This is a weird question but it's finals week and it's impossible to get in contact with my prof. I have no idea what he means by the last line where he says a C-O bond is formed for every C-C and C-H bond. Can someone just clarify this for me?

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  • $\begingroup$ The reaction mechanism is complicated. You just have to know that hot alkaline KMnO4 converts an aldehyde to a carboxylic acid. In case of formaldehyde, its converted to carbon dioxide. KMnO4 cleaves alkenes just like ozonloysis. If cold dil. KMnO4 is used, instead of forming a carbonyl compound, a syn dihydroxylation compound is formed. chemwiki.ucdavis.edu/Organic_Chemistry/Reactions/… $\endgroup$ – Aditya Dev Dec 17 '15 at 0:15
  • $\begingroup$ Yeah I know that part. Just confused about this rule of thumb stuff. If that's true CH2-CH2 would end up with 4 CO bonds?? $\endgroup$ – john clastings Dec 17 '15 at 0:44
  • $\begingroup$ Yes. It will end up with two molecules of carbon dioxide. First two molecules of formaldehyde which then becomes HCOOH and finally CO2. $\endgroup$ – Aditya Dev Dec 17 '15 at 0:58
  • $\begingroup$ ohhh got it. Thanks for the clarification. But if I extend the length of the alkene wouldn't this "rule of thumb" not really work anymore? eg CH3-CH2-CH2-CH-CH-CH3. Wouldn't there need to be 17 CO bonds? $\endgroup$ – john clastings Dec 17 '15 at 4:30
  • $\begingroup$ In your "rule of thumb", its given "alkene". But you are talking about an alkane. $\endgroup$ – Aditya Dev Dec 17 '15 at 4:33

Your professor obviously wants you to consider the $\ce{C-C}$ bonds of the $\ce{C=C}$ double bond plus any $\ce{C-H}$ bond to either of the alkene carbons.

The first step in double bond oxidation by permanganate will always be replacing a $\ce{C=C}$ by $\ce{C=O + O=C}$. If you consider the double bond two bonds, then for each of those bonds each carbon atom gets one $\ce{C-O}$ bond to again give a double bond.

After we have generated the inital carbonyl, we can turn our attention to the other bonds the former alkene carbon has. Remember that aldehydes are easily oxidised to carboxylic acids while ketones are rather resistant to oxidation. The difference is the presence of a $\ce{C-H}$ bond in the aldehyde; permanganate can oxidise this to another $\ce{C-O(-H)}$ bond. This is the second part of your rule.

If accidentally the carboxylic acid species generated is formic acid, which has another $\ce{C-H}$ bond, this can again be oxidised to give carbonic acid. Since carbon’s oxidation state is maximal in $\ce{H2CO3}$, this is truly the end of the oxidation.


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