Alcohols, Epoxides and Ethers
Ethers From Alkenes, Tertiary Alkyl Halides and Alkoxymercuration
Last updated: January 22nd, 2020 |
When The Williamson Doesn’t Work: Synthesis of Tertiary Ethers From Alkenes, SN1 Reactions, and Alkoxymercuration
In the last two posts we’ve been discussing the Williamson synthesis of ethers. As we saw, our discussion was essentially a complete re-hash of everything we’d already said about the SN2 reaction that was covered several chapters ago. That’s the thing with organic chemistry – things you learn in the early stages of the course often come back in different forms later in the course.
Today’s post is similar in that we’re just going to be going back to old reactions we’ve already seen and look at them in a new light.
Here’s a summary of what we cover today:
Table of Contents
- How To Make Ethers of Tertiary Alcohols When The Williamson (SN2) Isn’t An Option?
- Synthesis of Ethers via SN1 Reactions
- Three Examples of Ether Formation Involving Addition of a Tertiary Alcohol To A Carbocation
- Avoiding Carbocation Rearrangements With Alkoxymercuration
- Summary: Synthesis of Ethers via SN1 (and related) Reactions
We ended the last post by posing a question. How do we synthesize ethers like this one below (di-t-butyl ether) ? We saw that when we attempt to form ethers like this through a Williamson reaction, it fails miserably – giving us elimination products (via an E2) rather than the desired ether.
Let’s think about this for a second. Back when we covered substitution reactions, we learned that the SN2 was best for primary alkyl halides and poorest for tertiary alkyl halides, due to steric hindrance.
But there was a different substitution reaction we learned that was actually superior for tertiary alkyl halides versus primary alkyl halides – the SN1 – and it had to do with the greater stability of tertiary carbocations versus secondary versus primary carbocations.
In fact, we encountered carbocations not only in SN1 reactions but in another type of reaction as well. If we take an alkene and add acid, recall that we end up forming a new C-H bond on the least substituted carbon of the alkene and we form a carbocation on the more substituted carbon of the alkene (remember Markovnikov’s rule?).
This might get you to thinking – can we use either of these reactions to form ethers, via a carbocation intermediate?
We can form this carbocation two ways.
If we dissolve an alkyl halide in the appropriate alcohol solvent, eventually the leaving group will leave, forming the carbocation – which is then trapped by the solvent. After removal of a proton, we’re left with our ether.
Alternatively, if we start with an alkene in an appropriate alcohol solvent, and treat with a strong acid – ideally a strong acid with a poorly nucleophilic counter ion [ yes to H2SO4 and TsOH as acids, generally no to HCl, HBr, and HI] the carbocation will likewise be generated, which is then trapped via the same pathway as before.
Let’s look at three examples. The first one is a typical SN1 reaction. The second one is an alkene addition reaction. The third one is alkene addition… with a twist! [Note – I didn’t put the mechanisms of these reactions in because we’ve talked about these mechanisms so many times before. However you can click here to see the mechanism of these three reactions]
“Oh yes”, you might be saying at this point, like someone who suddenly finds themselves awkwardly face-to-face with an old ex-boyfriend or ex-girlfiend. “Rearrangements.” Yes, rearrangements again!
Anytime we deal with carbocation intermediates, rearrangements are going to be something to watch out for. If we form, for example, a secondary carbocation adjacent to a tertiary or quaternary carbon, expect a hydride or alkyl shift (respectively) that will result in a more stable carbocation.
There is, however, a way out!
In particular, there’s a way we can form ethers from alkenes in a way that doesn’t involve a carbocation intermediate. It’s also a reaction we’ve seen before: oxymercuration.
Oxymercuration involves dissolving the starting alkene in an alcohol solvent and adding a source of mercury(II) like Hg(OAc)2 . A “mercurinium” ion is formed, which is then attacked at the most substituted position by one of the molecules of alcohol solvent. After removal of a proton, we’re left with the product of “oxymercuration”. The mercury can then be removed by treatment with sodium borohydride (NaBH4). We often don’t cover the mechanism, but if you’re curious, here’s the NaBH4 mechanism
Note that we’ve succeeded in adding “CH3OH” in this example across the alkene without any rearrangement occurring.
To summarize, we’ve revisited three methods today for ether synthesis:
- Ether synthesis via SN1 reaction of tertiary alkyl halides
- Ether synthesis via acid catalyzed addition of alcohols to alkenes
- Oxymercuration of alkenes in alcohol solvent
These serve as a useful alternative to the Williamson in cases where we want to build ethers of secondary and tertiary alcohols.
Now that you’ve covered the basics of ether synthesis, the world is your oyster. Just wait until you learn about all the exciting things we can do with ethers now that we know how to make them.
The next post in this series is going to be so exciting, I’m having a very difficult time restraining myself from spilling the beans. Yet, I must.
Next Post – Ether Synthesis Via Alcohols And Acid