Ozonolysis

Why ozonolysis matters

• Cleaves C=C cleanly to two carbonyls. No other reagent cuts an alkene with the predictability of ozone: no over-oxidation, no rearrangement, no rearrangement of stereocenters elsewhere in the molecule. A trisubstituted alkene gives a ketone + aldehyde; a disubstituted alkene gives two aldehydes. The position of cleavage is exactly the C=C bond — making ozonolysis the gold standard for alkene degradation in structure determination.
• Industrial azelaic acid: ~30 kt/yr. Oleic acid (CH₃(CH₂)₇CH=CH(CH₂)₇COOH, the major fatty acid in olive oil) is ozonolyzed under oxidative workup (H₂O₂, 60-80 °C, continuous flow) to give azelaic acid HOOC(CH₂)₇COOH (~30 kt/yr) and pelargonic acid CH₃(CH₂)₇COOH. Azelaic acid is the diacid monomer for nylon-6,9 and a key acne-treatment active. The Emery process at the Cognis/BASF Cincinnati plant is the canonical industrial example.
• Dominant tool for structure determination of natural products (1900-1960). Before NMR, ozonolysis cleaved every C=C in a natural product into pieces that could be identified by mp/bp comparison with authentic carbonyl samples. Harries used ozone to map natural rubber's polyisoprene structure (1905), establishing for the first time that rubber is a polymer of isoprene. Hundreds of terpene and steroid structures were determined this way.
• Atmospheric chemistry hinges on Criegee intermediates. Ozone reacts with isoprene, monoterpenes, and other plant-emitted alkenes throughout the troposphere, generating Criegee intermediates that destroy SO₂, NO₂, and many volatile organic compounds. CH₂OO and (CH₃)₂COO are the dominant nighttime oxidants in air — Criegee chemistry is responsible for ~25-50% of atmospheric SO₂ oxidation, with direct climate implications.
• Tolerates many functional groups in cold dilute solution. Run at -78 °C in CH₂Cl₂/MeOH (3:1 to 9:1), ozone selectively cleaves only C=C and (slowly) C≡C. Aromatic rings, esters, amides, alcohols, ethers, halides, and nitro groups all survive. Selectivity for the most electron-rich alkene in a polyene allows site-selective ozonolysis with sub-stoichiometric ozone.
• Two oxidation states of products on demand. Reductive workup with Me₂S (or Zn/AcOH, PPh₃, NaBH₄ for further reduction to alcohols) stops at the aldehyde + aldehyde (or ketone) product. Oxidative workup with H₂O₂ pushes aldehyde to carboxylic acid (ketones unaffected). The same starting material gives different products in 2-vs-3-step protocols.
• Late-stage cleavage in total synthesis. A protected double bond installed early in a complex synthesis can be unmasked by ozonolysis at the very end to reveal an aldehyde. Many complex syntheses use a styryl (PhCH=CH-) or vinyl (CH₂=CH-) group as a CHO synthon, ozonolyzing in the last 2-3 steps. Tolerance for adjacent stereocenters is critical and well-documented.

Common misconceptions

• "Ozone cleaves everything." Aromatic rings (benzene, naphthalene) react slowly because π-electron delocalization lowers the alkene character. With excess O₃ and prolonged reaction time, even benzene cleaves to glyoxal — but at standard ozonolysis temperatures (-78 °C), aromatics survive. C-H bonds, C-O bonds, and C-N bonds are completely inert.
• "You can isolate the ozonide." Pure 1,2,4-trioxolanes detonate on warming above about 0 °C, on shock, or on contact with reducing surfaces. Lab practice is to never warm or concentrate the ozonide; reductive or oxidative workup is added to the cold reaction mixture immediately. The Criegee intermediate is even worse — never isolable.
• "Reductive and oxidative workup give similar products." They differ by two oxidation states for any aldehyde-bearing carbon. Disubstituted alkenes give 2 aldehydes (reductive) or 2 carboxylic acids (oxidative). Trisubstituted give an aldehyde + ketone (reductive) or carboxylic acid + ketone (oxidative). Choose carefully — the wrong workup can ruin a 30-step synthesis.
• "Cis and trans alkenes give different products." No. The cleavage destroys the alkene geometry information; both cis-2-butene and trans-2-butene give 2 acetaldehyde upon reductive ozonolysis. Stereochemistry at sp³ centers near the alkene is preserved.
• "You need a calibrated ozone generator." Standard practice is to bubble O₃-O₂ stream through the cold solution while monitoring the color: dilute alkenes turn the solution slightly blue once O₃ is in excess (CH₂Cl₂/Sudan III indicator turns from red to colorless when oxidized). The 'blue endpoint' is the visual signal for stop. Calibrated dosing is for larger-scale or mechanism studies.
• "Triphenylphosphine is the cheapest reductant." Me₂S (dimethyl sulfide) is cheaper, gives only volatile DMSO byproduct (easy removal), and is the most common industrial reductant. PPh₃ is fine but produces Ph₃P=O which is hard to remove. Zn/AcOH is the cheapest by far but works only on simple substrates and can over-reduce ketones to alcohols.

Mechanism of ozonolysis (Criegee)

The textbook mechanism — proposed by Rudolf Criegee in 1949-1953 and confirmed by isotope and direct spectroscopic studies decades later — proceeds in three [3+2]-cycloaddition stages plus a workup step. Step 1: 1,3-dipolar cycloaddition. Ozone, a bent 1,3-dipole, undergoes a concerted [3+2] cycloaddition with the alkene to form a five-membered 1,2,3-trioxolane — three contiguous oxygens plus the two original alkene carbons. This species is called the molozonide or primary ozonide. The reaction is fast (k ≈ 10⁵ M⁻¹s⁻¹ for terminal alkenes at -78 °C) and exothermic.

Step 2: retro-[3+2] fragmentation. The molozonide is highly strained because its three adjacent O-O bonds (each ~140 kJ/mol) destabilize the ring. It fragments by retro-[3+2] into two pieces: a carbonyl (R₂C=O) and a carbonyl oxide (R₂C=O⁺-O⁻, the Criegee intermediate). The Criegee intermediate is a 1,3-dipole — equivalent to a carbonyl with an extra oxygen bonded — and is the most reactive species in the sequence. Step 3: re-cycloaddition to ozonide. The Criegee intermediate engages the carbonyl in a second [3+2] cycloaddition. The resulting five-membered ring places the third oxygen across the ring rather than adjacent: this is a 1,2,4-trioxolane (the ozonide, also called the secondary ozonide). With only two adjacent O-O bonds, it is roughly 100 kJ/mol more stable than the molozonide. The ozonide can be detected by ¹H and ¹³C NMR at -78 °C in CDCl₃.

Step 4: workup determines product oxidation state. The ozonide is then opened by either a reductive or an oxidative reagent. Reductive (Me₂S, Zn/AcOH, PPh₃) cleaves the O-O bonds without further oxidation: the result is two carbonyls (one from each side of the original alkene). Oxidative workup with H₂O₂ instead oxidizes each carbonyl carbon further: an aldehyde C-H becomes COOH (one extra oxidation), while a ketone C is already at maximum oxidation state for the carbonyl level and remains unchanged. Stronger oxidants (KMnO₄, OsO₄) do not improve over H₂O₂ at this stage and can over-oxidize sensitive groups elsewhere.

Reductive vs oxidative ozonolysis workup

Property	Reductive workup	Oxidative workup
Reagent	Me₂S, Zn/AcOH, PPh₃, (NaBH₄ for alcohols)	H₂O₂ (often 30% aq.)
Disubstituted alkene RCH=CHR'	RCHO + R'CHO (two aldehydes)	RCOOH + R'COOH (two carboxylic acids)
Trisubstituted RR'C=CHR''	RR'C=O + R''CHO (ketone + aldehyde)	RR'C=O + R''COOH (ketone + carboxylic acid)
Tetrasubstituted RR'C=CR''R'''	RR'C=O + R''R'''C=O (two ketones)	RR'C=O + R''R'''C=O (two ketones, no change)
Typical temperature	-78 °C → rt during quench	-78 °C → 60-80 °C in aqueous H₂O₂
Typical yield	60-90%	50-85%
Industrial example	Many natural product totals	Azelaic acid from oleic acid (~30 kt/yr)
Cost	Me₂S ~$5/kg, Zn ~$3/kg	H₂O₂ ~$1.5/kg (30%)
Side products	DMSO, Zn salts, Ph₃P=O	H₂O

Famous ozonolysis applications

• Harries on natural rubber (1904-1905). Carl Dietrich Harries discovered ozonolysis in 1903 by reacting ozone with rubber. He cleaved the polyisoprene chain to levulinaldehyde (4-oxopentanal) and demonstrated that rubber is a regular polymer of isoprene units (CH₂=C(CH₃)-CH=CH₂). This was the first definitive structural assignment of any natural polymer and laid the foundation for synthetic rubber chemistry.
• Industrial azelaic acid (Emery process). Oleic acid from beef tallow or soybean oil is continuously ozonolyzed at 30-40 °C under H₂O₂ in a flow reactor at the Cognis/BASF plant. Output: ~30 kt/yr azelaic acid (HOOC(CH₂)₇COOH) for nylon-6,9 manufacture, plasticizers, and acne medication; ~30 kt/yr pelargonic acid (CH₃(CH₂)₇COOH) for fragrance and herbicide applications. The reaction has been operated continuously since the 1950s.
• Woodward steroid syntheses (1950s). R. B. Woodward and others used ozonolysis at multiple stages of cortisone, cholesterol, lanosterol, and reserpine total syntheses to cleave specific alkenes installed as 'masked' aldehydes earlier in the sequence. Ozonolysis tolerated the 30+ step skeleton without disturbing nearby stereocenters.
• Nicolaou Taxol (1994). Ozonolysis of a styryl group at C-2 of the Taxol core unmasked the aldehyde precursor to the C-2 ester functionality. The reaction was run at -78 °C in CH₂Cl₂/MeOH with reductive Me₂S workup at the third-to-last step of the 35-step total synthesis without epimerizing any of the eight existing stereocenters.
• Corey discodermolide and prostaglandins. E. J. Corey's prostaglandin syntheses (1969 onward) deployed ozonolysis on multiple substrates to convert vinyl groups to aldehydes for subsequent Wittig couplings. Corey's use of the styryl/vinyl-as-aldehyde-synthon strategy became standard practice.
• Cargill industrial ozonolysis of vegetable oils. Cargill operates ozonolysis plants to convert renewable oleic and erucic acids from rapeseed and other oilseeds into biobased dicarboxylic acids (azelaic, brassylic). The biobased route competes with petroleum-derived adipic acid for nylon manufacture; production exceeds 50 kt/yr globally for biobased monomers.