4.4: Oxidation of Alkenes- Oxidative Cleavage Reactions
- Page ID
- 448314
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- write an equation to describe the cleavage of an alkene by ozone, followed by reduction of the ozonide so formed with either dimethyl sulfide or zinc and acetic acid.
- write an equation to describe the cleavage of an alkene by ozone, followed by oxidation of the ozonide so formed with hydrogen peroxide.
- predict the products formed from the ozonolysis-reduction or ozonolysis-oxidation of a given alkene.
- use the results of ozonolysis-reduction or ozonolysis-oxidation to deduce the structure of an unknown alkene.
- identify the reagents that should be used in the oxidative cleavage of an alkene to obtain a given product or products.
Make certain that you can define, and use in context, the key terms below.
- molozonide
- ozonide
- ozonolysis
Ozonolysis refers to the treatment of an alkene with ozone followed by a suitable reducing or oxidizing agent to break down complex double-bond-containing compounds into smaller, more easily identified products. From the identity of the products formed, it may be possible to deduce the structure of the original double-bond-containing substance. Ozonolysis will feature prominently in many of the road-map problems that you will encounter in this course.
A molozonide is an unstable, cyclic intermediate that is initially formed when an alkene reacts with ozone.
Note within the summary of the following reactions that ozonolysis produces aldehydes and ketones (reductive workup) or carboxylic acids, ketones, and carbon dioxide (oxidative workup).
Ozonolysis is a method of oxidatively cleaving alkenes or alkynes using ozone (O3), a reactive allotrope of oxygen. The process allows for carbon-carbon double or triple bonds to be replaced by double bonds with oxygen. This reaction is often used to identify the structure of unknown alkenes by breaking them down into smaller, more easily identifiable pieces. Ozonolysis also occurs naturally and breaks down repeat units used in rubber and other polymers. On an industrial scale, azelaic acid and pelargonic acids are produced from ozonolysis.
Introduction
The gaseous ozone is first passed through the desired alkene solution in either methanol or dichloromethane. The first intermediate product is an ozonide molecule which reacts further to produce carbonyl products. This results in the breaking of the Carbon-Carbon double bond and is replaced by a Carbon-Oxygen double bond instead.
Reaction Mechanism
Step 1:
The first step in the mechanism of ozonolysis is a cycloaddition (we will learn about this type of reaction later in the semester) between ozone and the Carbon-Carbon double bond, which forms the molozonide intermediate. Due to the high energy of the molozonide, it breaks apart to form a carbonyl and a carbonyl oxide molecule.
Step 2:
The carbonyl and the carbonyl oxide reorient (molecular rotation) and react again to create the stable ozonide intermediate. Since some ozonides are explosive, it is immediately reacted with a reductive or oxidative workup to then convert the ozonide molecule into the desired carbonyl products. A typical reductive workup is either zinc metal in acetic acid or dimethyl sulfide (DMS). A typical oxidative workup is hydrogen peroxide. A variety of carbonyl products could result depending on the starting alkene. For example, a tetrasubstituted alkene would yield two ketone products (with either a reductive or oxidative workup), while a trisubstituted alkene would yield one ketone product and one aldehyde product with a reductive workup or one ketone product and one carboxylic acid product (oxidative workup).
While there are other options for oxidative cleavage of the double bond, this is the most commonly used reaction.
References
- Vollhardt, K., Schore, N. Organic Chemistry: Structure and Function. 5th ed. New York, NY: W. H. Freeman and Company, 2007.
- Shore, N. Study Guide and Solutions Manual for Organic Chemistry. 5th ed. New York, NY: W.H. Freeman and Company, 2007.
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Draw the structure of the product or products obtained in each of the following reactions:
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Draw the structure of the product or products obtained in each of the following reactions:
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