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Oxidation of alkene...

Please read my whole problem to know why my answer is different from mark scheme.


I know that primary alcohols are oxidised to acids and secondary alcohols are oxidised to ketones on refluxing, but I think as that diol is coming from an alkene, so the same would happen with it on refluxing with K2Cr2O7, as it happens with an alkene when it is completely oxidised by Hot, Concentrated, Acidified KMnO4.

I was taught that oxidation of alkene with Hot, Concentrated, Acidified KMnO4 happens like this:

Diol is also formed in this process and here 'what I think' is formed, so am I not right?
Or IS markscheme right because the diol is then oxidsied by K2Cr2O7?
(edited 13 years ago)
Reply 1
Avatar for cpchem
cpchem
14
Dichromate and permanganate are different oxidants - you can't necessarily expect them to do the same thing!
In Box C you have a molecule with a primary alcohol group and a secondary alcohol group.

The primary alcohol group get oxidised first to an aldehyde the a carboxylic acid.

The secondary alcohol group becomes a ketone.

Lastly, there are no double bonds in the molecule for clevage to occur/diols to form. You cant expect KMnO4 to work in the same manner as Acidifed dichromate.
Reply 3
Avatar for SalBhatti
SalBhatti
OP
13
Original post by Ari Ben Canaan
In Box C you have a molecule with a primary alcohol group and a secondary alcohol group.

The primary alcohol group get oxidised first to an aldehyde the a carboxylic acid.

The secondary alcohol group becomes a ketone.

Lastly, there are no double bonds in the molecule for clevage to occur/diols to form. You cant expect KMnO4 to work in the same manner as Acidified dichromate.

Nope. I'm pretty sure that when a double bonded carbon atom has one Hydrogen attached to it, it always forms acid, and if it has no hydrogen attached to it, it always forms a ketone. And if it has 2 hydrogens attached to it, it forms carbon dioxide and water. And this all happens with Hot, conc, acidified KMnO4. :/

This doesn't happens when a particular diol is oxidised by acidified Potassium dichromate.
(edited 13 years ago)
Your second scheme doesn't really reflect how permanganate oxidises alkenes :nah:

As said above the mechanims of oxidation are quite distinct between the two oxidants, permanganate is prone to causing oxidative cleavage whilst dichromate isn't. If you're feeling brave take a look at how permanganate acts :smile:
(edited 13 years ago)
Reply 5
Avatar for Tobia_s
Tobia_s
Reckon the mark scheme may be correct.

Your answer is wrong because you are missing out one of the carbons. Oxidation & reduction don't affect carbons; mainly oxygen and hydrogen atoms.

The alkene is first oxidised to a diol; but also has another -OH on the central carbon. When further oxidised; especially with dichromate under reflux, complete oxidation occurs. So the 2 -OH groups at both extremes of the molecule get oxidised to carboxylic acid groups. The -OH on the central carbon is bonded in a "secondary alcohol position" and so will also be oxidised, this time to a ketone group -CO.

Remember with reference to organic chem:
Oxidation = addition of O/removal of H
Reduction =addition of H/ removal of O
Reply 6
Avatar for SalBhatti
SalBhatti
OP
13
Original post by EierVonSatan
Your second scheme doesn't really reflect how permanganate oxidises alkenes :nah:

As said above the mechanims of oxidation are quite distinct between the two oxidants, permanganate is prone to causing oxidative cleavage whilst dichromate isn't. If you're feeling brave take a look at how permanganate acts :smile:


Oh, wow! Thanks a lot for that. :smile: I'm brave in the matter of studies. :yes:

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