1. Oxidative Rancidity A mechanistic look at this reaction Go to Slide Show mode and click to begin

2. CH=CH-CH-CH=CH H Reactive pentadiene unit in fatty acid chain

3. CH=CH-CH-CH=CH H Reactive pentadiene unit in fatty acid chain Reactive allylic bond

4. CH=CH-CH-CH=CH H Heat, light, metal ions cause bond cleavage

5. CH=CH-CH-CH=CH H Bond begins to break

6. CH=CH-CH-CH=CH. H. Bond is completely broken, forming a pentadienyl free radical

7. CH=CH-CH-CH=CH. CH-CH=CH-CH=CH. CH=CH-CH=CH-CH. Resonance forms for the pentadienyl radical The resonance hybrid will look more like one of the lower two structures because of the stability of conjugated double bonds

8. CH-CH=CH-CH=CH. O-O Reaction with atmospheric oxygen

9. CH-CH=CH-CH=CH. O-O Reaction with atmospheric oxygen

10. CH-CH=CH-CH=CH. O-O Reaction with atmospheric oxygen

11. CH-CH=CH-CH=CH O-O Bond begins to form

12. CH-CH=CH-CH=CH O-O Unstable hydroperoxyl radical.

13. CH-CH=CH-CH=CH O-O. HR Hydroperoxyl radical reacts with a molecule of the original lipid, removing a reactive allylic hydrogen (this is a propagation step)

14. CH-CH=CH-CH=CH O-O-H.R Returns to propagation

15. CH-CH=CH-CH=CH O-O-H Hydroperoxide (unstable, will decompose to form odor compounds)

16. CH-CH=CH-CH=CH O-O-H Bonds begin to break

17. CH O O-H CH=CH-CH=CH.... Bonds are broken, electrons pair up to form a carbon-oxygen double bond Return to propagation

18. CH O These aldehydes are the source of the rancid smell in lipid oxidation.