Alcohols, Epoxides and Ethers
Alcohols To Ethers via Acid Catalysis
Last updated: January 22nd, 2020 |
How To Make Ethers With Alcohols And Acid
Last post I got a little ahead of myself. I was all excited about getting into the reactions of ethers, and forgot that there’s one last method for ether synthesis that we haven’t covered.
It’s actually not that general so you can likely skip ahead. But for the sake of completeness, here it is.
Table of Contents
- Synthesis of Symmetrical Ethers Via Acid-Catalyzed Dehydration of Alcohols
- The Mechanism: Acid-Catalyzed Dehydration of Alcohols
- Summary: Symmetrical Ether Synthesis via Alcohol Dehydration
Remember when we said that alcohols often need a “kick in the pants” in order to participate in reactions? That is, we either add acid to protonate them (forming their conjugate acid, which has a better leaving group) or add base to deprotonate them (forming their conjugate base, which is a better nucleophile).
Today’s post is a perfect example. Here’s the summary.
Here’s the deal. If we take a simple alcohol – ethanol is a perfect example – and heat it in the presence of strong acid, ethers can form.
How does this work?
There are three key steps.
First of all, one equivalent of alcohol is protonated to its conjugate acid – which has the good leaving group, OH2 (water, a weak base).
Next, another equivalent of the alcohol can now perform nucleophilic attack at carbon (SN2), leading to displacement of OH2 (water) and formation of a new C-O bond. This is an SN2 reaction.
The final step is deprotonation of the product by another equivalent of solvent (or other weak base), resulting in our ether product.
Here’s a drawing of the mechanism:
So how important is this process, really?
Industrially, it’s very important process for the synthesis of diethyl ether, which is a commodity chemical and useful solvent for organic chemistry. Ethanol is cheap. Sulfuric acid is cheap. Heat, distill, and Bob’s your uncle. Over 10 million tons of the stuff annually via this process.
Practically – and I say this to you, undergraduate student of chemistry – from a synthetic perspective –it’s not a very general synthesis of ethers. First of all, it’s limited to symmetrical ethers,. Secondly, the temperature has to be carefully optimized, because there are lots of side reactions possible. For example the optimal temperature for the formation of diethyl ether is about 130-140 degrees C. Once the temperature gets to 150 degrees and above, elimination starts to compete, leading to the formation of ethylene gas. And this is for primary alcohols, which don’t form carbocations very easily. Once you get into the category of using this process for secondary and tertiary alcohols, carbocations are much easier to form and elimination becomes an even more significant destructive pathway.
You should know what the correct answer for this question is. And be able to draw the mechanism. That’s it.
Beyond that, unless you’re Sigma Aldrich and are planning to make several metric tons of an ether, you can comfortably omit this method of ether synthesis from your synthetic toolbox. The Williamson ether synthesis will do the job just as well, and can also be used to make unsymmetrical ethers to boot.
Okay . Finally, next post we get to write all about the different reactions of ethers. We’ve learned five (5) – count ’em – ways of making ethers, and now that we’re armed with all this knowledge, we’ll go out and talk about all the different things we can do!
Next Post – Cleavage Of Ethers With Acid
P.S. This synthesis of ethers is so practically straightforward that it lends itself to “How-To” videos. Don’t do this unless you know what you’re doing – ether is extremely flammable.