Oxidative cleavage

With oxidative cleavage, the carbon-carbon bond of an alkene is completely broken, and in many cases this will break the molecule into two pieces.  Alkenes can be cleaved by oxidation with ozone, O₃., using a process called ozonolysis.  The carbon-carbon double bond is broken, and the alkene carbons are converted to aldehydes:

Dimethyl sulfide or zinc is added in the work-up stage of the reaction in order to reduce hydrogen peroxide, which is formed in the reaction, to water.

The simplest and most widely accepted mechanism for ozonolysis involves formation of a molozonide by a direct cycloaddition of ozone to the double bond.1

Isomerization of the molozonide appears to occur by a fragmentation-recombination reaction, as shown in Equations 11-7 and 11-8:

Potassium permanganate (KMnO₄) is another very powerful oxidizing agent that will oxidize primary alcohols and aldehydes to carboxylic acids. KMnO₄ is also useful for oxidative cleavage of alkenes to ketones and carboxylic acids:

Alkynes can also undergo oxidative cleavage.  Internal alkynes form carboxylic acids (RCOOH) and terminal alkynes form carboxylic acids and CO₂.  The ozonide intermediate only requires water to decompose it to the cleavage products:

Exercises

 

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