Ozonolysis

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Ozonolysis

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Key Questions

  • It does a very common reaction called oxidative cleavage, which cleaves/breaks one #pi# and one #sigma# bond in a #"C"="C"# bond. The most prominent reaction in which oxidative cleavage occurs 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.

    GENERAL ALKENE OZONOLYSIS REACTION

    http://www.masterorganicchemistry.com/

    Note that the #"C"="C"# bond is broken and we are forming a carbonyl (#"C"="O"#) bond on each of the two carbons that originally composed the alkene.

    REDUCTIVE WORKUPS

    The second step in ozonolysis is called the "workup". There are two different types of workups, and the most common is referred to as reductive workup.

    In this step, we add a reducing agent (commonly, #"Zn"(s)# or #("CH"_3)_2"S"#) that decomposes the intermediate formed at the end of the ozonolysis reaction. 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 #"ZnO"# or #"DMSO"# (dimethyl sulfoxide).

    (For more details / mechanism everything is written out.)

    Using a "reductive workup" preserves all other aspects of the molecule except for the #"C"="C"# 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.)

    http://www.masterorganicchemistry.com/

    Note that although I’ve written #("CH"_3)_2"S"# as the reductant here, it’s essentially interchangeable with #"Zn"(s)# 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:

    http://www.masterorganicchemistry.com/

    If your molecule has multiple double bonds, 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:

    http://www.masterorganicchemistry.com/

    This isn’t the end of the story with ozonolysis.

    OXIDATIVE WORKUPS

    There’s a second type of workup called an oxidative workup.

    Instead of using #"Zn"(s)# or #("CH"_3)_2"S"#, we can use the oxidizing agent #"H"_2"O"_2#, which oxidizes any aldehydes that form into carboxylic acids (notice that the green #"C"-"H"# bond below is affected).

    http://www.masterorganicchemistry.com/

    An alternative to using ozone for oxidative workup is to use the reagent #"KMnO"_4#, especially in the presence of hot acid; this will lead to the same result.

  • An ozonide is the 1,2,4-trioxolane structure that is formed when ozone reacts with an alkene,

    The first intermediate in the reaction is called a molozonide.

    i714.photobucket.com

    A molozonide is a 1,2,3-trioxolane (tri ="three"; oxa = "oxygen"; olane = "saturated 5-membered ring").

    The molozonide is unstable. It rapidly converts in a series of steps to an ozonide.

    science.uvu.edu

    An ozonide is a 1,2,4-trioxolane. It rapidly decomposes in water to form carbonyl compounds such as aldehydes and ketones.

    The video below shows the formation of the molozonide and ozonide intermediates as part of the mechanism.

  • Ozonolysis does not tell you about any stereochemistry there may have been in the original alkene.

    Reductive ozonolysis converts an alkene into a pair of carbonyl compounds.

    upload.wikimedia.org

    If the R groups are different, we can have cis/trans or E/Z stereochemistry.

    classes.yale.edu

    If R₂ and R₄ are the groups with higher priority, 1 is a Z alkene. 2 is an E alkene.

    But both give the same ozonolysis products.

    You cannot distinguish between cis-but-2-ene and trans-but-2-ene by reductive ozonolysis.

    upload.wikimedia.org

    They both give acetaldehyde as the only ozonolysis product.

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