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jon
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cyanide became safe to handle??
did i just read that????
or am i on drugs?
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Sedit
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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)
Knowledge is useless to useless people...
"I see a lot of patterns in our behavior as a nation that parallel a lot of other historical processes. The fall of Rome, the fall of Germany — the
fall of the ruling country, the people who think they can do whatever they want without anybody else's consent. I've seen this story
before."~Maynard James Keenan
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watson.fawkes
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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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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. 
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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Reference
Anders Hoveland
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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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Melgar
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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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Jimmymajesty
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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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rrkss
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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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future625
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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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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.
Rest In Pieces!
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future625
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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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ksj_6808
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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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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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Lithium
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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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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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cold_electrician
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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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tetrahedron
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| 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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Magpie
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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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Dr.Bob
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| 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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DJF90
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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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hissingnoise
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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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hissingnoise
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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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cold_electrician
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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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Dave Angel
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| 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!
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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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hissingnoise
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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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