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Chemistry Forums for Students => Organic Chemistry Forum => Organic Chemistry Forum for Graduate Students and Professionals => Topic started by: kobie on June 28, 2017, 05:50:20 PM

Title: Acyl bromide and triethylamine in acetone
Post by: kobie on June 28, 2017, 05:50:20 PM
Hi everyone, I am a newbie here.
I am considering to perform atom transfer radical polymerization (ATRP) on a modified nylon surface and got issue with the very first step which involves the attachment of the initiator bromo isobutyryl bromate (BiBB) to the substrate’s OH groups. The first reaction in the figure below which I found online is pretty similar to the one I am doing. But instead of using THF, I use acetone.
This is HPLC grade solvent 99.7% pure which contains less than 0.3% water. I tried to make it further anhydrous using MgSO4. After filtration, the solvent is let stay over molecular sieve 3A overnight before use. However, upon the addition of equal molar of triethylamine (Et3N) and bromo isobutyrate bromide (Bibb), there is a significant amount of white precipitate. I think it is triethylamine bromide salt formed due to the reaction of BiBB with whatever residues left in the solvent. I suspect that there is still water left in the already dried acetone.
I repeated the process of drying acetone and got the same results. This actually surprised me. After drying with MgSO4 and molecular sieve, I expect there must be very very little water left. But the amount of triethylamine salt formed is quite significant. I really do not know what is going on.
How do I make sure that the acetone I use is completely free of water? Do I have to perform this step in an inert atmosphere? We do not have a glove box in the lab so maybe this will be tricky.
Another question is the role of triethylamine. I am aware that it is used as a base to neutralize the proton released in the form of HBr from the reaction of BiBB. However, I wonder what happen if triethylamine is not used? The acidic condition resulted from HBr releasing is detrimental to the proceeding of further reactions???
I really appreciate if somebody can point me in the right direction.
Thank you very much!
Title: Re: Acyl bromide and triethylamine in acetone
Post by: Babcock_Hall on June 28, 2017, 07:07:37 PM
Would you provide some indication of your level of experience in performing organic synthesis?
Title: Re: Acyl bromide and triethylamine in acetone
Post by: kobie on June 28, 2017, 09:55:46 PM
I am an analytical chemistry grad student. I do not have much of a background in organic chemistry other than the undergraduate o-chem courses I took in college.
Title: Re: Acyl bromide and triethylamine in acetone
Post by: wildfyr on June 28, 2017, 11:25:20 PM
You can't dry acetone that way. Sieves cause aldol condensation instead of drying. MgSO4 will not get things that dry. The way to totally dry acetone is to distill it over KMnO4 (ugh). Why not use THF? Its way easier to dry (use baked sieves like you were trying to). Basically any DRY polar aprotic solvent will work here. You should see the BiBB ester on the surface by IR at like 1740. Once you get the PFP ester on the surface, its pretty stable. It will also be obvious by IR (C=O at 1810)

I actually come from a lab where we did surface modification involving pentafluorophenyl esters, I'm almost sure you've read some of our papers if youre doing this procedure. Here is a lovely little secret, you don't actually need TEA to catalyst this reaction. It will autocatalyze itself with all the amines floating around since the equivalencies of amine are huge vs the surface. Its basically a click reaction. And highly hydrolytically stable

You do use 1 eq TEA for the acylations reaction though. You should dry the TEA over sieves too, it gets quite wet.

Aside from excluding water in the BBiBE part, this reaction should work beautifully. Ill point out that you should try use a soxlet extractor to get all the noncovalently bound dye out. It "automates" the washing and youll get many more cycles than spinning down and rinsing. Dyes like to stick in cellulose so it will take a lot of rinsing to clean it all out. From eyeballing the dye's solubility, I recommend THF (again).

Heres an idea... BBiBE is pretty cheap ($1/g), why not just do that step neat? Probably don't even need TEA.

A word from the wise: don't change procedures (like solvent choice) you read in papers without good reason. The authors probably made a thoughtful decision to perform the experiment in that manner e.g. acetone is hard to dry, almost everything else isnt.
Title: Re: Acyl bromide and triethylamine in acetone
Post by: wildfyr on June 28, 2017, 11:49:00 PM
Actually, something important just occurred to me. ATRP of pentafluorophenol acrylate was something we struggled to get working. Something about that monomer doesnt agree with ATRP. I see you are using the methacrylate, but there are not many examples of ATRP in the literature with that either (aside from the paper you are copying, this one:!divAbstract, and another random one I found Most people use RAFT for controlled polymerization of this monomer.

We switched to free radical photopolymerization because it turned out to be more dependable. Good luck with the ATRP, it can be fickle. Feel free to PM me during your project, I'm happy to help or at least put you in touch with students still in my old lab who work with similar systems every day.
Title: Re: Acyl bromide and triethylamine in acetone
Post by: kobie on June 29, 2017, 09:18:11 AM
Hi wildfyr,
Thank you very much for your detailed answers!
So far the procedures in the articles I have found reported the use of chloroform, DCM and THF for this step. I have tried chloroform for this step and it worked. However, it damaged my substrate, which is made of nylon (activated by formadehyde to produce the OH groups). I haven’t tried DCM but I found from literature that it is not compatible with nylon either. Besides the compatibility issue, I would like to pick some solvent that is less toxic to work with. DMSO, ACN, acetone and THF are all compatible with nylon.
- DMSO reacts violently with BiBB.
- I am not sure about ACN though. I mixed BiBB, TEA with ACN. The color slowly got darker over the course of a few hours. I read somewhere that the nitrogen in –CN is a very weak nucleophile. I wonder if it is reactive enough towards acyl halide.
- So it comes down to acetone and THF. I do not have access to THF at the moment in our lab. From what I read, THF is very hygroscopic so I assume it is more so than acetone. So I decided to use acetone, which is abundant in our lab as the solvent for the reaction.
I will try to get some THF and dry it with MgSo4 and molecular sieve the way I did as you suggested and will try it for the reaction with BiBB and TEA to see how it goes.
Regarding the use of BiBB neat, that is a really clever idea. I had to google “neat” though (That shows you how much experience I have with organic chemistry. LOL). BiBB is quite inexpensive. I found a couple of vendors selling 500g for around 150-160$. I will definite try using it neat. Thank you for this wonderful suggestion.
Regarding the use of TEA in the reaction with BiBB, would you mind explaining to me its role? Does it act as a catalyst? Or is it added just to mop up the HBr generated in the reaction? What if I do not use it at all?

Title: Re: Acyl bromide and triethylamine in acetone
Post by: kobie on June 29, 2017, 09:36:42 AM
The figure in the first post is not my scheme of reactions though. I don't use cellulose as my substrate. Instead, I used modified nylon so fortunately, I do not have to go through the procedures you described to remove the dyes.
As to the monomer, I use glycidyl methacrylate (GMA), which is quite reactive for ATRP. When I tried the initiator attachment step with chloroform, I did go all the way to the ATRP step with this glycidyl methacrylate and the polymerization worked fine. But the nylon was mechanically damaged after the treatment with chloroform in the first step and as a result, it is not usable.
So far, the problem I have is still the initiator attachment step. The choice of solvent, making sure that solvent is completely anhydrous to minimize the formation of triethylamine bromide at the beginning.
Title: Re: Acyl bromide and triethylamine in acetone
Post by: wildfyr on June 29, 2017, 10:02:06 AM
Acetonitrile and BBiBE should be completely compatible. The color darkening was probably just HBr forming as water reacted with the acid bromide. Dry toluene, xylene, or benzene are also choices for these reactions that you can consider. A google search said they are compatible with Nylon.

You are correct that TEA is there to mop up HBr. If the reaction medium is too acidic, it lowers the speed of or even stops, the acid chloride+alcohol--->ester reaction.

With regards to doing it neat, you may still want (dry) TEA just so the HBr being released doesn't hydrolyze the Nylon at all.
Title: Re: Acyl bromide and triethylamine in acetone
Post by: rolnor on June 29, 2017, 12:33:15 PM
I am thinking, the hydroxymethylgroups you get on the nylon when treating it with formaldehyde, these bonds will be very easy too hydrolyze, or? If so the TEA is very important.
Title: Re: Acyl bromide and triethylamine in acetone
Post by: kobie on June 29, 2017, 01:50:39 PM
Thank you wildfyr and rolnor!
You guys made a good point about the possible hydrolysis of nylon and the hydroxymethyl groups (formed by treatment with formaldehyde) by the released HBr. I did not think about this issue so thank you for pointing this out. I think that TEA is not avoidable then.
I have done some search about methods to dry triethylamine. Most involve the distillation process. I wonder if the molecular sieves alone will work. Our lab is not equipped with organic chemistry equipment for me to do the distillation.
Title: Re: Acyl bromide and triethylamine in acetone
Post by: wildfyr on June 29, 2017, 03:39:08 PM
I wouldn't worry about distilling them, you just need the water gone. Some other minor impurities that won't hurt you. Amines tend to turn colored when stored due to a variety of very minor side reactions. Most stay analytically pure for a long time though. Heres a reddit thread discussing it: