Ozonolysis of Alkenes

Chapter 6: Reactions of Alkenes : Addition Reactions

ozonolysis of alkenes
Reaction type: Electrophilic Addition

Summary

  • Overall transformation :  C=C to 2 C=O
  • Ozonolysis implies that ozone causes the alkene to break (-lysis)
  • Reagents : ozone followed by a reducing work-up, usually Zn in acetic acid. 
  • 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 C=C.
how to visualise ozonolysis
 What would be the products of the ozonolysis reactions of: 
(a) ethene ? 2 molecules of methanal, H2CO
(b) 1-butene ? two aldehydes, CH3CH2CO and H2CO
(c) 2-butene ? 2 molecules of CH3CHO
(d) 2-methylpropene ? a ketone, propanone CH3COCH3, and an aldehyde, H2CO

 
MECHANISM FOR REACTION OF ALKENES WITH O3
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 ozonide rearranges to the malozonide. 
Step 3:
On work-up (usually Zn / acetic acid) the malozonide decomposes to give two carbonyl groups.