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Author: Subject: Adding and stabilisers to/from ethers and halohydrocarbons
mycotheologist
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[*] posted on 20-4-2012 at 11:27
Adding and stabilisers to/from ethers and halohydrocarbons


I read that by adding ethanol to diethyl ether, the ethanol protects the ether against autooxidation and thus inhibits the formation of peroxides. I also read that ethanol similarly stabilises chloroform and prevents the formation of phosghene. In both cases the ethanol must make up at least 1% of the solution. I like the idea of this because you can never be too safe when it comes to chemistry but I assume ethanol would significantly alter the solvents properties and make polar compounds more soluble in them. Is ethanol removed each time before use? If so, how is it done? An idea that pops into my head is to add water until two layers form and discard the aqueous layer assuming that the ethanol, being way more soluble in water than chloroform, will be in it. Are there any flaws in my logic here?

Side note: I'm fascinated by this butylated hydroxytoluene (BHT) compound. According to this page I'm reading:
Quote:

A common stabilizer system for ethyl ether is butylated hydroxytoluene (BHT), which is typically added to ethyl ether at low ppm concentrations. This compound scavenges the free radical species responsible for peroxide formation and is a very effective suppressor of peroxide formation. BHT is incompatible with methods requiring high optical purity due to strong UV absorbance resulting from the aromatic functionality of this molecule.

Ethanol is another common stabilizer, which is added at much higher concentrations (1-2%) than BHT. Due to this relatively high stabilizer concentration, the presence of ethanol significantly increases material polarity and may affect certain applications. Sigma-Aldrich offers ethyl ether with BHT, ethanol and BHT / ethanol stabilizer systems.

doesn't say the exact concentration for EtOEt but for THF, they say 200-300 ppm is sufficient to stabilise the ether. Heres what the page says about chloroform:
Quote:

Chloroform is unstable and is combined with a variety of stabilizers to enhance product shelf life. Sigma-Aldrich offers ethanol and amylene stabilized chloroform. Like ether, ethanol must be added to chloroform at relatively high concentrations (~1%) in order to be effective. This will increase the polarity of the solvent and potentially impact certain applications. Amylene (2-methyl, 2-butane) is added to chloroform in order to scavenge free radicals such as chloride generated by chloroform decomposition. This compound is effective at levels of approximately 100 ppm.

Concentrations that low shouldn't interfere with the solvents properties so if I ever decide to store ethers or chloroform for long periods of time, I'll try and get my hands on these radical scavengers.

Side question: Whats so special about ethanol? Why does it prevent these reactions from occuring?
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AndersHoveland
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[*] posted on 20-4-2012 at 15:25


I thought that ethanol actually very slowly reacts with chloroform during long periods of storage.
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mycotheologist
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[*] posted on 21-4-2012 at 03:58


Not according to Sigma-Aldrich:
http://www.sigmaaldrich.com/chemistry/solvents/learning-cent...
I read that methanol works too. I wonder why these alcohols work? These autooxidation reactions occur via free radical intermediates so I can see why radical scavengers work but ethanol is a mystery to me.
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[*] posted on 21-4-2012 at 05:10


I've heard somewhere that ethanol is being added to chloroform for "historic reasons", and that it doesn't really do much or anything. One of the things people do "because".
I can't back that up with evidence, nor verify it. It's just something that got stuck in my ear long time ago.




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