blue_vitriol
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Oxidation of primary alcohols with KMnO4
I am looking for a specific procedure that will oxidize acohols to carboxylic acids using potassium permanganate. I have searched both science madness
and google. Some of the sources state pH should be above seven, and some say below. I know that bases can catalyze KMnO4 reactions such as the
chameleon reaction, but i am afraid they would react with the product.
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AJKOER
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Here is a stab at it. Passing HCl vapors and air into a heated solution of KMnO4 in H2SO4 forms HOCl vapors (a very old preparation that oxidizes HCl)
that can be collected. The latter, Hypochlorous acid, is capable of oxidizing many organic compounds.
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blue_vitriol
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Doesn't KMnO4 + H2SO4 produce Mn2O7? Do you know if the reaction would work with its pure form? Thanks for the reply
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mr.crow
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Quote: Originally posted by AJKOER  | | Here is a stab at it. Passing HCl vapors and air into a heated solution of KMnO4 in H2SO4 forms HOCl vapors (a very old preparation that oxidizes HCl)
that can be collected. The latter, Hypochlorous acid, is capable of oxidizing many organic compounds. |
That sounds like a terrible idea!!!
Double, double toil and trouble; Fire burn, and caldron bubble
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weiming1998
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| Quote: Originally posted by AJKOER | | Here is a stab at it. Passing HCl vapors and air into a heated solution of KMnO4 in H2SO4 forms HOCl vapors (a very old preparation that oxidizes HCl)
that can be collected. The latter, Hypochlorous acid, is capable of oxidizing many organic compounds. |
What is the purpose of that when KMnO4 (especially acid-catalysed) is extensively used as an oxidizing agent already? Not to mention that HOCl and its
decomposition products might mess around with the functional groups and stuff up the organic compound.
Anyway, back to the original question. Bases are not used as catalyst for the oxidation of sugar by KMnO4 in the chameleon reaction. The base is used
to both stabilise the MnO4(2-) ion that is formed as an intermediate (green colour) in the reaction and to activate the sugar to more readily react
with oxidizing agents. If a solution of acidic KMnO4 and sugar is mixed together, the mix would just turn brown and you wouldn't get the colour change
effect.
Basically what I am saying is that you need the pH to be very low. Generally, a strong, non reducing acid (generally H2SO4, DON'T USE HCl) is used to
catalyse the oxidation of the alcohol by KMnO4. You also don't want to add concentrated H2SO4 directly, as this could result in the mixture catching
fire. Instead, use dilute H2SO4 to catalyse the reaction.
Potassium permanganate is a very strong oxidant, so the oxidation reaction should start spontaneously without any heating. It also means that no
complex apparatuses are needed (just add some dilute H2SO4 to some KMnO4, then add the alcohol you wish to oxidise in directly). The solution should
turn murky-brown after a few minutes. Leave it to oxidise for a bit longer, then filter out the brown MnO2 and you have your carboxylic acid.
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Boffis
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KMnO4 oxidises ethanol to acetic acid and n-propanol to propanoic acid in the cold (room temp) vigorously giving the aqueous acid and a dark brown
MnO2 precipitate. If however you carry out the reaction in warm (60-70C) dilute sulphuric acid no MnO2 forms because under these condition the
manganese oxide is capable of carrying out the oxidation itself.
I have made propanoic acid by heating n-propanol with waste MnO2 from other KMnO4 oxidations and 5M sulphuric acid. When the MnO2 has dissolved the
reaction mixture is either steam distilled or fractionated, the excess propanol and any byproducts such as propanal distill of quickly and the acid
between about 135-145C. The residue in the flask can be worked up for manganese sulphate but it requires careful crystallisation because it always
contains some potassium manganese II double sulphate, the potassium is always present in MnO2 derived from potassium permanganate.
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AJKOER
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There are issues with KMnO4 to quote my source "harmful, environmental risk". He pushes the use of HOCl as to quote "Hypochlorous acid (HOCl), an
alternative oxidizing agent, is readily available, inexpensive, safe, and environmentally benign", but to each his own. I would sell the KMnO4 and use
HOCl.
Now, the best oxidizing agent per my source measured by how widely used in this synthesis historically is chromic acid. But, it is a suspected
mutagen, carcinogen, highly corrosive and does have strict EPA regulations on handling and disposal (a chromium compound), but nothing is perfect.
Reference link: http://chemistry.colorado.edu/Labs/Experiments/14%20-%20Oxid...
[Edited on 5-2-2013 by AJKOER]
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blue_vitriol
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If you view KMnO4 as dangerous why would you even think about a chromium compound? I obtained permanganate purposefully to avoid hexavalent chromium.
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AJKOER
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Both Chromic acid and KMnO4 present an environmental risk in the opinion of some.
I am not recommending either.
I am in agreement the authors recommendation of Hypochlorous acid.
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Boffis
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In the hands of an idiot anything can be dangerous.
The original thread was can you use permanganate to oxidize alcohols? Answer Yes. Once the permanganate or chromate is spent the lower oxidation state
compounds are fairely harmless once diluted. I buy my potassium permanganate from the garden centre where they sell it for treating timber planters
and staining exposed wood.
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weiming1998
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| Quote: Originally posted by AJKOER | Both Chromic acid and KMnO4 present an environmental risk in the opinion of some.
I am not recommending either.
I am in agreement the authors recommendation of Hypochlorous acid. |
Both chromium (VI) and manganese (VII) compounds can be reduced to something less harmful to the environment. I understand that chromates/dichromates
can be a bit hazardous but permanganates? If the permanganate is going to be used to oxidize alcohols, all that is going to be left is a muddy
precipitate of MnO2 (if the alcohol is in excess), which is relatively harmless to the environment and can also be collected and used for a different
reaction. Small amounts of leftover permanganate can also be reduced to MnO2 with a variety of mild reducing agents.
HClO is not quite as effective as an oxidizing agent as KMnO4. The formed HCl and Cl2 (as a result of the oxidation) will also react with organic
compounds and chlorinate them. Finally, it is quite unstable and will have to be freshly made every time.
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AJKOER
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OK, per this MSDS (http://www.sciencelab.com/msds.php?msdsId=9927406 ), to quote:
"Take care in handling as explosions may occur if it is brought in contact with organic or other readily oxidizable substances, either in solution or
in dry state. Explosive in contact with sulfuric acid or hydrogen peroxide. Potassium permanganate + acetic acid or acetic anhydride can explode if
permanganate is not kept cold. Explosions can occur when permanganates come on contact with benzene, carbon disulfide, diethyl ether, ethyl alcohol,
petroleum, or oganic matter...."
So, as a teacher with students to worry about, I see a possible issue. Note, HOCl is made on the spot and is less available to precipitate an
accident. However, there is also the comment:
"DEVELOPMENTAL TOXICITY: Not available. The substance may be toxic to kidneys, liver, skin, central nervous system (CNS). Repeated or prolonged
exposure to the substance can produce target organs damage. Repeated exposure of the eyes to a low level of dust can produce eye irritation. Repeated
skin exposure can produce local skin destruction, or dermatitis. Repeated inhalation of dust can produce varying degree of respiratory irritation or
lung damage."
"Special Remarks on Chronic Effects on Humans:
May cause adverse reproductive effects (Male and Female fertility) based on animal data. May affect genetic material
(mutagenetic) based on animal data."
"Products of Biodegradation:
Possibly hazardous short term degradation products are not likely. However, long term degradation products may arise."
So a teacher may have stronger feelings on using/suggesting KMnO4.
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blogfast25
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Of course you would (you twit).
Oxidising primary alcohols with KMnO4 is easy and not harmful at all. I've done it G-d knows how many times.
Keep the concentration of the KMnO4 fairly low and control stoichiometry to avoid oxidising even further. See titration of oxalic acid (as a primary
standard) to CO2 with permanganate for standardisation of KMnO4 titrant solutions.
Try and keep your HClO fetish where it belongs, for just once!
[Edited on 7-2-2013 by blogfast25]
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ScienceSquirrel
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I wish people would stop reading MSDS and the literature and just taking everything as gospel no matter what the quantities or concentrations.
Potassium dichromate and permanganate are perfectly safe if handled correctly and as far as I know there are no restrictions on disposal of chromium
III or manganese II salts in the public drainage system as long as it is in small lab scale quantities.
Chromium and manganese occur in the environment and by the time a few grams reaches the sewage works that treats thousands of litres of waste per hour
they would not even be detectable.
Table salt is toxic and harmful if you eat it by the spoonful but it is nice on chips. You have to apply common sense in your life and as a chemist.
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AJKOER
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OK, some clarifications.
The preference for the use of HOCl was solely the opinion of an educator (sorry, Blogfast, but his written words are his own).
My opinion on the proper chemicals to employ applies to the setting. Common sense, appropriate quantities and concentrations are better controlled in
a non-student environment. I remember a case of a student over doing a Nitrogen iodide experiment, and it exploded of course, marking his face purple
(so much for common sense and quantity controls).
In a lab setting, absolute agreement.
In a home lab setting, where you own the home, I would be more cautious. My rationale, a home lab fire caused by or extenuated by the presence of
oxidizers may result in no insurance coverage, or at least the costly employment of a lawyer. How? The insurance company can claim, for example, it
was an illegal lab, and as all illegal activities are not covered ever by insurance, no coverage. Or, the company could claim there was fraud in not
disclosing underlying risks, etc. One should always chose an insurance company by its claims-paying reputation and not premium. An interesting example
is that if the house is reduced to a pile of wood by a non-covered peril (for example, lightning) and starts to burn, and as you have only a fire
insurance policy on your residence (by assumption), your insurance company will write you a check for the cash value of the pile of wood burned, and
not the cash value of your home that created the pile.
Bottom line, in my opinion, what chemicals one employs/stores should be guided by common sense relative to the setting including available safety
equipment.
[Edited on 7-2-2013 by AJKOER]
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blogfast25
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| Quote: Originally posted by ScienceSquirrel | I wish people would stop reading MSDS and the literature and just taking everything as gospel no matter what the quantities or concentrations.
Potassium dichromate and permanganate are perfectly safe if handled correctly and as far as I know there are no restrictions on disposal of chromium
III or manganese II salts in the public drainage system as long as it is in small lab scale quantities.
Chromium and manganese occur in the environment and by the time a few grams reaches the sewage works that treats thousands of litres of waste per hour
they would not even be detectable.
Table salt is toxic and harmful if you eat it by the spoonful but it is nice on chips. You have to apply common sense in your life and as a chemist.
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AMEN! to that. A little knowledge is a dangerous thing. MSDS sheets are useless in the wrong hands.
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