Chapter 6: Reactions of Alkenes: Addition Reactions Ch 6 contents

Ozonolysis of Alkenes

ozonolysis of alkenes
Reaction type: Electrophilic Addition

Summary

  • Overall transformation :  C=C to 2 x C=O
  • Ozonolysis implies that ozone causes the alkene to break (-lysis)
  • Reagents : ozone followed by :
    • a reducing work-up, either Zn in acetic acid or dimethyl sulfide, (CH3)2S 
    • an oxidising work-up, usually H2O2 (under these conditions, carboxylic acids are obtained instead of aldehydes)
  • It is convenient to view the process as cleaving the alkene into two carbonyls
    • The substituents on the C=O depend on the substituents on the original C=C.
    • The work-up determines the oxidation state of the products

      •  
REDUCTIVE work-up
how to visualise ozonolysis with [R] work-up
OXIDATIVE work-up
how to visualise ozonolysis with [O] work-up
Question
  • What would be the products of the reactions of each of the following with O3 followed by Zn / acetic acid ?

(a)  ethene ? 2 molecules of methanal, H2CO
(b) 1-butene ? two aldehydes, CH3CH2CHO and H2CO
(c) cis or trans-2-butene ? 2 molecules of CH3CHO
(d) 2-methylpropene ? a ketone, propanone CH3COCH3, and an aldehyde, H2CO

 
MECHANISM FOR REACTION OF ALKENE OZONOLYSIS
Step 1:
The p electrons act as the nucleophile, attacking the ozone at the electrophilic terminal O. A second C-O is formed by the nucleophilic O attacking the other end of the C=C.
ozonolysis of C=C to C=O
Step 2:
The cyclic species called the malozonide rearranges to the ozonide. 
Step 3:
The ozonide decomposes on  work-up. Reductive work-up with (usually Zn / acetic acid) gives the two carbonyl groups.

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