Leah here from leah4sci.com and in this video, we'll look at the alcohol oxidation to aldehydes, ketones and carboxylic acids. An alcohol is a carbon chain which we'll show as an R group bound to the hydroxy or OH functional group. Because the carbon chain has one bond to oxygen, if we increase the number of bonds between carbon and oxygen, we wind up with an oxidation reaction. In order to determine the type of oxidation. You need to take into consideration the starting alcohol and the specific reagent that you're using.
Let's start with a primary alcohol using 1-butanol as our example. Notice that the carbon holding the oxygen is bound to another carbon atom but also bound to 2 hydrogen atoms and that's the key when you're trying to figure out what can happen in your oxidation reaction. Remember from the redox intro video.
that oxidation is increasing the number of bonds to oxygen and decreasing or removing hydrogen atoms. What we're looking for here is a hydrogen atom to remove and since we have two hydrogen atoms, the first will be one oxidation, the second could be a second oxidation reaction. For the first oxidation, we can remove the hydrogen that's on the OH and one of the hydrogens on carbon. This allows us to form a second bond between carbon and oxygen to form a carbonyl but we still have a hydrogen at the end.
The primary alcohol when oxidized once gives us the aldehyde but we still have a hydrogen atom which means we can oxidize it one more time. For the second oxidation, we no longer have a hydrogen and oxygen that we can remove so we're going to add a second oxygen atom to the molecule with a hydrogen on that. So it is oxidation because we increased the number of bonds to oxygen. And while it looks like we didn't change the hydrogen, pay attention to what the hydrogen is attached to.
The hydrogen in the aldehyde is attached directly to carbon and even though we still have a hydrogen in the final product, because it's not directly attached to carbon, the carbon atom lost a hydrogen which is oxidation and we prove that by recognizing that it also gained a bond to oxygen. And this gives us our carboxylic acid. There are a number of different reagents that can carry out this oxidation. The most common one that you'll see has some version of chromium in it and different professors will focus on the different forms or different aspects of chromium. You'll likely see chromic acid which is H2CrO4 and this can be made from a couple of different chromium combinations.
For example, you can see sodium dichromate which is Na2Cr2O7 or potassium dichromate which is the same thing using potassium instead of sodium. Your professor may simply write Jones and ask you to figure out what's going on. A Jones reagent is starting with chromium trioxide, Cr3 but mixing that with concentrated sulfuric acid in an acetone solvent will cause a reaction between the reagents to form chromic acid and carry out the same reaction.
Notice that they all have chromium. And the key here is oxygen, because there is a lot of oxygen we can get an oxidation reaction. This is one pattern to look out for but this reaction can also take place using potassium permanganate, KMnO4. KMnO4 is something you've seen in lots of different oxidation reactions, it'll also play a role when it comes to oxidizing a primary alcohol.
The problem is all of these reagents are very strong oxidation reagents. That means if you have a primary alcohol and you react it with any of these, yes it will go to the aldehyde but the reagents don't know when to stop so they'll go straight from the aldehyde to the carboxylic acid so that the aldehyde is not something you can even isolate with these reactions. In fact, if you want to stop at the aldehyde instead of taking your oxidation all the way to the carboxylic acid. You have to use a special reagent that is called PCC. PCC stands for pyridinium chlorochromate, let's draw that out.
Pyridinium comes from the conjugate acid of pyridine where nitrogen has a positive charge and the counter negative ion is chlorochromate. Chromate would be the CrO3 but it's bound to a chlorine atom with a formal charge of negative one. I like to think of pyridinium chlorochromate as a weakened or poisoned oxidation reagent. So it starts out the oxidation but doesn't take it all the way.
In fact, one way that my students remember this is that PCC starts with a P, it's a prude and it doesn't go all the way. It stops at the aldehyde where all the other oxidation reactions will go straight to the carboxylic acid for a primary alcohol. In the next video we'll look at the mechanism for how this works.
A secondary alcohol has the OH bound to a secondary carbon. If you're not comfortable with identifying primary secondary, make sure to study my pencil trick linked below. In this case, in addition to holding two carbon atoms and the oxygen, the carbon also has one hydrogen atom. That means we have one hydrogen atom to remove for a single oxidation reaction.
When you oxidize a secondary alcohol, You remove the hydrogen on carbon, the hydrogen on oxygen and this allows you to form a pi bond between the carbon and oxygen giving you a ketone as your final product. Since the secondary alcohol can only undergo a single oxidation, we can use a strong oxidizing reagent like KMnO4 or any of the chromic acid family but you can also use PCC because it's not a very strong oxidation and there's Nowhere else you can go unless you go to extreme conditions where you're actually breaking the carbon chain. And finally we have a tertiary alcohol, here we're looking at 2-methyl-2-butanol. The carbon that's holding the oxygen is attached to three other carbons and that means it has no hydrogen atoms.
If we try to carry out an oxidation reaction, we have no hydrogen to remove which means we can't do an oxidation. In fact, If you try to remove just the hydrogen atom and form a pi bond between carbon and oxygen, this violates the octet rule because we have a total of 5 bonds or 10 electrons to that central carbon atom. If you see a tertiary alcohol on your exam, don't draw a product, you simply write N slash R for no reaction.
Be sure to join me in the next video where we look at the mechanisms for alcohol oxidation including the mechanism for KMnO4, PCC and Chromic Acid. You can find this entire video series along with my Redox practice quiz and cheat sheet by visiting my website leah4sci.com slash redox.