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Author: Subject: Acetaldehyde synthesis
jon
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[*] posted on 23-6-2010 at 22:16


cyanide became safe to handle??
did i just read that????
or am i on drugs?
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[*] posted on 24-6-2010 at 07:59


Well you sort of read that but yes you are on drugs...

Peach was arguing the semantics on the usage of calling a compound to toxic(accumulates in the body such as Hg) and harmful(death on the spot like H2S or HCN)





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watson.fawkes
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[*] posted on 24-6-2010 at 08:11


Quote: Originally posted by jon  
cyanide became safe to handle??
did i just read that????
or am i on drugs?
You are illustrating why changing ordinary language usage into jargon is completely dangerous in this case. The reason for labeling compounds toxic and harmful is that these words are meaningful in an ordinary sense to ordinary people. If you use these meanings in some jargon-laden way, it not only fails to communicate, but communicates something that is other than truthful.
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peach
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[*] posted on 28-6-2010 at 08:48


They're now planning to use H2S for a form of suspended animation of seriously injured casualties on the way to surgery. They worked out that a very precise dose of H2S antagonizes (blocks) the sites that would be damaged by oxidation when normal homeostasis wasn't functioning. They've been testing it on mice and the results are so positive they're going to human trials. I think they'll be injecting it dissolved in solution. The 'victim' also needs cooling in an ice bath. :P

I don't want to start arguing, but it's somewhat ironic you're calling it jargon when you'd likely have a fit on me if I started using none standardized names for reagents and solvents. I know what you're saying, but I think jon is joking.

He's actually illustrating my own point perfectly well. In that people now assume harmful means safe to handle. Whilst genuine lab suppliers are still shipping things that should now be labeled toxic as harmful. It may as well not exist as a label.

And I know none of this has anything to do with aldehydes and I hope I haven't annoyed you. I didn't mean to do so if I did, I just don't like the blanket use of the word toxic, as it desensitizes people to the real risks. I'll shut up now.

[Edited on 28-6-2010 by peach]
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[*] posted on 18-7-2010 at 02:41


Just a thought, if you can get your hands on a Selenium compound,
acetone reacts with selenium dioxide to make an aldehyde.
O=CH-CO-CH3
http://www.transtutors.com/chemistry-homework-help/aldehydes...
You could protect the aldehyde by condensing with NH4OH, leaving the ketone vulnerable to be reduced (for example with hydrogen iodide) but then I am unsure of how to recover the aldehyde again, after it has condensed. Maybe reacting with NaOH will give off ammonia gas and make NaOCH(OH)-CH2-CH3 , then acidify to recover the aldehyde.
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[*] posted on 20-7-2010 at 19:28


Treating amino acids with bleach does indeed produce aldehydes, for example, alanine + bleach gives acetaldehyde. However, this reaction also produces CO2 and NH3, and the NH3 is chlorinated into one of the chloramines. Still not entirely sure how to get rid of those...
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[*] posted on 21-7-2010 at 11:29


I never succeeded in recovering the acetaldehyde from dilute solutions, I usually got an unpleasant smell, that is far away from the smell of pure acetaldehyde. Also the acetaldehyde does not like to be acidified/basified:) you will get either a yellow solution with and acrid smell or an orange sticky polymer with an apple like smell, depending on the pH that you disturb poor little CH3CHO molecule:)
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[*] posted on 23-7-2010 at 02:07


Quote: Originally posted by Hoveland  
What is the reaction with acetaldehyde when the pH goes to high or too low?
I found an excellent explanation about aldehydes: http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/aldk...
which did not seem to tell what these reactions might be, but as a side note, I did learn

"Although the addition of water to an alkene is exothermic and gives a stable product (an alcohol), the uncatalyzed reaction is extremely slow due to a high activation energy . The reverse reaction (dehydration of an alcohol) is even slower, and because of the kinetic barrier, both reactions are practical only in the presence of a strong acid."


Look up aldol condensation for part 1. Part 2 of your response has nothing do do with aldehydes or acetadlehyde.
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[*] posted on 8-1-2012 at 22:04
Practical Preparation


that I know of there are 3 practical benchtop methods of preparing acetaldehyde; Namely the dehydrogenation of ethanol, the dehydration of ethylene glycol and Oppenauer oxidation.

1. My favorite and first goto would be the Oppenauer oxidation of absolute ethanol with aluminum isoproproxide in acetone. I would make the abs. EtOH by drying Everclear over plaster of paris. One could get everything at retail around town!

2. Dehydration of glycol is a combustion tube method. vapor phase ~400`C. essentially it gives 65/35 either way of 1,4 dioxane and acetaldehyde. alumina and Al phosphate are catalysts. can be made selective for either product by catalyst choice.

3. Dehydrogenation of ethanol with oxidative option. essentially one passes ethanol vapor through copper tubing packed with copper wool or BB's about 30% is converted to acetaldehyde and hydrogen. the temperature is ~280-300`C or 260-290 etc this could be achieved w fiberglass cloth, nichrome resistance wire and then fiberglass batting for insulation. This is an endothermic reaction. preferred catalyst is copper sponge; copper oxide, and copper chromite are active as are others

3b the oxidative option, one blends air with ethanol vapor, the O2 consumes H2 and drives the hydrogenation equilibrium towards the products While supplying process heat to boot.

Overall, I highly recomend No. 1 over all others. Oppenauer oxidation is suitable for preps on the order of a Kg. methods 2 and 3 are barrel size batch benchtop preps, I wouldn't consider using them for batches under a gallon, and probably wouldn't bother unless batch size was greater than 5 gallons.



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Adas
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[*] posted on 9-1-2012 at 08:59


My favorite is to dehydrogenate ethanol - Put pressed copper wires into a glass tube, heat it using induction heating (to not waste gas + this is ineffective) and pass ethanol vapor through it. I find this method the best.



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future625
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[*] posted on 11-1-2012 at 12:48


Im a big fan myself. I've been thinking about how ethyl acetate in ethanol would work out, subjected to the copper tube.
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[*] posted on 13-9-2012 at 23:14


As for what ive read on the acetaldehyde the most practical way of making this would be either oxidation of ethanol through a mild oxidiser such as the dichromate in 23% sulfuric acid or oxidation through copper or platinum (not sure on this one though). Type in "Acetaldehyde synthesis erowid" in google and click on the one that goes to the erowid site. This seems very logical, basically a distillation apparatus is set up, and 23% sulfuric is placed in the boiling flask, then an addition funnel is connected to the boiling flask with ethanol/water/sodium dichromate, the sulfuric is heated just enough to boil and the solution in the sep funnel is slowly dropped in, and the heat can be turned off because the reaction makes its own sufficient amount of heat and the acetaldehyde/other products are distilled over, then the crude acetaldehyde is put into a reflux setup, basically just refluxing and bubbling the vapors off of it through ether. The wash bottle is basically an enclosure where a gas line leads to it from the top of the condenser and it bubbles through the solution of ether, then that bottles exit gas nipple is connected to another wash bottle with ether ect ect...just go read it. Anyways i've seen a red hot copper rod stuck into a beaker of ethanol and supposedly it converts the ethanol into acetaldehyde which might be too much oxidation...Im pretty damn sure the dichromate way works, i've read quite a bit on it. IF YOU CANT GET SODIUM DICHROMATE AND YOU LIVE IN THE USA, THEN YOUR EITHER DUMB OR SOMETHING, ITS CALLED EBAY!!! Also its sold in solutions for electroplating.
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weiming1998
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[*] posted on 14-9-2012 at 03:14


I have made acetaldehyde as a concentrated solution in water by adding calcium hypochlorite to excess ethanol in a flask, then adding a stopper on and heating a bit. The reaction goes very fast, and is exothermic, to the point that runaway can occur in large amounts. producing a fruity-smelling gas that dissolves fully in water. This gas is acetaldehyde because on reaction with further calcium hypochlorite, filtration, then boiling and acidification, formic acid is produced (the resulting liquid is very pungent and stings the nose).
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[*] posted on 14-9-2012 at 13:28


Yes, i too have performed the oxidation of ethanol with calcium hypochlorite. I think it is a very scary reaction in the respect that it can run away very easily. When i am performing it, i add solid 70% calcium hypochlorite to the flask and add the ethanol all at once. i frequently put the flask in a ice bath as i fear it will explode or start to boil the ethanol along with it. once the icebath is added, the reaction seems to stop, and no more acetaldehyde is produced. once the ice bath is removed and a small flame is applied, it vigorously restarts, producing copious amounts of acetaldehyde gas. this gas can be passed through a CaCl2 drying tube and then bubbled into a cold solution water, ethanol, dichloromethane. sometimes it is hard to see if all the Ca(OCl)2 has been used, as after the reaction has complete, there is a CaCl2 brick at the bottom of the flask. it seems it would be wise to utilize an addition funnel to slowly drip the ethanol into the flask, as to ensure even bubbling and smoother/safer reaction.



Li
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borrowedlawyer
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[*] posted on 8-10-2012 at 02:17


Quote: Originally posted by jon  
cyanide became safe to handle??
did i just read that????
or am i on drugs?


Cyanide compounds can be safely handled (this is true with almost anything), provided adequate precautions are taken. However, most are very toxic, as you already know.

[Edited on 8-10-2012 by borrowedlawyer]
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[*] posted on 8-10-2012 at 11:17
Help! PCC method...


Hello all, I'll begin with the formality of introducing myself as "a long-time lurker, 1st-time post-er". I have a question which should be simple, but I can't seem to find a proven, precise method on ANY search engines... Lots of theory and stoichiometric equations, but nothing specific enough for me to confidently proceed... So here goes:

I've succeeded with the pyridinium chlorochromate (PCC) prep, and am ready to oxidize my dry ethyl alcohol; I intend to use celite, silica, 4A sieves. I've read all the literature I could find for oxidizing primary alcohols to aldehydes, but it seems like nobody's bothered to describe a specific workup procedure, since its such a straightforward reaction. I don't want to waste my reagent! Not a fan of CrO3 :(

Can anyone point me in the right direction? I've got an arsenal of pure, dry solvents to chose from, EXCEPT that I lack diethyl ether... just have petroleum ether(s) available.

The original paper [Tetrahedron Letters, No. 31, Corey, E.J. and Suggs, W., 'Pyridinium Chlorochromate...'] describes a TINY-scale procedure, with 1.5mmol PCC in 2mL DCM, and '1 mmol in 0.5 to 1.5 ml of CH2Cl2' of Alcohol X...

Does anyone have experience with this? I'm not comfortable with blindly scaling-up the given molar ratios; I'm thinking more along the lines of ~1mol of PCC.

Any help would be greatly appreciated, and I'd be glad to post results and pics.




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[*] posted on 8-10-2012 at 12:53


Quote: Originally posted by cold_electrician  
I've succeeded with the pyridinium chlorochromate (PCC) prep, and am ready to oxidize my dry ethyl alcohol; I intend to use celite, silica, 4A sieves. I've read all the literature I could find for oxidizing primary alcohols to aldehydes, but it seems like nobody's bothered to describe a specific workup procedure, since its such a straightforward reaction. I don't want to waste my reagent! Not a fan of CrO3 :(

Can anyone point me in the right direction? I've got an arsenal of pure, dry solvents to chose from, EXCEPT that I lack diethyl ether... just have petroleum ether(s) available.


hi cold_electrician.

i think the reason you can't find a detailed write up is that the PCC prep is only a general method for preparing small quantities of aldehydes from new alcohol compounds. it certainly isn't meant to provide a routine acetaldehyde synthesis.

note that petroleum ether is not a true ether and is completely different in properties from diethyl ether. but if they say DCM anyway..
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[*] posted on 8-10-2012 at 17:29


Here's an excerpt from OrgSyn. You can find it yourself by searching with the term pyridinium chlorochromate:

B. General oxidation procedure for alcohols. A sufficient quantity of a 5% solution of dipyridine chromium(VI) oxide (Note 1) in anhydrous dichloromethane (Note 7) is prepared to provide a sixfold molar ratio of complex to alcohol, an excess usually required for complete oxidation to the aldehyde. The freshly prepared, pure complex dissolves completely in dichloromethane at 25° at 5% concentration, giving a deep red solution, but solutions usually contain small amounts of brown, insoluble material when prepared from crude complex (Note 8). The alcohol, either pure or as a solution in anhydrous dichloromethane, is added to the red solution in one portion with stirring at room temperature or lower. The oxidation of unhindered primary (and secondary) alcohols proceeds to completion within 5 to 15 minutes at 25° with deposition of brownish-black, polymeric, reduced chromium–pyridine products (Note 9). When deposition of reduced chromium compounds is complete (monitoring the reaction by GC or TLC is helpful), the supernatant liquid is decanted from the (usually tarry) precipitate, which is rinsed thoroughly with dichloromethane (Note 10).
The combined dichloromethane solutions may be washed with dilute hydrochloric acid, sodium hydrogen carbonate solution, and water, or filtered directly through a filter aid, or passed through a chromatographic column to remove traces of pyridine and chromium salts. The product is obtained by removal of dichloromethane; any pyridine that remains can often be removed under reduced pressure.


[Edited on 9-10-2012 by Magpie]




The single most important condition for a successful synthesis is good mixing - Nicodem
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[*] posted on 8-10-2012 at 19:29


Quote: Originally posted by cold_electrician  

Does anyone have experience with this? I'm not comfortable with blindly scaling-up the given molar ratios; I'm thinking more along the lines of ~1mol of PCC.


I wouldn't start at 1 mol, I would start at maybe .1 mol to test the chemistry first. Isolating the aldehyde will be tough, due to the BP, many people would just go ahead and use the solution of aldehyde in DCM after perhaps filtering through Celite and some silica gel to remove the PCC salts. You could distill the aldehyde and condense with a dry ice cooled condenser (or at least salt and ice.) if you want to purify it, but it is not easy to keep around.
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[*] posted on 9-10-2012 at 07:12


You can store acetaldehyde as one of the polymeric forms, either the trimer paraldehyde (produced with sulfuric acid catalysis IIRC) or the tetramer metaldehyde (with hydrogen halide catalysis). In some cases they can be used as an in-situ source of acetaldehyde.
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[*] posted on 9-10-2012 at 07:30


I don't know the metaldehyde content of slugkiller pellets but heating to ~80°C breaks the tetramer so that acetaldehyde might be (carefully) distilled from the material!
The pellet's fruity smell is that of the aldehyde . . .

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[*] posted on 9-10-2012 at 07:53


All things considered though, hydration of C<sub>2</sub>H<sub>2</sub> is most convenient if one doesn't have ethanol to hand . . .
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[*] posted on 9-10-2012 at 11:19


Many thanks! The OrgSyn description with Collin's Reagent is the best workup I've seen, but the reason I chose PCC was to avoid the "sixfold molar ratio of complex to alcohol", and of course, because of the resources which I personally have access to.

I've read that PCC can be used in near-stoichiometric amounts? I'd certainly use an excess, which is always recommended, but my impression is that one of the main advantages of PCC is that lower quantities are required.

Dr. Bob, I think I'll start out with the .1mol suggestion, and do a couple preliminary trials, "scientific method" style. You hit the nail on the head regarding the issue of isolation - I definitely want to use (immediately!) a solution w/ DCM - not trying to test the stability of acetaldehyde in storage.

And if all else fails, slug poison.




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[*] posted on 9-10-2012 at 12:13


Quote: Originally posted by hissingnoise  
I don't know the metaldehyde content of slugkiller pellets but heating to ~80°C breaks the tetramer so that acetaldehyde might be (carefully) distilled from the material!


I looked into this one a while back and will likely revisit. For current reference (as I'm sure it is known) you can buy pellets at somewhere between 1 and 3 % metaldehyde depending on brand, and I've found a concentrated suspension at 20 % in more serious garden centres.

I tried a couple of rough depolymerisations of the 20 % suspension and succeeded mainly in subliming metaldehyde into my condensor coil, probably by heating too rapidly. Actually, perhaps one would be wise to sublime the material first to obtain the pure metaldehyde in case there are impurities inhibiting depolymerisation.

Furthermore, I stored the depolymerisation mixes in a flask, intending to continue the attempt, left it for months and came back to find some acetaldehyde in solution. I managed to distill this out with some serious cooling (the likes of an ether condensor) and a heavily chilled ice bath for the receiver.

So essentially, one route is to dilute the metaldehyde in a low concentration of sulphuric acid, stick it in a bottle for months and forget about it, and then distill the result when you have a lab tidy. Caveat: this was very rough, undocumented work so YMMV. Thermal depolymerisation, as hissingnoise suggests, is likely a more elegant route.




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[*] posted on 9-10-2012 at 13:05


It's also, unfortunately, a slow process and the 20°C B.P. doesn't help.
Hydration of ethyne in dilute sulphuric acid with mercuric sulphate catalyst at ~80°C with a well chilled receiver to condense CH<sub>3</sub>CHO should work well!
Impurities in carbide-produced ethyne will eventually poison the catalyst but it's still a usable process in the lab.

BTW, I should say I haven't actually tried either process . . . yet . . .

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