[Big-Daddy]

Oxidation of a primary alcohol with the MnO₄^- ion (or MnO₂, equivalently, as MnO₄^- becomes MnO₂ in solution and this reaction only happens in solution, correct?) results in the C to which the OH group is attached gaining an additional =O with the loss of 2 H atoms that are on it. (By definition of the alcohol being primary, there are 2 H atoms on it.) This is finally a carboxylic acid group.

How will the MnO₄^- oxidize secondary or tertiary alcohols?

[discodermolide]

Normally the product of oxidation of primary alcohols is an aldehyde. With such an oxidant as permanganate it may then oxidise the aldehyde to the carboxylic acid.
Secondary alcohols when oxidised give ketones. Tertiary alcohols are not oxidised.

[Big-Daddy]

This is generally true but I thought the MnO₄^- ion was too strong to merely oxidize it to an aldehyde or ketone?

Would a secondary alcohol be oxidized to a ketone, only, even with MnO₄^-?

I am already familiar with the oxidizing of alcohols by less powerful agents such as chromate or dichromate ions.

[discodermolide]

Well with MnO4^- you might start breaking C-C bonds in the ketone.

[Big-Daddy]

Well with MnO4^- you might start breaking C-C bonds in the ketone.

OK - any way to predict which C-C bonds you will break?

[discodermolide]

Nope not really, MnO₄^- is not really a selective reagent for C-C bond breaking.

[Big-Daddy]

Nope not really, MnO₄^- is not really a selective reagent for C-C bond breaking.

Ok, so we will just end up with a random mixture of smaller organic molecules?

What if we oxidize the secondary alcohol with MnO₄^-? Surely it is more energetically favourable to donate an O than to break C-C bonds ...

[orgopete]

You have to be careful here. Permanganate can oxidize with different mechanisms and under different conditions. It is a very nice and selective oxidant if used under mild conditions. In these cases, you may write reasoned electron transfers, e.g. alkene dihydroxylation or cleavage. If heated, then radical mechanisms also participate, e.g. benzylic oxidation of ethylbenzene to benzoic acid.

[Big-Daddy]

You have to be careful here. Permanganate can oxidize with different mechanisms and under different conditions. It is a very nice and selective oxidant if used under mild conditions. In these cases, you may write reasoned electron transfers, e.g. alkene dihydroxylation or cleavage. If heated, then radical mechanisms also participate, e.g. benzylic oxidation of ethylbenzene to benzoic acid.

What do you mean by "cleavage"?

[FandangoDingo]

The way I understand this is that KMnO4 is a harsh oxidizer (unlike PCC or PDC for example), and will oxidize primary alcohols into carboxylic acids. Secondary alcohols can only be oxidized to a ketone, as there is no room on the carbon for another bond. Tertiary alcohols simply cannot be oxidized to aldehydes/ketones/carboxylic acids.

As for the cleavage, I believe that while you risk non-specific cleavage of carbon-carbon double bonds, it's often suffice to use cold, diluted KMnO4 to avoid this. Also, if you DO want to cleave the double bonds (which I don't believe was your question), you can just assure that the KMnO4 is hot and concentrated.

[orgopete]

As far as I know, 2-butene can react with KMnO4 (or O3 or OsO4) to give a cyclic intermediate. This intermediate may decompose with cleavage of the C-C bond or it may be hydrolyzed to a diol. Low temperatures and base favor the hydrolysis reaction.

If ethylbenzene and 2-butene are reacted together at low temperature, 2-butene will react while ethylbenzene will not. These are different reactions. However, if ethylbenzene is heated with KMnO4, a radical oxidation will oxidize the benzylic carbon. What happens after that is conjecture. Although one could write a ketone as intermediate, the oxidation goes further. The methyl group becomes carbon dioxide. Again, this is a different oxidation than the low temperature oxidation. Isopropylbenzene will also oxidize to benzoic acid even if a tertiary alcohol could be written as an intermediate.

I wasn't trying to confuse anyone. I was presuming part of this question arises because KMnO4 can oxidize certain alkylbenzenes to benzoic acid. I don't think of this as reflective of a greater reactivity per se, but a difference in the conditions. Chromic acid can also be used to perform these oxidations when heated. Many compounds are oxidized in large quantities with air (plus a variety of catalysts). High temperatures can enable different reactions than the solution reactions that predominate in textbooks.

I suggest it is useful to know the mechanisms of these oxidations, why aldehydes oxidize to carboxylic acids, why PCC may not, and why tertiary alcohols do not oxidize. Then it may be easier to recognize why some oxidants may seem to violate those rules.

[antimatter101]

If you  are interested in how KMnO₄ and how other oxidising agents oxidise, search up this www.youtube.com/watch?v=9pfYX5odIrE