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Author: Subject: The short questions thread (1)
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[*] posted on 27-1-2009 at 00:03


Yeh i didnt even take that into consideration never mind, may be useful for a few things but like you said it will pretty much cook anything at those levels
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[*] posted on 27-1-2009 at 15:08
Alumina TLC Plates


Can alumina TLC plates be used for efficient chromatography of polar molecules?

I could get a good deal on some alumina TLC plates, but have no experience w/ alumina. All of the chemistry I do is with fairly polar species (carbonyls, alcohols, amines etc.); would I get bad separation of polar compounds and/or smearing?

I would appreciate if anyone could share any TLC tips they have picked up using alumina plates.




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[*] posted on 27-1-2009 at 22:19


Quote:
Originally posted by smuv
Can alumina TLC plates be used for efficient chromatography of polar molecules?

I could get a good deal on some alumina TLC plates, but have no experience w/ alumina. All of the chemistry I do is with fairly polar species (carbonyls, alcohols, amines etc.); would I get bad separation of polar compounds and/or smearing?

I would appreciate if anyone could share any TLC tips they have picked up using alumina plates.


In addition to this question, does anyone have any experience on whether prepared tlc plates have a shelf life if stored appropriately (cool dry), i uncovered a box of various types and the like but most are dated in the early 80's. They appear fine.




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[*] posted on 28-1-2009 at 06:32


Several years ago I had to work with some old stock of alumina and silica TLC plates. The results I obtained were the same when carried out on brandnew plates. So, I think you can employ old plates with confidence. By the way, I was dealing with carotenoids :D
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[*] posted on 28-1-2009 at 08:28


i've just been making phthalimide from phthalic anhydride and ammonia. Even with googles, breather and holding it at arms length ammonia is pretty nasty stuff (dont have fume hood).
Next time i was thinking replacing ammonia with urea. Is this any less smelly?
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[*] posted on 28-1-2009 at 11:27


Alumina plates work very well for polar compounds, I use this routinely at work as most complexes I work on are sensible to silica gel.. Doesn't change much from silica plates from a practical point of view, the Rf are different but still similar. I guess as long as they are well stored they can be kept years with no problem!



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[*] posted on 28-1-2009 at 13:43


Would combining 32 percent hydrochloric acid and DCM give 2 layers, one of anhydrous HCl and one of water?
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[*] posted on 28-1-2009 at 13:57


Quote:
Originally posted by Aubrey
i've just been making phthalimide from phthalic anhydride and ammonia. Even with googles, breather and holding it at arms length ammonia is pretty nasty stuff (dont have fume hood).
Next time i was thinking replacing ammonia with urea. Is this any less smelly?

I have tried this method, using urea, and it works like a sharm. You are done with the synthesis in a few minutes, because you just have to melt it, and after some time the mixture suddenly froths up, indicating you are done.
Then I think you will have to purify from any excess urea, but I skipped this as it did not interfere with my next step.
And yes, carbon dioxide and ammonia are evolved. You will get all the ammonia in a short period of time, so capture it with a wash bottle. However, just work outside. Good thing about ammonia is that it will very quickly rise up (it's lighter than air), so it won't just hang around like bromine/chlorine/NO2 do.

Urea method is cheap, very easy, and uses not expensive reagents.
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[*] posted on 28-1-2009 at 17:51
acetylation of amine with acetyl chloride


Acetyl chloride seems much more reactive than acetic anhydride and so I'm planning to use it for an acteylation I plan. Question: do I expect the product amide to be an HCl salt or free amide?



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[*] posted on 28-1-2009 at 18:12


Combining 32 % HCl with DCM (or with any other organic solvent for that matter) will give 2 layers. Depending on the Distribution Coefficient (also called as partition coefficient) of the system, each layer will contain different proportions of HCl, water and the organic solvent.

HCl has extremely strong affinity towards water. it is difficult to strip HCl out of water short of consuming it by reaction with alkali.

gsd
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[*] posted on 28-1-2009 at 18:27


Quote:
Originally posted by chemrox
Acetyl chloride seems much more reactive than acetic anhydride and so I'm planning to use it for an acteylation I plan. Question: do I expect the product amide to be an HCl salt or free amide?


The amide group itself is not particularly basic (electron delocalization). Only if you have more basic groups present in your compound will you wind up with the salt.

sparky (~_~)




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[*] posted on 28-1-2009 at 19:04


Ahh .. yeah .. I forgot to mention the pyrrolidine moiety and that was the crux of the question.. woops



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[*] posted on 28-1-2009 at 19:23


Quote:
Originally posted by chemrox
Ahh .. yeah .. I forgot to mention the pyrrolidine moiety and that was the crux of the question.. woops


If so, yes, it comes out as the salt; more so if the sp<sup>3</sup> N is not tertiary.

sparky (~_~)




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[*] posted on 31-1-2009 at 08:35


Hello, I have perhaps very simple question of whether it is possible for this reaction

Glycine
H2N-CH2-CO2Et convert to Cl-CH2-CO2Et

with use of NaNO2 and HCl or if there is problem with forming this not wanted compound. May it be best to form alpha acid first?
Thank you if you can help me

[Edited on 31-1-2009 by Oboe]
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[*] posted on 31-1-2009 at 09:12


Quote:
Originally posted by Oboe
Hello, I have perhaps very simple question of whether it is possible for this reaction

Glycine
H2N-CH2-CO2Et convert to Cl-CH2-CO2Et

with use of NaNO2 and HCl or if there is problem with forming this not wanted compound. May it be best to form alpha acid first?
Thank you if you can help me


Apparently.

sparky (~_~)




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[*] posted on 31-1-2009 at 14:13


Yes that is alternative Mr sparkgap. Ester is target though but it seems to stabilize diazo compound to get stable compound I linked. Perhaps someone knows if is possible to go from this stable compound to chloride?
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[*] posted on 31-1-2009 at 14:34


you could first make the chloroacetic acid and then form the ester; It is easier to form esters with chloroacetic acid than acetic acid because it is more electrophilic.



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[*] posted on 1-2-2009 at 08:33


My school has recently given be two very old bottles (1985) of Phthalic acid and phthalic anhydride.
Both are in very good condition, and i see no reason why the phthalic acid is affected in any way by its age.
I am not sure about the anhydride however, could it have absorbed water to hydrate itself to form phthalic acid?
I am not sure how readily phthalic anhydride combines with water, i hear some anhydrides have to be refluxed with water for days to hydrate.
thanks,
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[*] posted on 1-2-2009 at 11:05


both are solids at STP... I don't see why there'd be a lot of interconversion if they've been kept in bottles in cabinets... you're not in Kentucky or Vietnam are you?



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[*] posted on 1-2-2009 at 14:19


I recently made acetic acid by distillation of sodium acetate and sulfuric acid. The sodium acetate had been dried in the oven at 500F and the sulfuric acid was technical grade. I took 150ml of this and added two spoonfuls of anhydrous MgSO4 to dry it then I filtered it. The resulting solution was distilled with another spoonful of MgSO4 and the first 125ml that came over were collected.

I have placed the acid in the refrigerator and although it is 7C it is not totally frozen. What did I do wrong or what can I do to make it anhydrous?




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[*] posted on 1-2-2009 at 14:35


Are you sure it did not 'supercool'. My GAA was still liquid at 5C, but when I shaked the bottle, it all solidified, a beautiful process to see.
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[*] posted on 1-2-2009 at 16:42


I shook it and it didn't freeze. also the acetic acid before purification partially froze in the refrigerator and froze solid in the freezer but after the dessication step it wont even freeze below 0C.

This is the result of several combined bathes in the later batches I used a slightly dirty sulfuric acid but the product looked the same maybe this has something to do with it?

I have no idea why it won't freeze it doesn't make sense.:mad:




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[*] posted on 1-2-2009 at 17:03


crazyboy, I have a feeling this is freezing-point depression at work. Your acid is not as pure as you think it is, and the "slightly dirty sulfuric acid" is a very probable suspect.

sparky (~_~)




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[*] posted on 1-2-2009 at 17:10


That is what I suspected. As I said the first batches froze partially at 7C and totally at 0C but after I combined them with my latest one nothing freezes it.

The first few times I was using drain cleaner sulfuric acid which had a yellow tinge. I ran out of that so I switched to my gallon bottle of the same brand but the color was much darker.

I'm assuming it is some sort of buffer in the acid. Any idea what to do or should I just toss the acetic acid?




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[*] posted on 1-2-2009 at 17:28


Thank you for mr sparkgap and mr smuv for help with my question :)

mr crazyboy. perhaps to add clean sulfuric and distill again?
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