OZONE

OZONE electric discharge or cosmic rays.. : : : EQUIVALENT RESONANCE STRUCTURES

OZONOLYSIS

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

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

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

molozonideozonide forms initiallyforms after rearrangement OZONIDE AND MOLOZONIDE STRUCTURES

SOME EVIDENCE FOR THE MECHANISM

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

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

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

WORKUP PROCEDURES FOR OZONOLYSIS OZONOLYSIS

WORKUP PROCEDURES FOR OZONOLYSIS Two types of work-up (decomposition of the ozonide) are possible : Oxidative WorkupAdd aqueous acid (H 3 O+) Reductive Workup Two methods : A) Zn, acetic acid or Zn, H 2 O B) Pd/H 2 followed by aqueous acid H 2 SO 4 + H 2 O After the ozonide is formed it is hydrolyzed (work-up). CH 3 COOH

OXIDATIVE WORKUP

Just add H 3 O + ( = H 2 0 and acid ) Aldehydes are oxidized to carboxylic acids. Formaldehyde is oxidized to carbon dioxide, which is lost as a gas. OXIDATIVE WORKUP (acid) These oxidations occur because H 2 O 2 is a hydrolysis product. The general hydrolysis mechanism follows. OO O OO H 3 O+ You do not have to known this mechanism.

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

Aldehydes are easily oxidized by the H 2 O 2 that is produced. oxidizing agent Ketones are not oxidized by hydrogen peroxide. HYDROGEN PEROXIDE IS A PRODUCT OF THE OXIDATIVE WORKUP OF THE OXIDATIVE WORKUP

If formaldehyde is produced, it is oxidized to CO 2 and H 2 O. OXIDATIVE METHODS DESTROY FORMALDEHYDE Carbonic acid is unstable and decomposes to CO 2 and H 2 O. two moles gas

REDUCTIVE WORKUP

METHOD A Add Zn and H 2 O (or acetic acid) METHOD B Reduce the ozonide with Pd / H 2, and then add acid ( H 3 O + ). With either method, aldehydes survive intact and are not oxidized. REDUCTIVE WORKUP There are two methods of reductive workup.

The zinc “scavenges” the peroxide before it can act. REDUCTIVE WORKUP - METHOD A With the REDUCTIVE WORKUPS, no H 2 O 2 is produced. Notice that the O-O bond is broken so that no H 2 O 2 is formed during the subsequent hydrolysis.

REDUCTIVE WORKUP - METHOD B With the REDUCTIVE WORKUPS, no H 2 O 2 is produced. The hydrogenation step cleaves the O-O bond. Since the O-O bond is broken, no H 2 O 2 is formed during the hydrolysis.

EXAMPLES O3O3 Zn / H 2 O 1) O 3 2) H 3 O + OR O3O3 1) Pd/H 2 2) H 3 O + REDUCTIVE WORKUP OXIDATIVE WORKUP aldehyde survives formaldehyde CO 2

USING OZONOLYSIS FOR STRUCTURE PROOF STRUCTURE PROOF

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

C 7 H 12 C 7 H 14 1) O 3 / CH 2 Cl 2 2) H 3 O + Pd / H 2 PROBLEM TO SOLVE 6-ketoheptanoic acid or 6-oxoheptanoic acid answer

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

OXIDATION OF ACETYLENES

ACETYLENES KMnO 4 or 1) O 3, CH 2 Cl 2 2) H 3 O + Oxidation of acetylenes, whether by KMnO 4 or ozone, normally yields carboxylic acids.

OZONE AND SMOG

SO 2 CO 2 NO 2 NO R-CH=CH 2 COOLER AIR WARMER AIR O3O3 temperature inversion traps pollutants Temperature Inversion:Air above land is cooler than air above. FORMATION OF SMOG - OZONE IS A COMPONENT H2OH2O incompletely burned hydrocarbons reacts with unburned hydrocarbons

Spruce, Cedar, Fir or Pine Forest Terpenes O3O3 NATURAL SOURCES reacts with terpenes temperature inversion traps bioemissions

OXIDATION OF SIDE CHAINS ON AROMATIC RINGS ON AROMATIC RINGS

BENZENE RINGS Under normal conditions of ozonolysis, or treatment by KMnO 4, benzene rings are not oxidized. They are quite resistant to reaction due to the presence of aromatic ring resonance. KMnO 4 / 50 o C /2 hours When using KMnO 4 on a benzene ring that has a side chain, the side chain is oxidized before the ring is affected. The exception is rings with -OH, -OCH 3, -NH 2 and similar O/N groups, which oxidize quite readily.

Ozone, will not attack the saturated side chain. BENZENE RINGS O 3 / CH 2 Cl 2 /0 o O 3 / CH 2 Cl 2 /20 o long time However, under more vigorous conditions the benzene ring can be ozonized. 5+

SELECTIVITY

SELECTIVITY H2O2H2O2 O 3 / CH 2 Cl 2 /0 o oxidizes aldehyde cleaves double bond H 3 O+ oxidizes aldehydes (oxidative work-up) more vigorous ozonolysis : cleaves benzene ring cleaves double bond oxidizes aldehydes 1) 2) O 3 / 20 o / CH 2 Cl 2 2 hours cinnamaldehyde cinnamic acid benzoic acid oxalic acid

MORE SELECTIVITY 1) OsO 4 2) NaHSO 3 H2OH2O 1) O 3 / CH 2 Cl 2 / 0 o 2) Zn / CH 3 COOH KMnO 4 H 2 SO 4 30 min aldehydes survive (reductive work-up) aldehyde survives (OsO 4 is mild) aldehydes are oxidized by KMnO 4 RCO 3 H cinnamaldehyde epoxidation benzaldehyde