OIL RIG which stands for Oxidation is loss, Reduction is
gain. I don't like this because it doesn't tell me what I'm gaining and losing and if
a mnemonic doesn't tell me what it is, I get confused. The second mnemonic I learned from
a student and I find that much more applicable. LEO the lion says GER and this mnemonic tells
me that the loss of electrons is an Oxidation reaction and the gain of electrons is a reduction
reaction. And this worked very well in General Chemistry when you look at batteries and circuits
and electrons moving on a wire but in Organic Chemistry you're looking at bonds being formed,
atoms moving from one molecule to the other. How can we apply this?
Let's take a step back and re-evaluate what we know about Oxidation and Reduction. Oxidation
is the loss of electrons, and if you think of an electron dragging a proton with it we
have hydrogen atom. So Oxidation is really the loss of a hydrogen atom. Reduction is
the gain of electrons and if we take the electron and bring a proton with it, we simply have
the gain of a hydrogen atom. But redox in Organic Chemistry often involves the Oxygen
atom. So think of Oxygen and Hydrogen as antagonists to each other. If we take an atom that has
hydrogen and put an Oxygen on it, we separated the carbon hydrogen bond so in losing that
hydrogen atom, we gain an Oxygen atom. So Oxidation has Oxygen in it. It can be the
loss of a hydrogen atom or the gain of an Oxygen atom. I'll often write Oxygen as Ox
instead of just O so we don't confuse it with a letter or a zero.
Reduction, which is the gain of hydrogen atom can also be the loss of an Oxygen atom. But
there's one more method you're used to recognizing for Oxidation or Reduction and that is the
oxidation number or the change in Oxidation number. For example, if I have the ion Fe2+
reacting to form Fe3+, you’ve probably learned that when the Oxidation number goes up that
is Oxidation. How do you know this? I think of it as the oxidation number having gone
up. Up with an O. When the charge goes up, it's in Oxidation. On the other hand, the
reverse reaction here has the oxidation number going down or being reduced and the word reduced
has reduction in it. So that's a third way to look at it. Oxidation is when we go up
in charge and reduction is when we reduce the charge.
We'll use a combination of these three approaches when looking at the Oxidation or Reduction
reactions of Organic compounds. I'll go through a couple of them in this video to give you
an overview but I'll cover them in much more detail in the remainder of this series including
step by step explanations and detailed mechanisms. You can find this entire series along with
the redox practice quiz and cheat sheet by visiting my website leah4sci.com/Redox.
Let's start with a simple reaction. An alkene can be both oxidized and reduced. If we take
this molecule and do hydration. For example, reacting it with H2SO4 in the presence of
H2O. You should recognize this as the acid catalyzed hydration of alkenes which will
break the pi bond and give me an alcohol on the more substituted carbon. In this case
it doesn't matter because they're both equivalent so I get an OH. We recognize this as an oxidation
reaction because we gain a bond to oxygen. The carbon here got oxidized, but now let's
prove it looking at the oxidation number. In general chemistry you learn to take all
the atoms, add up their oxidation numbers and that should equal to the charge of the
molecule. Which is fine when you're dealing with small ions but when you have large organic
molecules, it gets overwhelming.
Instead we wanna zero in on just the atom in question and see what happens to that atom.
For carbon in particular, if it's bound to another carbon, they're equal, their charge
is zero. If it's bound to a hydrogen atom as we have here, we'll think of the hydrogen
as having an oxidation number of plus one. If the molecule is net neutral and carbon
is bound to an oxidation of plus one, it has to be minus one. That’s what gives us the
neutrality. So we're starting with a negative one carbon and for the product, don't forget
we still have that hydrogen atom. Hydrogen is still plus one. Oxygen is more electronegative,
it's minus one, plus one and minus one equals zero so the carbon holding them is also zero
to give us a net neutral. What happened to the oxidation state of carbon? It went from
negative one up to zero. Zero is greater than negative one, and since it went up in oxidation
state or up in charge, we know it's an oxidation. The alcohol can be oxidized further, we can
get a ketone, we can get an aldehyde if it's terminal, we can cleave the molecule, oxidative
cleavage to give aldehydes or carboxylic acids, but let's hold off on that for now.
Another reaction that I can do here is to react the alkene in the presence of something
like H2 with a palladium carbide catalyst. You should recognize this as the alkene reduction
reaction. In addition to the two hydrogens initially on the carbon, we now have two more
hydrogen atoms from this reaction so that carbon is fully saturated. We recognize this
as reduction because reduction is the gain of electrons with protons, meaning the gain
of hydrogen atom. But let's take a look at the oxidation numbers as well. Each hydrogen
atom has an oxidation of plus one. What will balance plus 2 minus 2? So carbon here has
an oxidation number of minus 2 and minus 2, both carbon atoms reacted. What is the process
of going from negative one to negative two? We have to reduce the charge by one. We dropped
it, negative one down to negative two, and because it's reduced by one, this is a reduction
reaction.
As you're studying this in class, don't go through all these oxidation numbers. I'm simply
showing it to you to prove it to you but I want you to use the trick of gain hydrogen
atoms = reduction, gain hydrogen atoms = oxidation. Let's take a look at an alkyne which is the
same general structure but there is a triple bond between the two carbon atoms instead
of just a double bond. We can still carry out a series of hydration reactions, for example
if we react it with HgSO4 and H2SO4, you should recognize this as the oxymercuration of an
alkyne which will first give you an enol that'll tautomerize to give you a ketone. What type
of reaction is this? Well we had zero bonds to oxygen in the beginning. We gained two
bonds to oxygen, it has to be an oxidation reaction which it shows on oxygen in brackets.
But let's prove it by looking at the oxidation state of the carbon that changed. This carbon
is only bound to carbon, that means it's neutral and it's oxidation number, is zero. Since
carbon that's double bound to Oxygen is double bound to a more electronegative atom, we get
a negative, not for Oxygen but per oxygen bonds. So this bond here is negative one,
this bond is negative one, that means the carbon to balance it must be positive two.
From neutral zero all the way up with an O. If it goes up by two it's an oxidation reaction.
How do we know? we gained bonds to oxygen.
The alkyne can be oxidized in many different ways. We can get a diol, we can get a diketone,
we can get an aldehyde if it's terminal. Cleave it to get carboxylic acids and more, but the
concept is the same. Gaining bonds to oxygen is Oxidation. If we reduce the alkyne, we
can get a mix of an alkane, a cis-alkene, or even a trans alkene. The central carbons,
the carbons that start out triple bond, have no hydrogens. I want you to notice that each
of these products has more hydrogen atoms than what we had initially. We won't go through
the individual oxidation numbers because by now I hope you see the pattern and understand
gaining those hydrogen atoms is a reduction reaction. And we'll go into it in more detail
for the redox of alkenes and alkynes video.
Another common redox series you'll see starts or ends with an alcohol. If we start with
a primary alcohol, we can partially oxidize it to get an aldehyde. How do we know it's
oxidation? Carbon has one bond to oxygen, here it has two bonds to oxygen, but look
at what else happened. We have two hydrogen atoms here, we only have one hydrogen atom
here, so in addition to gaining a bond to oxygen, we also lost a bond to hydrogen. Both
of these are definitions for oxidation. We can oxidize this even further to a carboxylic
acid. So primary alcohol can form an aldehyde or a carboxylic acid and notice the carboxyl
group has an additional bond to oxygen on the carbon atom. If we take the same sequence
and reverse it, we lose one bond to oxygen for a single reduction. We lose another bond
to oxygen or another reduction all the way back to a primary alcohol. If we start with
a secondary alcohol, we can only do one simple oxidation before we start breaking the carbon
chain.
How do you know we can only do one oxidation? We only have one hydrogen atom on a carbon
to remove. In order to oxidize we need to remove hydrogen and add Oxygen. Getting that
second bond to oxygen gives carbon four bonds to either carbon on oxygen and that's as far
as they can go unless we go to extreme conditions and turn it into something like an ester.
This is the oxidation and if we reverse it by removing the bond to oxygen adding a hydrogen,
that would be a reduction. A secondary alcohol is oxidized to a ketone and that's it. Now
what happens if I want to oxidize a tertiary alcohol? The answer is I can't! In order to
oxidize, I have to remove hydrogen, carbon doesn't have any. I have to gain a bond to
oxygen, well how? If carbon already has four bonds and I try to add a fifth bond by creating
that double bond, we've just violated carbon's octet. It has five bonds when the max is four
and that can't happen. So you cannot oxidize a tertiary alcohol. If you see this on the
exam, just write n slash r for no reaction.
The trickiest aspect of redox, in my opinion, are all the reagents. Every time you learn
a reaction you learn 1,2,3,4 and it can get very very overwhelming. I don't want you to
focus on which reagent does what instead I want you to understand why and how they do
it. Let's look at some of the common oxidation reagents you're going to see. KMnO4, CrO3,
O3, any form of Cr2O72 minus, in this case we have Na2 or K2 to balance it. And now let's
look at reduction. Common ones you'll see are LAH which is lithium aluminum hydride,
NaBH4 Sodium Borohydride and more. Look at what we see in the reagents. If Oxidation
is gaining bonds to oxygen, should we not use a reagent that is absolutely overflowing
with oxygen atoms? So overflowing that it is available to donate it to the molecule.
If the reduction is a loss of Oxygen or gain of Hydrogen, should we not use a reagent that
is packed with hydrogen atoms? That is what you're looking for. So if you're trying to
figure out if it's oxidation or reduction, ask yourself, what atom do I see in my reagent?
How can that possibly influence my product?
Be sure to join me in upcoming videos where we break down the reagents, break down the
mechanisms and show you what happens with every specific type of oxidation or reduction
for all the common ones you see in Organic Chemistry. You can find the entire series
along with my redox practice quiz and cheat sheets by visiting my website leah4sci.com/redox