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Author: Subject: The short questions thread (2)
Nicodem
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[*] posted on 10-4-2009 at 00:13
The short questions thread (2)


The old thread has become too long so please continue here. Those who never received a proper reply in the old thread, please feel free to repost the question.
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sakshaug007
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[*] posted on 10-4-2009 at 18:39


Hello everyone,

I was wondering if anyone knows where to get teflon wool? I want to use it for packing my vigreux column for distillation of corrosive compounds.

Thanks a lot.
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[*] posted on 10-4-2009 at 18:54


Vigreux columns are not meant to be packed. Additionally, i think it would be difficult to pack a vigreux column without breaking it.

A hempel column is meant to be packed, and there is little reason pack it with teflon wool as raschig rings, lessing rings, copper wool, stainless steel wool and even glass wool are cheeper, more readily available options. There is nothing you can distill without eating through the glass column that at least one of these packing materials cannot stand up to.




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sakshaug007
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[*] posted on 10-4-2009 at 19:53


Quote: Originally posted by smuv  
Vigreux columns are not meant to be packed. Additionally, i think it would be difficult to pack a vigreux column without breaking it.

A hempel column is meant to be packed, and there is little reason pack it with teflon wool as raschig rings, lessing rings, copper wool, stainless steel wool and even glass wool are cheeper, more readily available options. There is nothing you can distill without eating through the glass column that at least one of these packing materials cannot stand up to.


I can easily pack my column and have with steel wool before, it gives that much more surface area. I'm interested in Teflon wool because I want the most chemically inert substance for packing i.e. acid distillations, but I suppose I could use glass wool in this case.

Thanks for the reply
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not_important
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[*] posted on 10-4-2009 at 21:33


We are talking about the same here, yes? Vigreux columns are indeed not meant to be packed, doing so greatly increases their holdup, increases the pressure drop across them (they work well for vacuum distillations), and may actually decrease increase their HETP value.

For packed columns the better the thermal conductivity of the packing, the better the column works. Metal packing is preferable over glass or plastic, and beads over wool. Fibrous packings (wools and the like) tend to increase the holdup over other types, lower density Teflon's porosity also increases holdup; I think that Teflon wool would be a poor choice for any situation except those where a great deal of material will be distilled, such as solvent reclaiming.

With any decently constructed Vigreux there's not too much room for packing, and getting wool type packings into the interior would be quite difficult. On most of my Vigreuxs there are almost no clear paths through the length, if any at all; several are designed where opposing horns are of different lengths so there is no central path clear. And because on most the horns are tilted slightly downwards you'd never get a wool type packing back out.

If you bought a Vigreux that is easy to pack, you were ripped off badly. It's just plain shoddily constructed, there should not be anywhere that much open space in it.


Vigreux_column.jpg - 23kB
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sakshaug007
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[*] posted on 10-4-2009 at 22:26


Quote: Originally posted by not_important  
On most of my Vigreuxs there are almost no clear paths through the length, if any at all; several are designed where opposing horns are of different lengths so there is no central path clear. And because on most the horns are tilted slightly downwards you'd never get a wool type packing back out.

If you bought a Vigreux that is easy to pack, you were ripped off badly. It's just plain shoddily constructed, there should not be anywhere that much open space in it.


Actually you know what I might have been ripped off, I purchased this vigreux column off of ebay and it does not have the obstructed horn pathways your referring to. On the outside it looks like the one you have pictured but the interior of the tube is completely open and only the walls are roughened (bumpy). Maybe I should have researched these columns more before making an impulse purchase.

Thanks

[Edited on 11-4-2009 by sakshaug007]
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[*] posted on 11-4-2009 at 04:43
Electrolysis of LiCl in DMSO


@ sakshaug007: I was very interested in your results with LiCl in acetone which you mentioned in the first short question thread. As far as solubility of LiCl in acetone, you stated that you dissolved .35 g LiCl in 100cc acetone. Did you mean 3.5g? CRC tables I have state a solubility of 4.1 g LiCl in 100 ml acetone at 25C.

I have used LiCl in DMSO (5g in 60 cc--result is a viscous solution) and electrolyzed with a graphite anode and aluminum cathode. Distance between electrodes was ~ 1cm, voltage 15 V and current .25A. A black amorphous deposit was formed on the Al cathode which was very reactive with water. Almost no visible activity (no visible bubbles) at the anode. There is some bubbling at the cathode.

If I use graphite for the cathode, it begins to decompose, fouling the solution.
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[*] posted on 11-4-2009 at 17:49


Quote: Originally posted by sakshaug007  
Hello everyone,

I was wondering if anyone knows where to get teflon wool? I want to use it for packing my vigreux column for distillation of corrosive compounds.

Thanks a lot.


NI is correct on every detail, in fact incredibly succinct, i don't suffer from economy of detail so let me blather on a bit here.
For solvent reclaiming teflon wool is an excellent packing material, especially given you simply make it yourself. Important though, as not important stated (ha! what a sentence), that it is high or medium density not the porous low density type.

Get any old scrap solid teflon block and on your drill press get a 10-20 mm drill bit and drill out holes slowly, the wool shaves off as one continuous strand, fill a bag with this and then shake up the bag to intertwine it all. Takes about 15min from start to finish (assuming you have some scrap teflon, a drill press, 15mm bit and a plastic bag)




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[*] posted on 11-4-2009 at 19:34


My questions in the previous thread seem to have been missed:

Quote:
1. Can diphenyl carbonate and diphenyl oxalate undergo Fries rearrangement?

2. Would the conditions for hydrogenolysis of a benzyl group reduce double and triple bonds too?


sparky (~_~)




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[*] posted on 11-4-2009 at 19:45


What's the easiest way of cleaning commercial glass wool meant for house isolation? I.e. removing the yellow color/contaminants. Acetone, sulfuric acid?
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[*] posted on 11-4-2009 at 22:17


Can someone tell me what the name of FeS2O8 is? And a possible synthesis if your feeling up to it.




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[*] posted on 11-4-2009 at 22:38


Persulfate AKA peroxydisulfate, and I suspect that it is not synthesed but a solution of Na/K/NH4 peroxydisulfate and a ferrous compound used.

while we're at it,

1. Can diphenyl carbonate and diphenyl oxalate undergo Fries rearrangement?

Quote:
In all instances only esters can be used with stable acyl components that can withstand the harsh conditions of the Fries rearrangement. If the aromatic or the acyl component is heavily substituted then the chemical yield will drop due to steric constraints.


So I doubt the oxalte will work to give standard Fries products, but diphenyl carbonate might yield the phenyl ester of hydroxybenzoic acids.




2. Would the conditions for hydrogenolysis of a benzyl group reduce double and triple bonds too?

Mostly likely, but there are some special reduction tricks around, so I can't say that with 100% confidence. Maybe use trityl instead?
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[*] posted on 12-4-2009 at 12:26


Does anyone know the best route or have a reference to 2-bromobenzamide? I imagine a halogenation of benzamide, but expect this may result in an o,p mix which may be difficult to separate.
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[*] posted on 12-4-2009 at 14:19


The o-bromo product is the "kinetic" product, whereas the p-bromo is "thermodynamic". Running the aryl bromination with Fe + Br2 at reflux in say ether, or maybe dioxane. I had a quick search of the literature, but this stuff is now so readily available (albeit not cheaply) that it's hard to find the old preparation. The other option I see is accessing the bromine via diazotization of 2-aminobenzamide.

[Edited on 12-4-2009 by Arrhenius]
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[*] posted on 12-4-2009 at 15:18


Arrhenius: If the target material is the kinetic product then how will carrying out the reaction at reflux help you, especially in dioxane (bp ~100C) :S Any kinetic product is generally favoured by low temperatures (generally below 0C, and usually at -78C) and short reaction times.

One possible route from benzamide is sulfonation at the 4 position, then mono nitration, desulfonation, reduction to 2-aminobenzamide and then a sandmeyer reaction of the diazo salt with CuBr.

Edit: Synthesis of benzamide has been covered here, and is a nice write up by smuv.

http://www.sciencemadness.org/talk/viewthread.php?tid=4201&a...



[Edited on 12-4-2009 by DJF90]
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[*] posted on 12-4-2009 at 23:10


Hello,

Does anyone know if the Kolbe electrolysis reaction can be used for producing alkyl halides? I ask because I recently stumbled upon an article addressing the synthesis of methyl iodide by electrolysis of aqueous potassium acetate in the presence of iodine or potassium iodide.

Perhaps I need clarification on the mechanism because I'm not exactly sure how free radicals can react with anions to form covalent bonds, wouldn't there be an extra electron to account for? i.e. CH3* + I- => CH3-I + e-?

Thanks a lot!

[Edited on 13-4-2009 by sakshaug007]
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[*] posted on 13-4-2009 at 02:39


Quote: Originally posted by Arrhenius  
The o-bromo product is the "kinetic" product, whereas the p-bromo is "thermodynamic".

The -CONR2 group has -M and -I electronic effects and as such it directs the electrophilic aromatic substitution on the meta positions. But anyway, electrophilic bromination of aromatic compounds is not reversible enough that an equilibration of the products toward the thermodynamic ones would be feasible under normal reaction conditions. This was already discussed in a thread in relation to bromination of naphthalene when the desired product is 2-bromonaphthalene instead of the kinetic product (1-bromo...).



@sakshaug007: Yes, in principle, if you have a proper oxidant like elemental halogens present during the Kolbe electrolysis you should obtain the appropriate oxidation products of the radicals (like R-X) as the major product instead their coupling products (R-R). Check the literature, surely there must be a few papers on the topic. However, your equation is on the wrong track. Radicals rarely react with nucleophiles in a simple manner (but they can get reduced by them, form an anion radical, or abstract other radicals from it). Yet, they do very easily react with oxidants, in this case with elemental halogens: R<sup>*</sup> + X<sub>2</sub> => R-X + X<sup>*</sup>. Of course, if you have KI then I<sub>2</sub> (actually I<sub>3</sub><sup>-</sup>;) forms at the same electrode as the decarboxylation of the RCOO<sup>*</sup> radicals is going on.

@sparkgap: Though, ArO-CO-OAlkyl esters in theory should not work simply in the acid catalysed Fries rearrangement due to carbocation fragmentation problems, I can't see why a diaryl carbonate does not or would not work. With ArO-CO-OAr fragmentation should not be an issue and one should obtain the corresponding 2,2'-dihydroxybenzophenone. Yet there is nothing in SciFinder or Beilstein. With oxalates intermediate fragmentation is even more of an issue, and I could also find no literature examples.
All I could find is a reference for a photochemical Fries rearrangement of a phenyl carbonate (ArO-CO-OEt, 5 examples): Journal of Organic Chemistry, 37 (1972) 3160-3163. But this is mechanisticaly different from the normal Fries rearrangement.
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[*] posted on 13-4-2009 at 07:31


Quote:

Yet there is nothing in SciFinder or Beilstein.


Just as I suspected. :( Thanks, n_i and Nicodem, back to the drawing board...

~~~~~~~~~~~~

Another quickie: am I correct in assuming that perfoming a Baeyer-Villiger oxidation on a ketone with remote (i.e. non-conjugated) double bonds will also epoxidize those double bonds?

sparky (~_~)




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[*] posted on 13-4-2009 at 07:55


Quote: Originally posted by sparkgap  

Another quickie: am I correct in assuming that perfoming a Baeyer-Villiger oxidation on a ketone with remote (i.e. non-conjugated) double bonds will also epoxidize those double bonds?

I guess it would really be difficult to avoid epoxidation with the common reagents like MCPBA or other such peracids, particularly since double bonds, nonconjugated to any carbonyl or other electron withdrawing group, can be pretty reactive toward peracids and I think Baeyer-Villiger oxidation generally requires conditions harsher than epoxidation. Maybe, you could avoid epoxidation by using a persulfate in aqueous solution (with acetonitrile as cosolvent). I think persulfates might be faster at Baeyer-Villiger oxidation than alkene epoxidation (just guessing based on the mechanism). But again, this is a problem calling for a thorough literature search (when time allows...).

[Edited on 13/4/2009 by Nicodem]
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[*] posted on 13-4-2009 at 14:37


Quote: Originally posted by Lambda-Eyde  
What's the easiest way of cleaning commercial glass wool meant for house isolation? I.e. removing the yellow color/contaminants. Acetone, sulfuric acid?


You could try boiling a piece in an excess of Piranha's acid for some time. That will oxidize organic compounds. But there might also be some other compounds in them.
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[*] posted on 13-4-2009 at 20:23


Im assuming he means insulation correct? Heating this material will turn it compleatly white but leave it a little brittle so its a bit of a trade off it you ask me. It still worked well enough for a filter pack if thats what you had in mind.




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[*] posted on 13-4-2009 at 21:54
N-alkylation of piperazine


I looked up an N-alkylation of piperazine in Org Syn and am puzzled by the rather straight forward procedure and results. I have two questions: why is it desireable to get the piperazine in its dihydrochloride form and why does the reaction with R-Cl yield the desired R-piperazine instead of a mix of monoalkylated and dialkylated products?

http://www.orgsyn.org/orgsyn/pdfs/CV5P0088.pdf

one of my professors was a checker on this reax so I feel especially confident in it.




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[*] posted on 13-4-2009 at 23:04


When you add equal molar quantities of the hexahydrate and the dihydrochloride to the solution then there is only enough HCl "in solution" to protonate half the nitrogens. This should cause one nitrogen in each piperazine ring to be protonated and thus non-nucleophilic, and hence alkylation only occurs once.

[Edited on 14-4-2009 by DJF90]
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[*] posted on 16-4-2009 at 18:09


Will dehalogenation of an aliphatic compound by catalytic hydrogenation , using Pd/C also work in removing a Flouride, in the similar way as in the removal of Iodine, Bromine and Chloride....i know the Flouride is very reactive, hence it might be a challenge.....solo



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[*] posted on 17-4-2009 at 10:22


Hello everyone,

I have recently become very interested in Kolbe electrolysis particularly concerning alkyl halide synthesis. I want to attempt a kolbe electrolysis of a solution of sodium acetate and potassium iodide to form methyl iodide and would like to ask your advice for optimizing efficiency of the electrochemical cell so that I get the most methyl iodide with the least amount of ethane and methanol byproducts. For instance, should the solution be hot or cold? Should I have it stirring or not? Should I have divided cells? Should I use a dilute or saturated solution of iodide and acetate (I assume saturated)? What voltage, current density etc.

I have read previous posts concerning reaction conditions for Kolbe electrolysis and it was stated that the highest yields are obtained using smooth platinum electrodes (which I of course don't have). I will instead use graphite electrodes which have shown to produce moderate yields of the kolbe products (i.e. ethane). I thought about platinized titanium electrodes but they are apparently detrimental to the reaction (not sure why). Does anyone know where I can get "cheap" (Ha ha!) platinum?

If anyone could provide references or textbook information regarding Kolbe electrolysis would be very helpful as well.

Thank you very much




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