Today’s post represents not so much a pattern in alkene reactions, so much as it does a very common reaction that bears mentioning along with the rest. What makes this reaction special is that it does not simply break the carbon-carbon π bond, as we have been accustomed to seeing, but additionally breaks the C-C σ bond as well.
This type of reaction is known as oxidative cleavage [i.e. cleavage of bonds, occuring with oxidation] and the most prominent example of an oxidative cleavage reaction is ozonolysis.
As mentioned on one Reagent Friday back in the day, ozone does more than absorb UV radiation in the upper atmosphere and cause breathing problems in traffic-clogged cities. It’s a powerful oxidant, and since its discovery in the mid 1800’s by (Schönbein) has found use in the cleavage of carbon-carbon multiple bonds.
Here’s the pattern for the reaction of alkenes with ozone:
Note that the carbon-carbon double bond is broken and we are forming a carbon-oxygen double bond on each of the two carbons that originally composed the alkene. The second step in ozonolysis is called the “workup”. There are two different types of “workup”, and the most common is referred to as “reductive workup”. In this step, we add a reducing agent (commonly zinc metal or dimethyl sulfide) that decomposes the intermediate formed at the end of the ozonolysis reaction (called an “ozonide” by the way). If you’re wondering where the third oxygen of ozone went – it’s now attached to what used to be our reducing agent (making either zinc oxide (ZnO) or dimethyl sulfoxide (DMSO). [For more details / mechanism everything is written out in this post.]
Using “reductive workup” preserves all other aspects of the molecule save the double bond. So if we start with, say, a trisubstituted alkene, as in the example below, we will end up with a ketone and an aldehyde. [What happens if the alkene carbon is attached to two hydrogens? It becomes formaldehyde, which is then further converted to carbon dioxide]
Note that although I’ve written (CH3)2S as the reductant here, it’s essentially interchangeable with Zn for our purposes.
An interesting consequence of ozonolysis is that if the alkene is within a ring, you end up with a chain containing two carbonyls:
If your molecule has multiple alkenes, then you will end up with more than two fragments. For many years ozonolysis was used as a method for the structure determination of unknown molecules. By analyzing the fragments it is then possible to deduce what the original structure was, through “stitching” together the fragments. [This was particularly important in the case of unsaturated molecules known as terpenes]. Here’s one example:
This isn’t the end of the story with ozonolysis. There’s a second type of workup that can be used, referred to as oxidative workup. Instead of using Zn or S(CH3)2, if we use the oxidant hydrogen peroxide [H2O2], any aldehydes that form will be oxidized to give carboxylic acids. Like in the example below – notice that the green C-H bond is oxidized to C-OH [but all the other hydrogens remain intact ].
An alternative to using ozone for oxidative workup is to use the reagent KMnO4 , especially in the presence of hot acid; this will lead to the same result.
This is the last category of important alkene reactions we’ll cover for now in this series; in the next post we’ll wrap up the reactions of alkenes with a summary post.
NEXT POST: Summary of Alkene Addition Reactions