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Author: Subject: Why are organic compounds more vulnerable to oxidation in alkaline solution?
Biochemscientist
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[*] posted on 17-1-2013 at 01:25
Why are organic compounds more vulnerable to oxidation in alkaline solution?


I've read from numerous sources that certain organic compounds (particularly phenolics) tend to be more vulnerable to autoxidation (atmospheric oxidation) at high pH, but I haven't come across any references which explain why or how this happens.

For instance, one reference states:

"The molecular structures most likely to oxidize are those with a hydroxyl group directly bonded to an aromatic ring (e.g., phenol derivatives such as catecholamines and morphine), conjugated dienes (e.g., vitamin A and unsaturated free fatty acids), heterocyclic aromatic rings, nitroso and nitrite derivatives, and aldehydes (e.g.,
flavorings)... Oxidation is catalyzed by pH values that are higher than optimum, polyvalent heavy metal ions (e.g., copper and iron), and exposure to oxygen and UV
illumination. "

Wikipedia states the following about the phenolic compound Pyrogallic acid:

"When in alkaline solution, it [Pyrogallic acid] absorbs oxygen from the air, turning purple from a colourless solution. It can be used in this way to calculate the amount of oxygen in air, notably via the use of the Orsat apparatus."

Can anyone explain the mechanism for this reaction or point me towards an article or reference with information about this topic?
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Valentine
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[*] posted on 17-1-2013 at 06:31


About the oxidation of pyrogallol:
A kinetic study of the air oxidation of pyrogallol and purpurogallin
International Journal of Chemical Kinetics, Volume 21, Issue 6, pages 465–476

The attached document contains some information about the reactive oxygen species.
Attachment: 217037.pdf (960kB)
This file has been downloaded 1580 times

[Edited on 17-1-2013 by Valentine]
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Nicodem
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17-1-2013 at 09:45
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[*] posted on 17-1-2013 at 10:04


Quote: Originally posted by Biochemscientist  
I've read from numerous sources that certain organic compounds (particularly phenolics) tend to be more vulnerable to autoxidation (atmospheric oxidation) at high pH, but I haven't come across any references which explain why or how this happens.

This is nothing general. Phenols are more prone to oxidation in basic solutions because the more basic the medium the more of the phenols are deprotonated into the more oxidation prone phenolate anions.

Please read the forum guidelines. Quoting without citing the source is not an acceptable behaviour on a science forum.




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12AX7
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[*] posted on 17-1-2013 at 17:48


Know anything about the Lewis octet rule and resonance structures?

Phenol is quite stable all alone: it has two resonance structures (the degeneracy favors stability). In basic media there is a third resonance structure where the negative charge moves from the oxygen atom onto the carbon ring. If this conformation were locked by further oxidation, it would be a chemical dead end, so that even though the two or three resonance structures available to the molecule are more favorable, oxygen gets its foot in the door and breaks that. And in the end, oxygen gets its way.

I forget, is aniline (think: phenol with a more alkaline functional group attached) more oxidizable in basic or acidic solution? I recall it's easy to nitrate, but that's not strictly an oxidation. That said, the driving force in ring oxidation has little to do with the functional group itself -- just that it allows oxygen to get a foothold.

Tim




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