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Author: Subject: Esters of Chromate/Dichromate?
Ramium
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[*] posted on 24-6-2020 at 22:39
Esters of Chromate/Dichromate?


Scouring through random books at a uni library today, and came across an entry on Di-t-butyl Chromate, used as a selective oxidizing agent. A red oil at room temperature apparently. It can be prepared from reaction of t-butanol with CrO3, or Chromyl Chloride. I have a bit of a special interest in obscure, unlikely compounds like this, so I thought I might have a go making the Diethyl ester. Just slowly Chromyl Chloride to excess of ethanol, reflux, and distill of ethanol or Diethyl Chromate, how hard can it be?

Would this reaction be likely to occur readily and simply like this?

Mmm.. volatile hexavalent Chromium.. how nasty is the product likely to be? surely no worse than Chromyl Chloride

Anyone have any additional information on physical properties/reactivity of dialkyl Chromates?


Any thoughts would be great




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[*] posted on 24-6-2020 at 22:59


You can't make the diethyl ester- it will oxidize to acetic acid. You can only make chromate esters of tertiary alcohols.



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[*] posted on 25-6-2020 at 00:44


I even think that mixing ethanol with CrO2Cl2 may lead to fire. A well-known demonstration is spontaneous ignition of ethanol, when it is dripped on solid CrO3. So, be careful with this, there is a good chance of accidents.



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[*] posted on 25-6-2020 at 02:43


It's not a reaction I'd like to try but... it seems you can do it in pentane at room temperature.

Prkparation d'un ester chromique dkriuk d'un alcool
tertiaire liquide
La rnethode suivante qui constitue une adaptation de
la methode genQale dkrite par Wienhaus (25) s'est
avQBe la plus efficace (sauf pour le tert-BuOH oh on
doit faire reagir le Cr03 et l'alcool selon le rapport
molaire 1.1 et 2 respectivement).
L'alcool tertiaire (0.3-1 g) a esterifier est dissous dans
25 ml de pentane pur contenu dans un ballon de 50 ml;
la reaction se fait a la temperature de la piirce. Tout en
maintenant l'agitation (au moyen d'un agitateur ma&-
tique) une quantit6 Bquimoleculaire de Cr03 anhydre
(conserve dans un dessicateur) est ajoutee. I1 y a coloration immediate de la solution en rouge-orange et apparition de gouttelettes d'eau au fond du ballon. L'agitation
est poursuivie durant 3-5 rnin a la temperature ambiante.

From
https://www.nrcresearchpress.com/doi/pdf/10.1139/v75-438
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[*] posted on 25-6-2020 at 08:51


Quote: Originally posted by woelen  
I even think that mixing ethanol with CrO2Cl2 may lead to fire. A well-known demonstration is spontaneous ignition of ethanol, when it is dripped on solid CrO3. So, be careful with this, there is a good chance of accidents.


Because the ethanol can boil on surface of solid CrO3 and the vapours be oxidized by solid CrO3 surface?

CrO2Cl2 density is 1,91. Much denser than ethanol.
Therefore, when chromyl chloride is slowly added to excess ethanol, any excess reaction heat would just result in chromyl chloride sinking to bottom and ethanol and ethanal vapours bubbling up at not above 78 Celsius. Not warm enough for autoignition.

That´s goal 1 - not lighting a fire.
How about goal 2 - selectively producing ethanal rather than ethanoic acid?
Excess of ethanol may help. But a common measure to favour oxidation to aldehydes only is said to be use of pyridine for reaction environment.

Finally, goal 3.
Since secondary, primary and methyl chromates are unstable already at room temperature, refluxing off ethanol is not a way to separate them.

Yet some people were somehow able to measure speed of chromate ester decomposition, finding that the reaction was 6 times slower when it was deuterium not protium at alpha position.

Since chromate ester decomposition is one molecule reaction, its speed should be affected mainly by temperature. Ester formation speed should also depend on temperature, but other stuff as well.
At which conditions would you speed up chromate ester formation while keeping its decomposition reasonably slow?
Chromyl chloride freezes at -96 Celsius, and ethanol at -114 Celsius.
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[*] posted on 25-6-2020 at 11:05


Quote: Originally posted by chornedsnorkack  

Yet some people were somehow able to measure speed of chromate ester decomposition, finding that the reaction was 6 times slower when it was deuterium not protium at alpha position.



The clever bit is that you can do that without measuring either rate of reaction.
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[*] posted on 25-6-2020 at 15:17


Allright, feeling like a bit of a fool right now :). CrO2Cl2, CrO3 are strong oxidizing agents so of course they will just oxidize ethanol - nothing else. Tertiary alcohols would form Chromate esters because they cannot be oxidized, and so this would be the only reaction pathway.



Quote: Originally posted by unionised  
It's not a reaction I'd like to try but... it seems you can do it in pentane at room temperature.

From
https://www.nrcresearchpress.com/doi/pdf/10.1139/v75-438



If pentane is needed as a solvent, that would suggest to me the reaction of tertiary alcohols with CrO3 could be pretty energetic (Though definitely not as much so as oxidation of primary alcohols etc). I might try this..


I hadn't considered thermal instability of Chromate esters, and I guess if decomposition is 0th order (only one molecule reactant), temperature would be only factor for decomposition.

I was thinking perhaps Grignard with Ethylmagnesium Bromide and CrO2Cl2 would lead to the Diethyl ester, but now I think about it, this would form (C2H5)2CrO2, which is itself probably unstable.

If Chromate esters of primary alcohols could be made however, would they be relatively stable and able to be isolated at room temperature?




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[*] posted on 25-6-2020 at 16:50


Quote: Originally posted by Ramium  

If Chromate esters of primary alcohols could be made however, would they be relatively stable and able to be isolated at room temperature?


No, they'd just be intermediates in the oxidation.




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[*] posted on 25-6-2020 at 20:52


Quote: Originally posted by Ramium  
I was thinking perhaps Grignard with Ethylmagnesium Bromide and CrO2Cl2 would lead to the Diethyl ester, but now I think about it, this would form (C2H5)2CrO2, which is itself probably unstable.


Grignard is a reducer. You could instead react the chromyl chloride with sodium ethoxide.
Of course, the ester is unstable at room temperature. How good, generally, is reaction between alcoholate and acid halide to produce esters?
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