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plasma
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Acetaldehyde synthesis
Does anybody have a synthesis for this, besides megalomanias. (I don't have sodium dichromate)
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vulture
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Maybe oxidation of ethanol with hydrogenperoxide? While I think of it, this must require a catalyst.
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Nick
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C2H5OH + H2O2 does work, if you add a catalyst. I used a few grains of K2Cr2O7, but a little KMnO4 would work.
Trouble is, yields are low, and if you use conc. reagents then runaway can occur.
Another way is passing air and ethanol vapour over hot copper, then distil the ethanal from the ethanol and ethanoic acid.
Maybe you could reduce ethanoic acid with Na2S, or mix it with HCl and slowly add zinc dust (not quite enough to react with all the HCl),
then distil off the ethanal.
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plasma
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Will this work :
HCHO + CH3OH -> CH3CHO + H2O
Thanks !
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madscientist
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Sorry plasma, that won't work; organic chemistry isn't as straightforward as inorganic. :-/
Nick, are you sure that using KMnO4 will work as a catalyst in a H2O2 / CH3CH2OH reaction that is to form CH3CHO? KMnO4 oxidizes H2O2 readily, causing
both to decompose... I have tried adding two or three grains of KMnO4 to a test tube 1/4 of the way filled with 27.5% H2O2, and the result was a
violent, hot, oxygen gas / steam forming reaction. All that was left was some very hot water, and Mn3O4 (I believe that's what the dark brown
precipitate was).
I weep at the sight of flaming acetic anhydride.
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madscientist
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I have more to say:
CH3CHO can also be prepared by the dehydrogenation of CH3CH2OH. I think there is some discussion on this method of preparing CH3CHO somewhere on this
board. This is the reaction that occurs when hot ethanol vapors are passed over hot copper metal in the absence of oxygen:
CH3CH2OH --(Cu)--> CH3CHO + H2
I'm certain that ethanoic acid and sodium sulfide will react to form merely hydrogen sulfide and sodium acetate.
Is this how the proposed CH3COOH / HCl / Zn process is supposed to work?
CH3COOH + 2HCl + Zn --> CH3CHO + H2O + ZnCl2
If so, I'm almost completely certain that it won't work. :-/
Edited to add: http://www.sciencemadness.org/talk/viewthread.php?tid=9
There is some discussion on preparing CH3CHO via dehydrogenation of CH3CH2OH in that thread.
[Edited on 4-6-2002 by madscientist]
I weep at the sight of flaming acetic anhydride.
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Nick
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Yes, I am certain that KMnO4 will work. Only use a TINY amount, or the H2O2 will be decomposed too fast. Copper turnings may be better, basically
anything that decomposes the H2O2 will work.
I'm not surprised you got a hot, violent reaction when adding KMnO4 to 28% H2O2! Like I said, conc. reagents = runaway. I used 9% H2O2 and vodka, and
it was nice and gentle but not very high yielding.
Ethanol and hot copper works without oxygen, too? That's interesting.
(D'oh! Yes, sulphide + acid --> salt + H2S, I forgot...)
I haven't seen HCl + Zn used to reduce carboxylic groups before, but I know it works on nitro groups. It might be worth trying. Basically, the HCl and
Zn react to produce atomic hydrogen, which reduces the thing you're trying to reduce. You need quite a large excess of HCl and Zn, because the atomic
hydrogen will quickly react to form molecular hydrogen if it isn't oxidised soon enough.
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Polverone
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Dichromates
It is possible, though perhaps not convenient, to prepare dichromates from more readily available precursor materials. K2CO3 + 2KNO3 + Cr2O3 + lots of
heat = 2K2CrO4 + 2 NO + CO2. I tried this today and it works fairly well, but be warned that fairly intense heat is needed (a good propane flame), and
you may also need some extra carbonate/nitrate if they decompose before acting on the Cr2O3. I believe that this should work when substituting the
corresponding sodium salts also. The addition of enough sulfuric or nitric acid to turn the mixture somewhat acid (around pH 4?) will convert a
chromate solution to dichromate. There is an obvious color change. In most applications, one wishes to separate the nitrate/sulfate that is also
formed in this process. But for oxidizing ethanol, I don't think potassium nitrate/sulfate would much interfere, since you're going to distill anyhow.
K2CO3 and Cr2O3 are available cheaply as ceramic materials. NaNO3/KNO3 sources are obvious. Na2CO3 is readily obtained for many purposes or made by
heating baking soda.
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vulture
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How about strongly heating copper wool until it gets black (CuO) and than quickly dumping it into ethanol?
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Rhadon
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CuO and C2H5OH does work, but if you just use copper wool you'll get problems if you want to make larger amounts of acetaldehyde.
It would be a better idea to use pure CuO which can be made by electrolysis of Cu.
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vulture
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Doesn't the CuO need to be hot in order to split it's oxygen?
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Rhadon
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Hmm, I think you're right. Anyone knows how hot the CuO will have to be?
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T. Bond
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Vulture and Rhadon, there is another way to skin the CuO.
Copper Oxide can be reduced with H2(g) yeilding Cu(s).
CuO + H2(g) ==> Cu(s) + H2O
I am wondering if its possible to reduce acetaldehyde back to ethanol other than thru NADH reaction? Any suggestions?
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vulture
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Err, Mr. Bond, if I may call you that way, the problem was not making copper!
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plasma
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I have pure CuO. I think I'll just heat some of it and drop in the right amount of C2H5OH. Does any body know the required temp ?
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a_bab
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For this reaction the CuO should to be red hot. It can be heated in a tube than passing the alcohol vapors thru the tube. BUT it'll work only with
methanol; I tryed once both and it didn't worked with ethanol. With methanol works perfectly.
It's enough to heat a copper wire (whool) in order to oxydise it and if you put it quickly in a test tube containing methanol; the vapors will quickly
decompose the copper oxide leaving the wire in a nice red color of pure copper.
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PrimoPyro
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Dudes..........come on now, do you all love the hard ways or what?
The simplest way to make acetaldehyde with little byproducts is simple treatment of ethylene glycol (common radiator antifreeze in the USA) with
dilute aqueous H2SO4. This is called the Pinacol Rearrangement.
Pinacols are vicinal glycols, meaning 1,2-diols of course.
HO-CH2-CH2-OH + H+ ---> HO-CH2-CH2-OH2+ ---> HO-CH2-C+H2 ---> -O-C+H-CH3 ---> CH3-C(H)=O.
Basically, one alcohol gets protonated, then the dehydrating abilities of H2SO4 remove water, forming a carbocation, which rearranges to the more
stable carbocation, which is the one on the carbon with the other hydroxyl, and then the alcohol extrudes a proton (so the protonic acid is a
catalyst, not a reactant) and the O-C bond strengthens to a double bond.
The Pinacol Rearrangement is used all the time in clandestine labs, for rearranging 1-phenyl-propan-1,2-diols to 1-phenyl-2-propanones, which are
direct precursors for amphetamines and methamphetamines.
The Pinacol rearrangement also works on epoxides because the acid opens the epoxide ring to the diol with the water present.
All that need be done is heat some ethylene glycol and dilute H2SO4 together for awhile. There are nearly zero side reactions. Zero that I know of,
but Im covering my ass just in case. :p
Come now, this is so much cheaper than using all these inorganic reagents.
PrimoPyro
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vulture
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Carbocations?
Are you sure there are carbocations intermediates? I thought these only could be created by superacids? Or maybe this is a nomenclature problem
again......
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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PrimoPyro
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Organic VS. Inorganic
I have noticed that organic chemistry has slightly different definitions of certain things than inorganic chemistry, and this can cause problems
during communication, possibly this is an example.
Yes, I mean carbocations, as in C+ species. And no, not just superacids create them. In fact, nearly any chemical can create a carbocationic
intermediate. It is the responsible intermediate for nucleophillic substitutions, several eliminations, and many other concepts as well.
An insanely huge list of reactions and reagents are used every second to form carbocations in reality, for an equally insanely large list of reasons.
I would say that the carbocation intermediate is directly responsible for MORE than 50% of all organic reactions, in one way or another. There really
is no method for me to convey just how often they are encountered in organic synthesis.
The intermediates are not isolatable. They exist for perhaps only microseconds, before they react in the desired reaction to form the product or yet
another intermediate.
As for what you thought I was saying, I do not know. If you explain what you don't understand or agree with, perhaps I can comment further.
PrimoPyro
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vulture
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figured it out....
... I "misphrased" over there, I think superacids are the only species to isolate carbocations in stable complexes, no?
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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BLAST_X
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acetaldehyde synthesis
Have somebody try out this method,
to form acetaldehyde in 3 steps:
1. synthesis of acetylene (C2H2)
2. synthesis of vinyl alcohol [C2H4O]
3. decomposition to acetaldehyde
a example:
In a first flask with addition funnel drop
slowly H2O to calcium carbide (CaC2).
The forming C2H2 gas letting-in a second flask in a catalyst-mixture of H2SO4/HgSO4 which should coolled below > 15 C.
The forming ethanal can be separated from the mixture by distilling with a water-bad at >21 C, perhaps under vacuum.
Acetaldehyde can be used for instance to
synthesize pentaerythritol tetranitrate, when mixed with paraformaldehyde and then nitrated directly.
all you need is C - H - O - N
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Alchemist
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Acetaldehyde from Ethylene Glycol
Hello all,
I read somewhere that Acetaldehyde can be made from Ethylene Glycol and Sulfuric Acid (I think it was on the HIVE)! I tried it this week end and all I
get is Dioxane (Diethylene Ether)! Has any one elese ever heard of getting Acetaldehyde rather than Dioxane? If you what have I done wrong! I believe
the reaction is the called Pinacol rearrangement. I have read some on it, but have found no clues that Ethylene Glycol can be converted to
Acetaldehyde.
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TripleDamn
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Synthesis
I believe you would talking about synthesis of 1,2-TPEGDN ?
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vulture
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If you're really in for a challenge, you can try reacting acetylene and water in the presence of a Hg salt.
2C2H2 + 2H2O -> 2CH3CHO
Why is subscript so f***** up?
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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Organikum
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ethylene glycol is nonsense
this is from the MERCK index:
Title: Dioxane .
CAS Registry number: 123-91-1
Additional name(s): 1,4-Diethylene dioxide
Molecular formula: C4H8O2
Molecular weight: 88.11
Percent Composition: C 54.53%, H 9.15%, O 36.32%
Literature references: Prepd by distilling ethylene glycol with dil H2SO4 .
......
Thanks Alchemist I would have believed the nonsense. Ok a dioxane snthesis isn´t as bad.
ORG
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