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Author: Subject: A choice of ethers
bbartlog
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[*] posted on 11-1-2010 at 19:35
A choice of ethers


So I synthesized some 2-chloropropanol using IPA+ZnCl2+HCl, with a view towards making diisopropyl ether via Williamson ether synthesis (combining it with NaOH+isopropyl alcohol). Now, however, it also occurs to me that I could just as easily use NaOH+ethanol and end up with ethyl isopropyl ether. So I have a few questions:

- is ethyl isopropyl ether as prone as DIPE to forming peroxides with air?
- it's asymmetrical and therefore slightly more polar. Is this actually enough to be significant, assuming that one of my goals here is to have a rather non-polar solvent?
- has anyone used this solvent and if so, any interesting observations?

I'm leaning towards it, partly because the lower boiling point (51C) is attractive, and partly because I have more ethanol than IPA at the moment. But it doesn't seem like a widely used solvent, so I'd be substituting it for more ubiquitous compounds.

Finally, has anyone here synthesized chloroethane using the reagents (Lucas reagent) I used for 2-chloropropane? The chloropropane synthesis went very smoothly by my standards, no gunk or anything, though I did reflux for six hours. But ethanol should be tougher to chlorinate and I'm not sure that normal pressure can get the job done in a reasonable amount of time.
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[*] posted on 11-1-2010 at 20:28
An easyer way?


Hello bbartlog.
If diisopropyl ether is your intended product, and you plan on using it as a solvent there is a much simpler method to obatain your ether in a larger less labor intensive process. The process im thinking of is the dehydration of IPROH with sulfuric acid, and distilling the resulting ether out of the H2SO4/ IPROH mixture. I personally have never done this with isopropyl alcohol, but I have done this a few times with ethanol to make ethyl ether. And if I remember correctly a few members discused the synthesis of DIPE somewhere, but I cant find the thread at the moment.

Now ethyl chloride on the other hand...
I think the bigest problem you are going to encounter is the low boiling point of the ethyl chloride. I tried making ethyl chloride once from ammonium chloride, sulfuric acid, and ethanol with horrible yields (less than 1 or 2 ml). I have made ethyl bromide a few times with much better results, though I am not sure ethyl bromide is usable for what you want. Finally I would suggest you have a look at this thread ( http://www.sciencemadness.org/talk/viewthread.php?tid=10758 ), smuv and a few other members do quite a good job demonstrating the synthesis of a few alkyl halides including ethyl chloride.
I hope I have been helpful, and please post your results!
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bbartlog
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[*] posted on 11-1-2010 at 21:55


Dehydration via sulfuric acid is simpler (one step), but from the reports I've seen in this forum on making diethyl ether this way it's not so easy to get a good yield, and you get a lot of junk in addition. Of course, I haven't yet actually tried the Williamson synthesis, so we will see if it is actually any smoother.
It's also possible that the dehydration would go better when using IPA, but in my limited experience bigger molecules means even more opportunities for unwanted side reactions. However, the thread you linked is very helpful inasfar as it describes lots of examples another way of making the alkyl halide component of the synthesis.
The low boiling point of ethyl chloride would certainly be something to be mindful of. Plain old ice water works fine for condensing 2-chloropropane, but for ethyl chloride I'd probably aim for something like -20C in my condenser, via salt and ice.



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[*] posted on 12-1-2010 at 16:41


Good news!
If you are concerned about tars and what not than I do not think dehydration of isopropyl alcohol should be to much of a problem. The reason being is that the only tars and gunk formed are going to be in the flask where the sulfuric acid meets the isopropyl alcohol (in the boiling flask) , and not in the final product. Also most of the gunk is going to be in a solution of hot sulfuric acid, so any tar formed should not stick to your equipment.

One of my Organic chem books has a few pages on ether synthesis so if I get the chance some time next week I hope to try an ether synthesis. So staytuned until next time! ( And if all goes well I might post pictures)
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[*] posted on 13-1-2010 at 09:03


I would not be so sure you can get much DIPE from the reaction of isopropanol with H2SO4. I would expect mostly propene to form. Industrially, DIPE is made by passing vapours of isopropanol (and/or propene) over acidic forms of zeolites, clays, alumina or titania at 200°C or higher, preferably under pressure (see WO2009133423 for one such example). Maybe it is possible to do in amateur setting, but requires some equipment (like a tube furnace, for example).

Also, because 2-chloropropane is a secondary alkyl chloride, you can not react it with alkoxides and get ethers as main products. Utmost only trace amounts of ethers would form, the main reaction pathway being E2 leading to propene. With NaOH/EtOH you might get up to 10% of the ether (most likely <5%), but with NaOH/iPrOH it is unlikely that the nucleophilic substitution would occur at a more than ~1% extent.
Alkyl chlorides tend to give more elimination products than the corresponding alkyl bromides anyway, while secondary alkyl halides already as such give mostly elimination products with basic nucleophiles. So with 2-chloropropane you are doubly handicaped in any attempt of a Williamson ether synthesis.

PS: How about posting your preparation of 2-chloropropane?




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[*] posted on 13-1-2010 at 11:08


Thanks for the information, Nicodem. Saves me the effort of making the attempt with the alkoxide. I hadn't realized that the Williamson synthesis was not so general. I suppose I should see what else I might do with the chloropropane.
I'm planning to do some more runs of the 2-chloropropane synthesis (this time with photos) as well as a redistillation of the product from the first run before I post a detailed writeup, as there were some shortcomings of the first preparation and I have some other ideas I want to try. Briefly though:

3.3 moles of HCl as 31% aqueous solution (360g), 3 moles of isopropyl alcohol as 91% aqueous solution (198g), 1 mole of ZnCl2 as ~80% aqueous solution (169g), and 15g of activated alumina[1] were added to a 1l flat-bottomed boiling flask fitted with a distillation adapter and thermometer, this leading to a tube going through a bucket of ice water. This was put on a hot plate and the temperature adjusted to keep the vapor temperature in the head under 80C[2]. 171g of crude product was collected in a receiver and washed twice (shaken in a separatory funnel) with 50ml portions of 1M aqueous Na2CO3 solution[3].

(Further edit): I redistilled this (151g after the two washes), adding 100ml of distilled water to avoid any possibility of boiling the flask dry. Collecting the fraction that passed over at head temperature 33-39C yielded 96g, corresponding to a 41% yield. Interestingly there was a remaining fraction (still forming a separate phase from the bottom layer of water) which began to pass over around 45C; just based on the boiling point this would seem to be 1-chloropropane but I have no idea whether such a rearrangement product is plausible.

[1] - initially, I had hoped to do this synthesis with simply IPA, HCl and ZnCl2. This doesn't work, at least at temperatures where the reagents will remain liquid outside a pressure vessel. However, I noticed while cooling the solution that small bubbles (sort of what like one would see coming up from the bottom of a champagne glass) were forming at a couple of points (microscopic flaws) in the flask even at lower temperatures (50C), and I vaguely recall that this reaction is catalyzed with montmorillonite in industry. So I added some activated alumina that I had reconditioned from a commercial water filter, and immediately the solution became turbid with tiny bubbles, remaining so even after any gas trapped in the alumina would have been accounted for. So the reaction was carried out with alumina as catalyst (I also have silica, which I may try separately). However, the alumina became poisoned or exhausted towards the end of the six hour period (adding a few more grams of new catalyst noticeably increased the generation of gas); I think it fares better as a gas phase catalyst...

[2] - I didn't succeed in controlling the temperature especially well, as it rose to 82C on multiple occasions. Some isopropyl alcohol passed over along with the chloropropane. Future tries will be done using a Hempel column packed with some sort of glass, which should make maintaining the head temperature at a low enough level much easier.

[3] - this is close to being saturated, and as a result both phases became slightly turbid as some of the Na2CO3 came out of solution. I'll probably use a 0.5M solution (or even more dilute) next time. No noticeable gas was evolved, suggesting that very little HCl had passed over; however the aqueous phase did become larger as some IPA (verified by smell) was drawn out of the 2-chloropropane.


[Edited on 13-1-2010 by bbartlog]

[Edited on 14-1-2010 by bbartlog]
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[*] posted on 13-1-2010 at 11:31


Serious technical error: M and moles do not mean the same thing; it should be 3.3 moles of HCl as 31% aqueous solutoion (~12M). See the difference? Same of course applies to the 3 moles of IPA and 1 mole of Zinc chloride.

By the way, did you use Vogel's prep as a basic procedure for the reaction (I think he uses 1-butanol though, same in OrgSyn)?

[Edited on 13-1-2010 by DJF90]
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[*] posted on 13-1-2010 at 12:11


Thanks, I fixed that error.
Now that you mention it, I had seen that Vogel prep before and may even have unconsciously recalled some of it while planning this synthesis, but I was only thinking of what I had read about the Lucas reagent. Rereading it I see that they use quite a large excess of ZnCl2 and HCl, and that the ZnCl2 is anhydrous. Also of course the use of butanol means that this prep can be run about 20C hotter than the one for isopropyl alcohol.
Their ratio of reagents is also quite different than mine; I have something like 10:9:3 HCl:alcohol:ZnCl2, while they have more like 9:5:10. Funny to see the catalyst outweigh the reagents. But their yield certainly beats mine, and I'd been thinking about increasing the amount of HCl in my mix in any case.
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