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Hi!
I’m Deboki Chakravarti and welcome to Crash Course Organic Chemistry!
Oxidation-reduction reactions, also known as redox reactions, are all around us from charging a cell phone to avocados turning brown.
And they happen inside of us too.
Redox reactions let us strip electrons off the food we eat, and add them onto the oxygen we breathe to give us the energy we need to survive.
Speaking of electrons, in Crash Course General Chemistry, we defined oxidation as the loss of electrons and reduction as the gain of electrons.
Or as I remember it, LEO the lion says GER:
Losing Electrons is Oxidation and Gaining Electrons is Reduction.
These rules are true for organic molecules, but it can sometimes be easier to track the number of carbon-oxygen bonds we gain or lose.
For example, the molecule methane can be oxidized by replacing carbon-hydrogen bonds with carbon-oxygen bonds until we get carbon dioxide, the most oxidized form of a single carbon atom.
On the flip side, carbon dioxide can be reduced by replacing oxygen atoms or oxygen-carbon bonds with hydrogen atoms.
So, we can also define oxidation as gaining bonds to oxygen, and reduction as losing carbon-oxygen bonds.
In this episode, we'll consider the oxidation and reduction reactions of alkenes, and see what types of oxygen-containing compounds we can make from alkenes.
[Theme Music]
Oxidizing agents are molecules that oxidize organic compounds.
They accept electrons from the organic compound, and in the process, the reagents are reduced.
Oxidizing agents are usually the culprits behind multiple carbon-oxygen bonds forming from an alkene!
We'll see two types of oxidizing agents in this episode:
ones with oxygen-oxygen bonds, and ones with metal-oxygen bonds.
Like we worked on last episode, a handy way to see patterns and predict products in addition reactions is by asking three key questions.
Our three-part secret handshake.
Question Number 1: What are we adding across the double bond?
Question Number 2: Where will the groups add on an asymmetrical molecule?
In general, this question is asking about regioselectivity.
Markovinkov’s rule is an example of regioselectivity.
And Question Number 3: What is the expected stereochemistry of the added groups?
If they're added to the same face of the alkene, it's syn addition, or if they're added to opposite faces, it's called anti addition.
In this episode, we’ll look at these questions in the context of reactions that oxidize our alkene friends.
We saw an example of an oxidizing agent at the end of episode 16 with hydroboration-oxidation, when hydrogen peroxide replaced the boron atom we added to the alkene.
We can also use peroxides to perform a reaction called epoxidation.
An epoxide is a three-membered ring with an oxygen, and we can make these from alkenes!
So Question One is pretty straightforward:
epoxidation adds one oxygen molecule across both atoms of the double bond.
Usually we do epoxidation by reacting an alkene with mCPBA, meta-chloroperoxybenzoic acid.
You might see this reaction written out in a simplified way like this:
But if you draw everything out, you can see how mCPBA transfers one of its oxygen atoms in the process:
To answer our other questions, we need to look at the reaction mechanism.
Like hydroboration, we have another concerted reaction that happens all at once -- so there's a lot of electron pushing to help us get to the transition state.
In fact, if you can get this mechanism down, you’ll be doing great!
This reaction has some of the most complex arrow pushing we’ll see in a one-step organic reaction.
Basically, the alkene's double bond forms a strained bridge with the blue oxygen at the same time the other bonds break on mCPBA.
I like to start with the double bonded oxygen in mCPBA, and attack the proton.
Then, I can use the electrons from the oxygen-hydrogen bond to attack the alkene.
The alkene attacks back, and the bond to the blue oxygen that breaks forms a new carbonyl.
And the transition state looks like this.
Because we're only adding one atom and it's bridged, the answer to Question Two is…
it doesn't matter.
Essentially the same group is added to each side.
And because everything is all attached at once in a concerted reaction, if we look at the stereochemistry, we always have a syn addition.
That's our answer to Question Three.
The oxygen bridge can form on either side of the double bond, so we get two different enantiomers as products in a racemic mixture,
which, as you might remember, means equal amounts of each one.
Overall, epoxidation has a pretty straightforward combo.
We can sum it up with another card.
So now we know how to make an epoxide, which is a fun little ring shape!
Earlier in this series we looked at the bond energy of cyclopropane, and learned that three-membered rings have a lot of strain, so they're fairly unstable.
And instability means the chance for some exciting chemical reactions.
Specifically, we can use epoxides as a gateway to anti-dihydroxylation, a name that hints at the stereochemistry of this reaction:
spoiler alert, it's going to be anti addition.
This name also hints at what we're going to end up with, two hydroxyl groups, also known as alcohol groups.
Yes, hydroxyl and alcohol are interchangeable, in case organic chemistry nomenclature wasn't tricky enough.
The anti-dihydroxylation reaction happens the same way for both enantiomers of an epoxide, so we’ll just look at one stereoisomer.
Remember, a strong acid in water forms hydronium ions.
So when we add aqueous hydrochloric acid to an epoxide, the nucleophilic oxygen will attack our electrophilic hydronium ion, which makes a positively charged oxonium ion.
This next step is where it gets fun.
A water molecule will swoop in as a nucleophile in an anti-attack and open the epoxide.
In this particular case, the water molecule can attack either of the epoxide carbons.
The final step of epoxide opening is that typical one-two punch pattern that happens under acidic conditions,
a water molecule adds first, and the oxonium ion we get is deprotonated by a second molecule of water.
This makes the racemic mixture of 4,4-dimethylcyclohexan-1,2-diol.
And that's it!
Here's our three-step summary of anti-dihydroxylation.
The overall effect of the anti-dihydroxylation process is the addition of two alcohol groups on opposite sides of the substrate.
But if we wanted to add them to the same side we'd need a totally different reaction, logically called syn-dihydroxylation.
And we actually have two options for oxidizing agents here!
Like fraternal twins, they're related but have some distinguishing features.
One metal catalyst is osmium tetraoxide, which is a wonderful source of oxygen but also super toxic.
It’s safer to use less of it, so we can add other oxidizing agents like tert-butyl peroxide or N-methylmorpholine-N-oxide (also known as NMO) into the reaction.
NMO helps remake osmium tetraoxide over the reaction and lets us more safely use a catalytic amount of the toxic metal.
The first step in this reaction is the formation of an osmate ester, which is why our hydroxyl groups end up syn.
The osmium reagent approaches the double bond from one face.
In the reduction step, the sodium bisulfite binds to the osmate ester, and helps it break apart in water.
If we use osmium tetraoxide with NMO, the NMO does two things.
It helps break up the osmate ester and gives up one of its oxygens to regenerate osmium tetraoxide, making the reaction catalytic in osmium.
The nitty-gritty of these reactions is inorganic chemistry and a little beyond the scope of this series.
What's most important for this one is which reagents to use for syn-dihydroxylation and generally why they work.
So with that in mind… the other twin, uh, reaction, happens in a similar way.
The metal catalyst involved in the first oxidation step is potassium permanganate.
And that second step needs cold, basic conditions like sodium hydroxide floating around in an icy cold solution.
And here’s our card to sum up syn-dihydroxylation.
Up until this point, we’ve been adding alcohol groups to alkenes which is all fine and good but lacks some drama, y'know?
Sure, all chemical reactions are cool in their own unique ways, but so far we've left our carbon chains intact.
What about something that blasts a molecule into smaller pieces?!
Entropy!
Turns out, we can oxidize an alkene and break the double bond completely using a reaction called ozonolysis.
The name comes from our main reagent, ozone, and "lysis" which is Greek for breaking something down.
To understand how this happens let's look at the reaction mechanism!
The first step is our alkene reacting with ozone so it adds across the double bond.
This makes an intermediate, which then breaks apart and rearranges into a molecule called an ozonide.
After we've got an ozonide, the next step is a quick reduction reaction by adding DMS or zinc in acid -- the referees that break up the reactants!
And our products are two different molecules, each with a carbonyl group.
Now these products might be a little hard to visualize, but we can think of ozonolysis as pair of oxidizing scissors that snips the double bond in half while adding a double bonded oxygen to each side.
On paper, I draw an alkene in pencil, use an eraser to "snip" the double bond in the center, and then add in oxygen caps to both double bonded ends.
Since we’re splitting the double bond in half in ozonolysis we don’t need a notecard.
We don’t need to worry about stereochemistry and regiochemistry.
Now we know lots of ways to oxidize alkenes, and we also need to learn how to reduce them.
Adding hydrogen across a double bond is called hydrogenation, and makes an alkane.
However, the activation energy needed to get the party started is really high because, other than the double bond, there aren't really any reactive sites in the alkene or molecular hydrogen…
they're pretty stable and boring as far as reactants go.
So to help a hydrogenation reaction along,mwe need a catalyst to lower the activation energy -- usually a metal, like platinum or palladium.
First, the hydrogen forms a complex with the surface of the metal.
The alkene approaches the metal-hydrogen complex, and hydrogen is added with its electrons across the double bond.
The alkene gains electrons, so it’s reduced, and the metal surface makes the hydrogens add to one face -- a syn addition!
Here’s our hydrogenation notecard.
In the past few episodes, we've talked about a lot of different reactions involving alkenes, building intuition about products along the way.
But, let's face it, in organic chemistry we need tons of practice.
One way of practicing is filling in a wheel of chemical reactions to see how they're all connected!
Basically taping all our reaction mechanisms together into one big map of The Island of Alkenes.
You can pause the screen here if you want to take notes!
Here's the reaction wheel our team drew up with 1-methylcyclohex-1-ene in the center….
Look at all we've learned in the past few episodes alone!
And there's still so many exciting things to come.
But in this episode, we:
Learned about oxidation (the addition of oxygen) and reduction (the addition of hydrogen) in relation to organic molecules
Added two alcohol groups to an alkene, both syn and anti
And used ozonolysis to cleave a double bond
All our reaction knowledge will keep building, and we'll even see some familiar reagents next time when we explore reduction reactions with alkynes!
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