alking
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isopropylamine alkylation w/bromoethane not going as expected
I have been trying to make so diisoproplyethylamine. Initially I tried to reductively aminate acetone, which worked, but it wasn't especially well
yielding so I'm now trying to start with isopropylamine from glycophosphate weed killer.
Initially I extracted about 1.1 mol of isopropylamine from a liter of 41% roundup. I isolated it by fractional distillation, but did not dry it, I
assume the only significant contaminate was water. It came over at 32C with the tail end ending up around 75C or so (though the temp was measured
below the still head so likely lower still).
Next I refluxed it with a stoichiometric amount of bromoethane with about 2eq of anhydrous isopropanol as solvent. Initially I held it at below
boiling, maybe 50-60C, for about 15 hours and little change occurred. There was slight yellowing and a small amount of crystals around the top of the
solution. I thought maybe this was not hot enough so I turned it up just enough so that small bubbles formed. It's been about 5 hours since and it's
significantly yellowed, but there are no more crystals than before.
Now it could be that there is more water present than I thought which is solvating the majority of the formed diisopropylamine bromide, but I really
wouldn't think so, even if there is a decent amount of water it should be pushed out by the isopropanol and unreacted reagents I would think anyway.
So this, of course coupled with the unfavorable color, makes me think the majority of my product is forming some kind of side product, perhaps due to
oxidation. I do recall reading at one point that alkylating diisopropylamine with EtBr results in a lot of unsaturated side products and does not work
too well, but I can't find the reference to confirm what actually is going on, if it even delves into it.
So, I'm wondering where to go from here. Does anyone have any experience or advice? How long should I let this react? Initially I was planning to wait
until tomorrow morning which would be 30+hrs, that should be sufficient for the majority of it to react, but given the yellowing I'm now concerned if
I'm just destroying my reagents and/or product and if I should change course. If this isn't viable at all then could anyone suggest a better approach?
I'd imagine iodoethane might work better, but I have no economical source for iodine so BrEt is really preferred if I can make this happen.
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clearly_not_atara
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Shouldn't there be a catalytic base for this? Also I wouldn't be surprised if isopropylethylammonium bromide and/or diethylisopropylammonium bromide
are soluble in isopropanol; chlorides are generally insoluble in alcohols but bromides are soluble. Some degree of oxidation is usually a possibility
but while a little oxidation might change the color of the rxn mixture it shouldn't ruin your product.
I think you'll have to basify and distill to know if there were any changes. TLC would be pretty helpful here I think.
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alking
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A base catalyst is preferred, yes, but as far as I understand it's not required, the reaction will simply take longer. The amine should deprotonate
the EtBr as it is and then as n-ehtylisopropylamine is formed it will sort of self catalyse, or at least that's what I'd expect. You should end up
with mostly ethylisopropylamine bromide with a small amount of n,n-diethyl-n-isopropylamine bromide, and any unreacted starting products.
I'd expect some yellowing due to oxidation, but I was concerned by how rapidly it seemed to happen. Maybe it's because of the higher reflux the
isopropylamine is more prone to it with more in the vapor phase.
That's also a good point about the solubility. I don't know if that's true or not, but that's something I had not considered that may well be
relevant. The only reference I could find of someone trying this they used EtI instead of EtBr. They said the reason was that a patent they were
following, as I think I mentioned earlier, resulted in a lot of unsaturated side products when using EtBr. That was ethylating diisopropylamine
however and not i-PrNH3 to Eti-PrNH3, which I thought might be a better bet. They were ultimately making the same product though,
diisopropylethylamine, I just chose to do so in a different order hoping it would work better.
I could run a TLC plate, but I have nothing to visualize the spots with so it's not too useful unfortunately.
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Sidmadra
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Try to run your reaction and then evaporate it. If it worked, there should be some kind of salt remaining. This will eliminate any sort of question
whether the salt is remaining dissolved after the reaction. If there is no salt remaining, then the reaction didn't happen.
I actually tried to conduct a similar alkylation with Ethyl Bromide dissolved in Heptane, and an Amine, puzzled as to why it didn't seem to be
working, but what I'm guessing happened is that my condenser wasn't cold enough, or that my Ethyl Bromide simply escaped from the reaction pot somehow
due to its high volatility. As you probably know this stuff is seriously volatile. I try to weigh out 10g of it, and if I don't use it immediately,
the stuff evaporates like a gram per minute, and that's just sitting in a small tiny beaker covered with aluminum foil, not stirring.
I got around the the issue by doing the alkylation in a screw top flask designed for pressure.
Just a thought. The simplest way for you to quantify whether the reaction worked or not is to just evaporate whatever is left at the end and see if
you have any sort of salt in there.
[Edited on 16-8-2017 by Sidmadra]
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