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Author: Subject: K2Cr2O7 oxidising amounts
BenZeen
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[*] posted on 29-9-2010 at 02:22
K2Cr2O7 oxidising amounts


Hi,
I want to oxidise alcohols to aldehydes with acidified potassium dichromate but i am unsure about the amounts needed and i dont want to use too much and end up with the carboxylic acid. Even though this question seems simple, I thought i would ask because of the toxicity of potassium dichromate i want to minimize its use (finding the perfect ratios to use etc). So if anyone could give me some figures such as grams per litre of K2Cr2O7 and H2SO4 needed to make up a mild oxidising mix i would be much obliged.
Thanks
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bahamuth
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[*] posted on 29-9-2010 at 06:12


H2SO4/dichromate would be anything but mild in my opinion for oxidizing alcohol to aldehyde.

Do you know how to calculate moles? If not you should learn it so that you might calculate the mole ratio so you wont end up with the carboxylic acid. Also you would need to distill the aldehyde off as soon as it is formed or you would get bad yields of the aldehyde.

Hypochlorite may be a good way to go for green and mild oxidation of alcohols to aldehyde/ketone.



Ref.

Tetrahedron Letters "Phase transfer catalyzed oxidation of alcohols with sodium hypochlorite" Volume 39, Issue 40, 1 October 1998, Pages 7263-7266

"Oxidation of alcohols using calcium hypochlorite and solid/liquid phase-transfer catalysis" John W. Hill , Jeffrey A. Jenson , Charles F. Henke , Joseph G. Yaritz and Richard L. Pedersen J. Chem. Educ., 1984, 61 (12), p 1118




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madscientist
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[*] posted on 29-9-2010 at 07:04


The best way to oxidize alcohols to aldehydes is to remove the aldehyde as it is formed via distillation (they tend to have lower boiling points).



I weep at the sight of flaming acetic anhydride.
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BenZeen
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[*] posted on 29-9-2010 at 22:37


Hi, thanks for the replies
I do know how to calculate mols but i was just unsure because i have seen 0.1 molar solutions being used etc.
So your saying just keep the amounts stoichiometric and it should stop at the aldehyde?
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woelen
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[*] posted on 29-9-2010 at 23:14


If you keep the amounts according to stoichiometry, then you get mostly aldehyde, but you will also obtain some acetic acid and there will also be some unreacted ethanol. A perfect 100% conversion of ethanol to acetaldehyde, while no acetic acid is formed or someethanol remains unreacted simply is not possible. By boiling off the formed acetaldehyde as soon as it is formed you suppress the formation of acetic acid and any dichromate which is not used for formation of acetic acid remains available for oxidation of other erhanol.

So, I would use a slight excess amount of ethanol (e.g. 1.3 times stoichiometric value) and I would use a fairly large excess amount of H2SO4. Dichromate is a strong consumer of acid in its redox reaction.




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BenZeen
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[*] posted on 29-9-2010 at 23:21


Thanks
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[*] posted on 30-9-2010 at 06:14


To make carboxylic acids you have to reflux, so I'm assuming that it's a fairly slow process and that you shouldn't get too much contamination from them. The carboxylic acid will have a much higher boiling point than the aldehyde anyway (due to H bonding) so your best bet would be to redistill after you've done the oxidation.
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[*] posted on 1-10-2010 at 06:23


Aldehydes are much more succeptible to oxidation than alcohols, so almost all oxidizers will preferentially oxidize aldehydes before alcohols. It really is necessary to remove the aldehyde as it is formed. Yields will be poor otherwise.



I weep at the sight of flaming acetic anhydride.
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