Reagent Friday: Ozone (O3)

by James

in Aldehydes, Alkenes, Ketones, Organic Chemistry 1, Organic Reagents

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”.

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.

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.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.

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.)

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!

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{ 3 comments… read them below or add one }

azmanam September 27, 2011 at 1:24 pm

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. :)

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echemie December 19, 2013 at 7:45 pm

The word “ozein” has a Greek etymology. It is not German.

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James Ashenhurst December 27, 2013 at 12:49 am

Fixed. Thank you!

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