[pantone159]

According to my book, cold dilute KMnO4 does not react with pure alcohols (forming MnO2), but often does react with impure alcohols that contain aldehydes.

I took four alcohol samples I had available,
1 - 99% methanol
2 - 95% ethanol ("Everclear" for drinking)
3 - 91% isopropanol (drug store stuff)
4 - 99.953% isopropanol (for cleaning circuit boards)
and added a few drops of 0.031 M KMnO4, to 1-2 ml of the above alcohol samples, at room temperature.

I expected to see no result (purple color remained) for most of these, with 3 (and to a lesser extent 2) seeming the most likely to be 'impure'.

All four eventually formed MnO2.  This did take an hour or so, there was not any immediate reaction.  Are all of these impure then?  Or does a 'positive test' mean a very fast reaction (e.g. seconds, not tens of minutes)?

[Alberto_Kravina]

All four eventually formed MnO2.  This did take an hour or so there was not any immediate reaction

Maybe the alcohols got partially oxidized to aldehydes with O₂, they were "pure" when they were in the closed recipient and got impure when they got exposed to air and/or light.
Hint:Try again, but put the alcohols in a sealed recipient, so that there can't be any interferences.

My 100th post!

[Will]

My guess is that a positive result would have to appear within a few minutes, not tens of minutes. Try again as Alberto_Kravina said, but also try the reaction with a mixture of the alcohol with a bit of aldehyde. Does the book give a reason for cold dilute KMnO₄ not oxidising pure alcohols?

Good luck!

[limpet chicken]

SWIM has experience with this reaction, oxidising pseudoephedrine to a ketone, ephedrone (NOT reducing to methamphetamine, and this was NOT done, I don't make excuses, I genuinely, unlike my standard personality, did not do this, neither, for that matter, did SWIM)

COLD, dilute solution, added to the alcohol in question, after both being chilled, the alcohol to just about 1 degree C, likewise with the very dilute KMnO4, and left in the cold, oxidised the alcohol to a ketone, any more concentrated, or even room temperature for a minute or two, and you end up with the pseudoephedrine oxidised to benzaldehyde.

Acidified dichromate is better to yield ketones, reflux with NaMnO4 or KMnO4 is far better for producing aldehydes.

[science2000]

Can permanganate oxidize an alcohol to a carboxylic acid?

[AWK]

Can permanganate oxidize an alcohol to a carboxylic acid?

Even more - cyclohexyl alcohol can be oxidized to adipic acid (dicarboxylic)

[pantone159]

I haven't got around to repeating my test (lazy, I know) but now I have some formaldehyde so I can compare the methanol reaction to that with some formaldehyde.  (I don't have acetaldehyde, however.)  I'll also make sure that the KMnO4 is cold, not just room temperature.

[FeLiXe]

the thing to keep in mind is that it is an auto-catalysed reaction.

in the beginning KMnO4 will probably be able to oxidize the alcohol a little bit. So if you wait long enough, eventually there will be enough aldehyde to catalyse the oxidation.

[pantone159]

the thing to keep in mind is that it is an auto-catalysed reaction.

in the beginning KMnO4 will probably be able to oxidize the alcohol a little bit. So if you wait long enough, eventually there will be enough aldehyde to catalyse the oxidation.

Thanks.  That is likely why I got a slow reaction (when I expected none).