/ Mild (bacterial) oxidation of alcohols
Can anyone on here help with this please?
It is well known that acetobacter will cause dilute solutions of ethanol to be oxidised to ethanoic acid when left standing exposed to air; this being the reason that wine left standing in air will spoil and taste vinegary.
It is also reported that different strains of the bacteria can produce other compounds such as dihydroxyacetone and gluconic acid from other starting materials. The bacteria are therefore clearly versatile.
This suggests that in addition to ethanol a wide variety of alcohols in dilute aqueous solution may be oxidised to carboxylic acids when exposed to the atmosphere which will normally contain these bacteria.
However I cannot find any specific references to support this broader suggestion, neither in my textbook collection, which is substantial, nor on the internet. There are articles and statements which come close but I can’t find an authoritative source (except one which I am doubtful about) which states categorically that all or most alcohols in dilute aqueous solution are oxidised to carboxylic acids when simply left standing in air (i.e. without a catalyst added).
My questions are these:
1. In particular I am interested to know whether or not aqueous solutions of propanol and butanol are oxidised to their carboxylic acids by simply standing exposed to air.
2. Do all, or at least most other, alcohols also oxidise to their carboxylic acids when in dilute aqueous solution and simply exposed to air?
Sadly I no longer have access to a lab or I would carry out some experiments myself.
Finally: I appreciate that the bacteria are inhibited or possibly killed by high concentrations of the alcohol. It is dilute aqueous solutions I am interested in. Nevertheless if you have good information as to the indicative concentration levels at which the bacterial oxidation is prevented I’d be interested.
I thought your question was interesting so I forwarded it to somebody much cleverer than me in our Analytical lab (aromatic molecules company). His reply is:
"Over a short period of time I suspect not without a catalyst, however if they were left for an extremely long period of time (years) I would expect to see some partial oxidation since oxygen (in air) is very reactive.
Alcohol oxidation is an important organic reaction. Primary alcohols can be oxidized either to aldehydes or to carboxylic acids, while the oxidation of secondary alcohols normally terminates at the ketone stage. Tertiary alcohols are resistant to oxidation. Tertiary alcohols don't have a hydrogen atom attached to that carbon. You need to be able to remove those two particular hydrogen atoms in order to set up the carbon-oxygen double bond
Ethanol also undergoes bacterial oxidation to ethanoic acid. This form of oxidation is a problem for wine producers. Air contains a large proportion of bacteria called Acetobacter. Acetobacter bacteria use atmospheric oxygen from air to oxidise ethanol in wine, producing a weak solution of ethanoic acid (vinegar). So once a bottle of wine has been opened it can quickly turn to vinegar due to the large number of bacteria in the air. Wines with a high concentration of alcohol or fortified wines such as sherry and port are very resistant to bacterial oxidation. This is because the ethanol concentration is too high for the bacteria to tolerate.
Vessels used for fermenting beer and wine at home are fitted with airlocks. These allow carbon dioxide gas released from the fermenting process to escape but prevent air from entering the vessel."
I can see that some of this duplicates what you've written but I hope other bits of it are of use.
Not my area but isn't this about the substrate-specificity of the bacterial dehydrogenases involved?
Might this be useful?
Information on EC 126.96.36.199 - alcohol dehydrogenase (quinone) and Organism(s) Acetobacter aceti
Only described in acetic acid bacteria where it is involved in acetic acid production. Associated with membrane. Electron acceptor is membrane ubiquinone. A model structure suggests that, like all other quinoprotein alcohol dehydrogenases, the catalytic subunit has an 8-bladed ‘propeller’ structure, a calcium ion bound to the PQQ in the active site and an unusual disulfide ring structure in close proximity to the PQQ; the catalytic subunit also has a heme c in the C-terminal domain. The enzyme has two additional subunits, one of which contains three molecules of heme c. It does not require amines for activation. It has a restricted substrate specificity, oxidizing a few primary alcohols (C2 to C6), but not methanol, secondary alcohols and some aldehydes.
Many thanks for your response. This is quite a specialised question probably of no interest to the vast majority of visitors to this site but within a couple of hours of posting my very obscure question I get two worthwhile responses. What a marvellous website this is. Three cheers for UKC.
I am inclined to agree with your colleague's opinion, thank you for taking the trouble to pass it on.
In my literature search on this matter I came across some very interesting catalysts which I'd not previously been aware of. I sometimes think I may have missed a vocation as a research scientist but it's way too late now!
Thanks for your response Dave. Please see also my response to Rigid Raider.
The link you referred me to was very interesting and relevant. Biochemistry at the level of detail in your link is not really my field either but the link is very worthwhile nevertheless.
Thanks again for taking the trouble to respond in such detail.
You're welcome. I only dabble these days.
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