A Primer On Organic Reactions
By James Ashenhurst
Introduction to Oxidative Cleavage Reactions
Last updated: May 21st, 2019
The four posts on acid-base, substitution, addition, and elimination covered the 4 main reactions in organic chemistry I. In this second series of posts we go beyond these to introduce a few of the less common (but still important) reactions you learn in organic chemistry 1. We’ve talked about rearrangements and free-radical substitution: we finish up here with lots of cleavage (oxidative cleavage).
As I’ve said with everything in this series, the point is not to understand why just yet, but to be able to see from the diagrams what bonds are broken and formed. You need to understand how to read line diagrams and also the concept of isomers. But other than that no further skills are required. The point here is to be able to follow the plot – to see what is happening. A later series of posts will go into more detail as to why things happen.
So what do I mean by “oxidative cleavage reactions”.
When certain types of compounds: [1,2-diols (vicinal diols), alkenes, and alkynes], are treated with certain reagents, carbon-carbon bonds are cleaved, and carbon oxygen bonds are formed. When we increase the number of C-O bonds at the expense of C-C or C-H bonds, that’s referred to as “oxidation“.
Let’s have a look at diols (carbons with alcohols on adjacent carbons). When diols are treated with NaIO4 (sodium periodate), the carbon-carbon bond between the two diols is cleaved, and we form two new carbon-oxygen double bonds. This can give us either aldehydes or ketones, depending on what our starting diol looks like.
Note the pattern: break C–C, form C–O.
Here’s a different oxidative cleavage reaction using our friend from the upper atmosphere, ozone (O3). Treatment of an alkene with ozone results in cleavage of the carbon-carbon double bond, and formation of two new carbon-oxygen double bonds. It’s like you take a pair of scissors and cut the bond in half, and cap each half with an oxygen. It’s not a good time here to explain the purpose of O3 or why it does what it does or why the Zn is important (plug: they’re in the Reagent Guide) but they all have their purpose.
Note that again, aldehydes or ketones are formed, depending on the pattern of the diol we start with. And if we start with alkynes, we end up cleaving all three C-C bonds and obtain carboxylic acids.
Here’s the final example. Some reagents will not only oxidize C-C bonds, but they will oxidize C-H bonds too. In particular, ozone – when treated with hydrogen peroxide – does this, and so does KMnO4 (potassium permanganate). In both cases we end up with carboxylic acids instead of aldehydes. Note that if we don’t have any hydrogens attached to the alkene carbon, we still end up with a ketone.
One of the frustrating things about these reactions is that it’s really hard to explain exactly WHY these reagents work the way they do, and HOW it came to be that we use *these* reagents, and not others. The cold, unsatisfying truth is that organic chemistry is very much an experimental science, and long ago it was found that these reagents just work really well at doing these reactions. A deep understanding of exactly *why* and *how* these reagents worked the way they do came much later, and is sadly, beyond the scope of your standard introductory course. This is part of the reason why I found introductory organic chemistry extremely frustrating. Asking your introductory organic instructor about why these reagents work is like a 7-year old asking their mom about where babies come from. You get nothing but lies and half truths because they’ve decided you’re not ready for that information yet.
On the plus side, there’s not much to worry about in the way of stereochemistry or regiochemistry. It’s all about keeping track of the bonds being broken and formed.
On that happy note, we’re now done with the essential reactions of Org 1. Now on to a new series where we explore the next question to ask about a reaction – but it looks like Jess has already asked it!