Organic Reagents

By James Ashenhurst

Reagent Friday: Ozone (O3)

Last updated: January 29th, 2020 |

Ozone (O3), A Powerful Oxidant In Organic Chemistry

In a blatant plug for the Reagent Guide and the Reagents App for iPhone, each Friday  I profile a different reagent that is commonly encountered in Org 1/ Org 2. 


Ozone is a molecule that most people are familiar with hearing about, either because it is missing (in the high atmosphere, where it absorbs UV radiation) or because it is present (as a toxic component of smog). But ozone is also very useful in a chemistry lab.

The blue color of excess ozone demonstrates that this ozonolysis is complete. Thanks to @azaprins for the photo

What it’s used for: As a reagent, ozone is used to cleave alkenes and alkynes to give carbonyl compounds such as aldehydes, ketones, and carboxylic acids. The type of products obtained depends upon the “workup” used – that is, the way it is treated after the reaction is over. The process of breaking a carbon-carbon multiple bond to form carbonyl compounds is called “oxidative cleavage”.

Ozone (O3) In the Oxidative Cleavage Of Alkenes (With Reductive Workup )

When alkenes are treated with ozone and subjected to “reductive workup” with either zinc (Zn)  or dimethyl sulfide  (Me2S) [triphenylphosphine (Ph3P) also sees use], the carbon-carbon double bond is cleaved to form ketones or aldehydes, depending on the structure of the alkene.


Ozone (O3) In The Oxidative Cleavage Of Alkenes (With Oxidative Workup)

If H2O2 is used instead of Zn (“oxidative workup”) the aldehyde will instead be oxidized to a carboxylic acid. In other words the C-H bond attached to the alkene becomes a C-OH bond. Note that ketones – which lack a C-H bond on the carbonyl carbon – are obtained regardless of whether workup is reductive or oxidative.

Oxidative Cleavage Of Alkynes

Ozone can also be used to oxidatively cleave alkynes, in which case carboxylic acids are formed. If the alkyne is a “terminal” alkyne (that is, has a hydrogen attached to one of the sp-hybridized carbons) then the product is CO2.

Mechanism For The Oxidative Cleavage Of Alkenes With O3 (Ozone) And Reductive Workup With Zn Or DMS

The mechanism for oxidative cleavage is a fairly lengthy one and the actual details of the experiments used to determine the proposed mechanism are fascinating, but for our purposes we’ll just mention that the first step is referred to as a “cycloaddition”, (sometimes a “3+2 cycloaddition”) resulting in the formation of a 5 membered ring (“molozonide”). The molozonide is unstable, and undergoes fragmentation followed by rearrangement to give an isomeric 5-membered ring called an “ozonide”.


At the temperatures at which these reactions are done (usually dry-ice/acetone, or –78°C) ozonides are fairly stable, but will break down to form acyclic compounds when warmed. To obtain the final products (as well as to get rid of any excess ozone), the workup is performed. In reductive workup, a reagent is added that will cleave the O-O bond. Warming of the solution then results in the desired aldehyde/ketone. In oxidative workup, the ozonide is allowed to decompose in the presence of hydrogen peroxide, which will oxidize aldehydes to carboxylic acids.  (This is already a long post, so the images will have to wait for now.)

Ozonolysis is discussed in more detail here (See post –  Alkene Reactions: Ozonolysis)

Real life advice: One of the cool things about using ozone in the lab is that it has a very distinctive beautiful blue color. Back in the day, I used an old-school ozone generator to bubble ozone through my reaction solution, and when there was no more of the starting alkene left, the blue color of the excess ozone served as an indicator that the reaction was done. The smell is very distinctive too – sharp and metallic, a smell you might recognize if you’ve stood around transmission power lines. In fact, Schönbein, who discovered ozone in 1840, coined the name “ozone” from the German Greek “ozein”, meaning “to smell”. Pretty cool that one atom make such a difference to the nose! (although one wonders how much of it is merely the sensation of your olfactory receptors getting zapped).

Left – Christian Schonbein, discoverer of O3. Right – O3 generator (Univeristy of Alberta Lipid Chemistry Group)

P.S. You can read about the chemistry of ozone and more than 80 other reagents in undergraduate organic chemistry in the “Organic Chemistry Reagent Guide”, available here as a downloadable PDF. The Reagents App is also available for iPhone, click on the icon below!


(Advanced) References and Further Reading

  1. Ueber die Einwirkung des Ozons auf organische Verbindungen
    Just. Lieb. Ann. Chem. 1905, 343 (2-3), 311-344
    DOI: 10.1002/jlac.19053430209
    The first paper describing the oxidative cleavage of unsaturated compounds with ozone in solution.
  2. OZONE
    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.
  3. 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.
  4. Notes- A Convenient Method for Reduction of Hydroperoxide Ozonation Products
    Knowles and Q. Thompson
    The Journal of Organic Chemistry 1960 25 (6), 1031-1033
    DOI: 10.1021/jo01076a044
    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.
    Ronald E. Claus and Stuart L. Schreiber
    Org. Synth. 1986, 64, 150
    DOI: 10.15227/orgsyn.064.0150
    This procedure in Organic Syntheses demonstrates how ozonolysis can be used to quickly generate differentiated bifunctional compounds.
  6. 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:
  7. New evidence in the mechanism of ozonolysis of olefins
    G. Klopman and C. M. Joiner
    Journal of the American Chemical Society 1975 97 (18), 5287-5288
  8. 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
    DOI: 10.1021/ja00809a006
  9. 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
    DOI:  10.1021/ja00812a043


Comment section

12 thoughts on “Reagent Friday: Ozone (O3)

  1. I remember distinctly the smell of ozone wafting from our ancient ozone generator feeding the gas into ancient fume hoods. My wife always knew when I worked with ozone in the lab, because my clothes apparently still smelled like ozone. :)

  2. Sir, what would happen if ozonolysis of alkene is used at room temperature? What product would one obtain? Thank you.

  3. What I cannot understand is how such a low-efficiency synthesis method in use since 1857 hasn’t been superseded by anything better ─ I mean, M van Marum was generating ozone by E. discharges before about 1800?

    1. Assuming you have an ozone generator, the reaction is super easy to run. Bubble ozone through the reaction until you see a persistent blue color, add the reductant, and let it warm. A lot simpler than most organic reactions you encounter.
      It’s like the Fischer esterification. Old as the hills, but still works.

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