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OZONE

OZONE. OZONE. electric discharge. or. cosmic rays. :. -. +. +. :. :. -. EQUIVALENT RESONANCE STRUCTURES. OZONOLYSIS. Ozonolysis. FORMATION OF AN OZONIDE. unstable. HYDROLYSIS OF THE OZONIDE (WORKUP). aldehydes or ketones. FORMATION OF AN OZONIDE - MECHANISM. -. UNSTABLE. :.

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OZONE

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  1. OZONE

  2. OZONE electric discharge or cosmic rays .. .. .. .. : .. .. .. - + + : : .. .. - EQUIVALENT RESONANCE STRUCTURES

  3. OZONOLYSIS

  4. Ozonolysis FORMATION OF AN OZONIDE unstable HYDROLYSIS OF THE OZONIDE (WORKUP) aldehydes or ketones

  5. FORMATION OF AN OZONIDE - MECHANISM - .. .. UNSTABLE : : O O .. .. C H C l 2 2 + : : O O .. o 0 C : O O .. .. ozone Addition is concerted molozonide carbonyl oxide .. .. .. + + O O .. .. O C C .. : : O O .. .. - - : O : : O : .. C O .. : O C .. ketone or aldehyde ozonide

  6. OVERALL RESULT OF THE REACTION WITH OZONE The double bond is replaced with the ozonide ring.

  7. OZONIDE AND MOLOZONIDE STRUCTURES molozonide ozonide forms initially forms after rearrangement

  8. SOME EVIDENCE FOR THE MECHANISM

  9. EVIDENCE FOR THE MECHANISM When ozonolysis is performed in an aliphatic hydrocarbon solvent (hexane) dimers of the carbonyl oxide intermediate sometimes form. insoluble hexane carbonyl oxide dimer This dimerization proves the existence of the carbonyl oxide intermediate.

  10. YET MORE MECHANISTIC EVIDENCE If a foreign ketone is placed in the solution (e.g., benzophenone) it becomes incorporated into a portion of the ozonides formed. EXPECTED + benzophenone traps the carbonyl oxide Can you explain this with a mechanism ? HINT: The benzophenone reacts with the carbonyl oxide intermediate. TRY THIS ON YOUR OWN

  11. MORE EVIDENCE When an unsymmetrical alkene undergoes ozonolysis, it is not uncommon for three different ozonides to form. EXPECTED The alkene must break in two and recombine. Can you draw mechanisms that explain the formation all three ozonides ? cis & trans TRY THIS ON YOUR OWN

  12. WORKUP PROCEDURES FOR OZONOLYSIS

  13. WORKUP PROCEDURES FOR OZONOLYSIS After the ozonide is formed it is hydrolyzed (work-up). Two types of work-up (decomposition of the ozonide) are possible : Oxidative Workup Add aqueous acid (H3O+) H2SO4 + H2O Reductive Workup A) Zn, acetic acid or Zn, H2O Two methods : CH3COOH B) Pd/H2 followed by aqueous acid

  14. OXIDATIVE WORKUP

  15. OXIDATIVE WORKUP (acid) Just add H3O+ ( = H20 and acid ) O O H3O+ O O O Aldehydes are oxidized to carboxylic acids. Formaldehyde is oxidized to carbon dioxide, which is lost as a gas. These oxidations occur because H2O2 is a hydrolysis product. The general hydrolysis mechanism follows. You do not have to known this mechanism.

  16. OXIDATIVE WORKUP (acid + water) continued …...

  17. HYDROGEN PEROXIDE IS A PRODUCT OF THE OXIDATIVE WORKUP Aldehydes are easily oxidized by the H2O2 that is produced. oxidizing agent Ketones are not oxidized by hydrogen peroxide.

  18. OXIDATIVE METHODS DESTROY FORMALDEHYDE If formaldehyde is produced, it is oxidized to CO2 and H2O. gas two moles Carbonic acid is unstable and decomposes to CO2 and H2O.

  19. REDUCTIVE WORKUP

  20. REDUCTIVE WORKUP There are two methods of reductive workup. Add Zn and H2O (or acetic acid) METHOD A Reduce the ozonide with Pd / H2 , and then add acid ( H3O+ ). METHOD B With either method, aldehydes survive intact and are not oxidized.

  21. REDUCTIVE WORKUP - METHOD A With the REDUCTIVE WORKUPS, no H2O2 is produced. The zinc “scavenges” the peroxide before it can act. Notice that the O-O bond is broken so that no H2O2 is formed during the subsequent hydrolysis.

  22. REDUCTIVE WORKUP - METHOD B With the REDUCTIVE WORKUPS, no H2O2 is produced. The hydrogenation step cleaves the O-O bond. Since the O-O bond is broken, no H2O2 is formed during the hydrolysis.

  23. REDUCTIVE WORKUP EXAMPLES aldehyde survives O3 Zn / H2O OR O3 1) Pd/H2 1) O3 2) H3O+ 2) H3O+ OXIDATIVE WORKUP formaldehyde CO2

  24. USING OZONOLYSIS FOR STRUCTURE PROOF

  25. “At one time” = before spectroscopy. AT ONE TIME OZONOLYSIS WAS WIDELY USED FOR STRUCTURE PROOF BY DEGRADATION Simpler Known Compounds Broken apart ( or degraded ) to simpler pieces that are easier to identify. Unknown compound The original structure can be deduced by reassembling the pieces.

  26. PROBLEM TO SOLVE 6-ketoheptanoic acid or 6-oxoheptanoic acid 1) O3 / CH2Cl2 C7H12 2) H3O+ Pd / H2 C7H14 answer

  27. oxidative workup oxidized during work-up WHAT WAS THE ORIGINAL STRUCTURE ?

  28. OXIDATION OF ACETYLENES

  29. ACETYLENES 1) O3, CH2Cl2 2) H3O+ KMnO4 or Oxidation of acetylenes, whether by KMnO4 or ozone, normally yields carboxylic acids.

  30. OZONE AND SMOG

  31. O3 FORMATION OF SMOG - OZONE IS A COMPONENT WARMER AIR temperature inversion traps pollutants COOLER AIR SO2 CO2 H2O reacts with unburned hydrocarbons NO2 NO R-CH=CH2 incompletely burned hydrocarbons Temperature Inversion: Air above land is cooler than air above.

  32. O3 NATURAL SOURCES temperature inversion traps bioemissions reacts with terpenes Terpenes Spruce, Cedar, Fir or Pine Forest

  33. OXIDATION OF SIDE CHAINS ON AROMATIC RINGS

  34. BENZENE RINGS Under normal conditions of ozonolysis, or treatment by KMnO4, benzene rings are not oxidized. They are quite resistant to reaction due to the presence of aromatic ring resonance. When using KMnO4 on a benzene ring that has a side chain, the side chain is oxidized before the ring is affected. KMnO4 / 50o C /2 hours The exception is rings with -OH, -OCH3, -NH2 and similar O/N groups, which oxidize quite readily.

  35. BENZENE RINGS Ozone, will not attack the saturated side chain. O3 / CH2Cl2 /0o O3 / CH2Cl2 /20o long time However, under more vigorous conditions the benzene ring can be ozonized. 5 +

  36. SELECTIVITY

  37. SELECTIVITY more vigorous ozonolysis : cinnamaldehyde H2O2 O3 / 20o/ CH2Cl2 oxidizes aldehyde 2 hours 1) O3 / CH2Cl2 /0o oxalic acid 2) H3O+ cinnamic acid cleaves benzene ring cleaves double bond oxidizes aldehydes benzoic acid cleaves double bond oxidizes aldehydes (oxidative work-up)

  38. MORE SELECTIVITY RCO3H epoxidation 1) OsO4 cinnamaldehyde KMnO4 2) NaHSO3 30 min H2SO4 H2O 1) O3 / CH2Cl2 / 0o 2) Zn / CH3COOH aldehyde survives (OsO4 is mild) aldehydes are oxidized by KMnO4 benzaldehyde aldehydes survive (reductive work-up)

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