Alkene Reactions: Ozonolysis
Last updated: July 9th, 2019 |
Ozonolysis of Alkenes
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.
Table Of Contents
- Ozone (O3) Is A Powerful Oxidant For Cleaving Alkenes To Carbonyl Compounds
- Ozonolysis With “Reductive Workup” : All C–H Bonds Are Preserved
- Ozonolysis Of A Ring Results In A Chain With Two Carbonyls
- Ozonolysis Of A Compound With Multiple Bonds Results In Several Fragments
- Ozonolysis With Oxidative Workup Converts Aldehydes To Carboxylic Acids
- Summary: Ozonolysis of Alkenes
- Further Reading
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
- Ueber die Einwirkung des Ozons auf organische Verbindungen
Just. Lieb. Ann. Chem. 1905, 343 (2-3), 311-344
The first paper describing the oxidative cleavage of unsaturated compounds with ozone in solution.
I. Smith, F. L. Greenwood, and O. Hudrlik
Org. Synth. 1946 26, 63
This procedure from Organic Syntheses, a source of reliable, reproducible and independently tested organic chemistry laboratory experimental procedures, provides a detailed explanation of how to build a laboratory ozonizer.
- The Preparation of Aldehydes, Ketones, and Acids by Ozone Oxidation
Albert L. Henne and Philip Hill
Journal of the American Chemical Society 1943 65 (5), 752-754
This paper shows that carboxylic acids are formed in good yields from aldehydes when the ozonolysis reaction mixture is worked up in the presence of excess hydrogen peroxide.
- Notes- A Convenient Method for Reduction of Hydroperoxide Ozonation Products
Knowles and Q. Thompson
The Journal of Organic Chemistry 1960 25 (6), 1031-1033
Although the current practice is to use dimethyl sulfide in a reductive ozonolysis workup, trimethyl phosphite can also be used, as this paper from Nobel Laureate W. S. Knowles demonstrates.
- OZONOLYTIC CLEAVAGE OF CYCLOHEXENE TO TERMINALLY DIFFERENTIATED PRODUCTS: METHYL 6-OXOHEXANOATE, 6,6-DIMETHOXYHEXANAL, METHYL 6,6-DIMETHOXYHEXANOATE
Ronald E. Claus and Stuart L. Schreiber
Synth. 1986, 64, 150
This procedure in Organic Syntheses demonstrates how ozonolysis can be used to quickly generate differentiated bifunctional compounds.
- Mechanism of Ozonolysis
Dr. Rudolf Criegee
Angew. Chem. Int. Ed. 1975, 14 (11), 745-752
This is an account by Prof. Rudolf Criegee on work done towards determining the mechanism of ozonolysis. Criegee himself carried out extensive work in this area – the ‘Criegee intermediate’ in ozonolysis is named after him.
The following papers are further mechanistic studies on ozonolysis:
- New evidence in the mechanism of ozonolysis of olefins
Klopman and C. M. Joiner
Journal of the American Chemical Society 1975 97 (18), 5287-5288
- Mechanism of ozonolysis. (a) Microwave spectra, structures, and dipole moments of propylene and trans-2-butene ozonides. (b) Orbital symmetry analysis
Robert P. Lattimer, Robert L. Kuczkowski, and Charles W. Gillies
Journal of the American Chemical Society 1974 96 (2), 348-358
- Microwave and mass spectral studies of the ozonolyses of ethylene, propylene, and cis- and trans-2-butene with added oxygen-18 formaldehyde and acetaldehyde
Charles W. Gillies, Robert P. Lattimer, and Robert L. Kuczkowski
Journal of the American Chemical Society 1974 96 (5), 1536-1542