Ch. 12 - Alcohols, Ethers, Epoxides and ThiolsWorksheetSee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins

Condensation reactions join two smaller molecules together to form a single, larger molecule.

  • Only forms symmetrical ethers

Concept #1: The Mechanism of Alcohol Condensation. 

Transcript

So another way to make ethers is through a reaction called Acid-Catalyzed Alcohol Condensation. I know this sounds really complicated, but it's not that bad. As you guys will learn later in orgo two, a condensation reaction is simply a reaction that takes two molecules and makes them into one bigger molecule. I'm just going to say it's a reaction that takes two smaller molecules and then it turns them into one bigger molecule. That's the definition of condensation.
What we're going to be doing here is we're going to be taking two alcohols, it's an alcohol condensation, so we're going to take two alcohols, we're going to put them together. We're going to condense them and they're going to turn into one ether.
How does this work? Let me just go ahead and just draw the mechanism for you. The way this works is you have alcohol in the presence of acid and heat. What's going to wind up happening is that the acid's going to protonate one of the alcohols.
Let's go ahead and just draw this part really quick. I've got my H3O+ that I'm going to write like this because it's easier to deprotonate that way. Same thing as H3O+, I'm just writing it a little bit different. So my OH is going to grab an H from the acid and what I'm going to wind up getting is something that looks like this. I have a protonated alcohol now.
Now what's going to happen is that that protonated alcohol just turned into a good leaving group. Water is a good leaving group. So my other equivalent of alcohol, the one that did not get protonated is going to do a backside attack on this good leaving group. We're basically going to get an SN2 reaction where I get this attacking that carbon and kicking out the good leaving group.
So now what we're going to wind up getting is – let me just draw it in the same colors that I used. The black alcohol that still has an H on it, but now that's going to be attached to the two-carbon chain from the red alcohol. On top of that, there's going to be a water that just left by itself. Does that make sense so far?
So we've got the black one attacking the red one. This looks like an ether, but we've got a problem. There's a formal charge. So what can we do about that formal charge? Remember, this is called acid catalyzed for a reason. That means that you always have to end up with the same amount of acid that you started off with because it's a catalyst. It can't be consumed or destroyed in the reaction.
What that means is that I use the water to pick up the proton. And what I'm going to wind up getting at the end is I'm going to get an ether plus the same H3O+ that I started off with. There you have it. We just condensed an ether out of alcohol.
Now there is going to be a significant limitation for this synthesis. Can anyone tell me? It's only going to yield a certain type of ether. Actually, there's a typo here that I will correct in your notes. This should not say alcohols. It should say ethers. But it's going to form only symmetrical ethers.
The reason is because we're always going to be reacting acid and alcohol and you're going to have an abundance of alcohol. What that means is that one molecule is going to react with another molecule of the same alcohol and you're going to wind up getting the same R groups on both sides. That's why I'm saying that it's symmetrical because you're always going to get the same R groups on both sides.
Sometimes you want that. For example, you wanted an asymmetrical ether. It has to be asymmetrical, maybe Williamson Ether Synthesis would be a better choice because that one it doesn't matter. You can just add R groups as you want.
So let's move on and keep talking about ethers.