Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: mir on July 27, 2007, 08:50:20 AM
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I have developed an interest in reducing agents after working with hydrogenperoxide and hypochlorite in organic synthesis. While monitoring a reaction with GC, I often need to quench the solution to get accurate kinetic data. But finding a good reducing agent that attacks the oxidant selectively and leaves the product untouched has proved difficult. Everyone I know has pointed out thiosulfate as a reducing agent, but it produces a basic environment during the reduction. Lately I have investigated sodium nitrite as a potential reducing agent. But I am worried if it will react with dichloromethane to produce some sort of a chloro-methane-nitrite (J. Org. Chem. 2004, 69, 6907-6908).
First question: Do you know of any sidereaction between nitrite and functionalgroups like ketones, aldehydes, halogens etc?
Second: Do you have an idea of a good reducing agent for quenching of oxidants like hydrogenperoxide and hypochlorite in a neutral to basic environment?
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Mir,
Sodium nitrite, particularly in an acidic medium, reacts with primary amines, secondary amines, epoxides, and alcohols to give either nitrosamines or nitroalcohols. As an aside, sodium nitrite is also a component in the antidote for cyanide poisoning (the other component is sodium thiosulfate). I know of no synthetic utility for the reaction of nitrite with the other functional groups that you list, but there may well be side reactions.
Since you are concerned about selectivity, why not just stay away from reducing agents altogether? For example, to quench the hydrogen peroxide quickly, you could use a disproportionation catalyst. Consider manganese dioxide, Lindlar's catalyst, or even catalase enzyme (the latter will work in 50% acetonitrile/50% water). As for the hypochlorite, you can effectively quench it by adding an excess of cyanuric acid but this would make the solution acidic. However, I think that even cyanurate salts will react with free chlorine to effectively quench the hypochlorite. Depending on your solvent, you could also precipitate the excess perchlorate with lithium chloride, as is commonly done in the industrial synthesis of lithium perchlorate.
Please let us know how things work out! Good luck,
Nick Conley, Principal
Conley Chemical Consulting
www.conleychem.com (http://www.conleychem.com)
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Addition of pretty much any phosphorus(III) compound (ideally triphenylphosphine or something more water soluble like triethylphosphite) will quench those oxidising agents, and many more (both H2O2 and hypochlorite can be used for oxidising P(III) -> P(V) on a preparative scale; but diiodine, hypervalent iodine, oxone, mCPBA and other peracids are also used). The required P(III) compounds are readily available, and both listed above are air and moisture stable, comparatively non-toxic, and can be handled without special equipment.
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I guess the only option for reducing agents besides metal-salts (I dont want any metals in my reactionmix, since it may catalyse other reactions) is:
phospite
nitrite
sulphite
Anyone of these reaction wont change the pH and thats good:
NaNO2 + NaOCl --> NaCl + NaNO3
Triphenylphospine was a really good idea! :)
I dont know about Cyanuric acid, I will check into it.
About that catalase-enzym: I wish somebody had some in my institute. I have been asking everywhere for it.
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Hi,
in the pass I used to add low boiling point olefines to reduce excess NaOCl and get rid of the diols by simple water washing.
g.luck