This post is about a reaction of alkenes that we haven’t talked about yet. It’s called oxidative cleavage.
What we do is use a reagent called ozone (O3) – yes, it’s the same molecule that forms the protective layer in our atmosphere! – which breaks the carbon-carbon double bond, and caps each carbon with a new double bond to oxygen.
It’s “oxidative” because we’re breaking C-C and forming C-O, and it’s “cleavage” because, well, the alkene is being broken apart.
The two variations are called the “reductive workup” and the “oxidative workup”.
The “reductive workup” is fairly straightforward. We break the C-C double bond, and “cap” each carbon of the alkene with a C=O . Here, we add O3 and then zinc (Zn). That’s it! (Sometimes you’ll see dimethyl sulfide – Me2S – used instead of Zn in this process).
Depending on whether our alkene has carbons or hydrogens directly attached, we’ll either get ketones or aldehydes.
In the oxidative workup, instead of adding Zn or CH3SCH3 at the end, we add H2O2. This does exactly the same transformation as before – break the C=C, form two new C=O bonds – but it also does some extra “surgery”. It will also break any C-H bonds that are attached to the alkene, and replace them with C-OH.
(We can also use KMnO4 and acid for this instead of H2O2).
This means that if a carbon of the alkene is attached to two alkyl groups, we’ll get a ketone. But if it’s attached to any hydrogens, those hydrogens are replaced with OH. So in the example below, instead of an aldehyde, we get a carboxylic acid.
A student of mine calls these the “good surgeon” and the “bad surgeon” reactions. The “good surgeon” will just cut you in half. But the “bad surgeon” will not only cut you in half, but also cut off extra limbs. You have to admit, it’s a vivid metaphor!
Thanks for reading! James
P.S. Reaction guide – oxidative cleavage (reductive workup)
P.P.S. Reaction guide – oxidative cleavage (oxidative workup)