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Author: Subject: A simple macroscale acetylation: N-acetylglycine (video)
valeg96
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[*] posted on 18-10-2021 at 11:18
A simple macroscale acetylation: N-acetylglycine (video)


For the first time ever, I've made a chemistry video. It's my first attempt at this stuff. Any comments or suggestions?

A simple macroscale acetylation: N-acetylglycine


[Edited on 18-10-2021 by valeg96]

[Edited on 18-10-2021 by valeg96]





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[*] posted on 18-10-2021 at 13:28


I liked the video. Nicely concise and clear about procedure, like a good writeup. Good music too.

Excuse my ignorance, but what is the benefit of using water as a solvent in this reaction? It was my understanding that acetic anhydride is hydrolyzed by water to acetic acid - not immediately, but fairly quickly.




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Tsjerk
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[*] posted on 18-10-2021 at 13:57


Haven't you made glycine acetate?

At least the use of acetic anhydride in water is a waste. The anhydride reacts with water in the blink of an eye.
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[*] posted on 18-10-2021 at 14:21


I can't hide my skepticism, but it's a literature procedure, as you can see from the video. Also, I've seen similar procedures in water with anilines (and they work) but yeah.

The advantage is most likely that in water it's easier to purify and wash the product at the expense of some extra reactant lost. Working in glacial acetic acid is a viscous pain... Also, as always, I used less solvent than the original instructions.

http://www.orgsyn.org/demo.aspx?prep=CV2P0011

Also, to my knowledge, acetic anhydride does not react with water that fast. If I'm not mistaken one test to see if it hasn't hydrolysed is to put some drops in water, and they should sink to the bottom, like a different phase. If they dissolve, then it turned to acid during storage.

[Edited on 18-10-2021 by valeg96]

[Edited on 18-10-2021 by valeg96]





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[*] posted on 18-10-2021 at 15:02


Not immediate, as shown by DougsLab near the end of this video:

https://www.youtube.com/watch?v=g9sPlMkYEIw

But 30mins at 50C?

I would expect glycine acetate to be very soluble in water, probably deliquescent. Since it's recovered from the aqueous soln by filtration, the product is probably not glycine acetate.

@valeg96, have you done a m.p.?




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[*] posted on 18-10-2021 at 15:21


The fine white ppt crashes out after like 5 minutes, but the original procedure says to keep it stirring for 20+ minutes. I chose to extend reaction times for good measure (also to destroy leftover acetic anhydride). Also the product is not very soluble in water and not deliquescent at all. As you predicted, acetylglycine is about 10x less soluble in water than glycine.

I tried to do some TLCs with ninhydrin but I had some technical difficulties. I'll whip out some TLC procedures for the identification of aminoacids; I do not have a m.p. apparatus but only a Thiele tube, and it's probably not a good idea to heat it to 230+°C





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[*] posted on 19-10-2021 at 00:41


If Orgsyn says so, I immediately believe you made acetyl glycine, apparently the acetylation can compete with the hydrolysis of the anhydride.
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[*] posted on 19-10-2021 at 11:18


Quote: Originally posted by Tsjerk  
If Orgsyn says so, I immediately believe you made acetyl glycine, apparently the acetylation can compete with the hydrolysis of the anhydride.

Second that, its orgsyn after all.
Never had anything fail on me that they give out as procedure, its the ultimate check(in my opinion) for really working peer reviews.




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[*] posted on 19-10-2021 at 13:54


Well, I managed to check it via TLC (my first successful AA TLC with butanol and ninhydrin!) and it's definitely a compound with a secondary amine, as it's not revealed by nihydrin unlike other aminoacids.

A bit messy but from left to right: G (glycine) AG (acetylglycine) W (waste solution) E (monosodium glutamate) Q (glutamine). These last two were added for the sake of it, just to see how they'd show up.

Using sodium glutamate was a mistake as the acid eluent protonated it to the less soluble acid and smeared it all over a bigger area. Also, above the red streak of Q there is a streak of E that spilled by accident. The relevant columns are G (reactant) and AG and W. The fact that ninhydrin doesn't show the spots means they don't have a primary amine anymore. It also means there is no glycine left in the mother liquors, and the product isn't sensibly contaminated.

IMG-20211019-WA0022.jpeg - 961kB

[Edited on 19-10-2021 by valeg96]





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