Ozonolysis O3 H2o2

Alkene Reactions: Ozonolysis using O3 and H2O2

Oxidative ozonolysis (O3 then H2O2) of alkenes

Ozone captures an alkene in a 1,2,3-trioxa five-membered ring (the molozonide), rearranges to the familiar ozonide, and the oxidative peroxide workup drives each vinylic carbon to the highest accessible oxidation level. Any vinylic hydrogen ends up on oxygen to give a carboxylic acid; fully substituted carbons stall at ketones. Terminal CH2 fragments can go all the way to CO2. The mechanism below keeps reagents valence-neutral in every frame so the electron flow is easy to follow.


Quick Summary

  • Reagents/conditions: O₃ (often at −78 → 0 °C in O₂); oxidative workup with H₂O₂/H₂O. (Compare to reductive workups such as Me₂S or Zn/HOAc.)
  • Outcome: Each vinylic carbon becomes a carbonyl carbon. Vinylic hydrogens yield carboxylic acids; fully substituted centers become ketones; terminal CH₂ groups can oxidize to CO₂.
  • Key intermediates: Molozonide → Criegee pair → ozonide/peroxide adduct; proton relays mediated by H₂O₂/H₂O set up the elimination.
  • No rearrangements: The pathway is polar/closed-shell—no carbocations, so no shifts.
  • Remember: Oxidative workup pushes aldehydes to acids; reductive workups stop at aldehydes.


Mechanism (9 Steps)

Class: Polar cycloaddition + peroxide-mediated oxidative cleavage.

Step 1: ozone engages the alkene to form the molozonide
Step 1 — [3+2] cycloaddition gives the molozonide.

Ozone adds across the C=C to furnish the 1,2,3-trioxolane (molozonide). Both new C–O bonds form in one polar step.

Step 2: molozonide fragments into a carbonyl and a Criegee carbonyl oxide
Step 2 — Retro-[3+2] collapse to a carbonyl + Criegee pair.

Lone pairs reorganise, cleaving the weak O–O bond and generating a neutral carbonyl plus a zwitterionic carbonyl oxide (Criegee) fragment.

Step 3: carbonyl oxide attacks the carbonyl to create a peroxy hemiacetal
Step 3 — Carbonyl oxide attacks the aldehyde, giving a peroxy hemiacetal.

The anionic oxygen attacks the electrophilic carbonyl carbon, closing into a peroxide-linked adduct.

Step 4: lone pairs form the ozonide
Step 4 — Lone pairs re-form the ozonide-like trioxolane.

Internal electron shifts give the classical ozonide, with separated positive and negative oxygen centres.

Step 5: electron dance collapses the peroxide bridge
Step 5 — The ozonide rearranges to position the peroxide for attack.

The ozonide remains intact but is now oriented so that the peroxide-rich workup can cleave the three-oxygen bridge.

Step 6: hydrogen peroxide and water coordinate the fragments
Step 6 — H₂O₂ and H₂O add across the fragments.

The ozonide encounters peroxide/water, opening to peroxidic adducts that set up the oxidative workup.

Step 7: proton transfers stage the leaving groups
Step 7 — Proton relays prepare the peroxide for departure.

Proton transfers shuffle charge so the leaving groups are activated for cleavage under the oxidative conditions.

Step 8: divergent acid/ketone paths driven by vinylic substitution
Step 8 — Decision tree: vinylic hydrogens give acids; fully substituted carbons give ketones.

Each carbon is evaluated dynamically. Under H₂O₂ workup, any aldehyde fragment is oxidized to a carboxylic acid, while fully substituted carbons remain as ketones. The overlays simply show the deprotonation/cleavage sequence that leads to those outcomes.

Step 9: final products drawn after redrawing the carbonyl geometry
Step 9 — Products redrawn with fresh 2D coordinates.

The code removes the appropriate protons/peroxides and redraws the carbonyl fragments so the final frame matches hand-drawn references.


Mechanistic Checklist

  • Concerted ozone [3+2] addition → molozonide → Criegee rearrangement.
  • Carbonyl oxide recombines; ozonide forms.
  • Hydrogen peroxide/water add, followed by proton shuttles.
  • Vinylic H? → acid path (water grabs the proton, O–O collapses).
  • No vinylic H? → ketone path (hydroperoxide leaves, C=O forms).
  • Final redraw ensures neat carbonyl geometry.


Worked Examples

Substrate: CCC1=CCCCC1
Substrate (ring opening)
Reagents: O3, H2O2
O3; H2O2, H2O
Products: chain di-carbonyl after ring opening
Ring opens to give an open-chain diacid under oxidative workup.
Substrate: CC\C=C\C(C)C
Acyclic alkene with two vinylic hydrogens.
Reagents: O3, H2O2
O3; H2O2, H2O
Products: two carboxylic acids
Both vinylic positions carry hydrogens → two carboxylic acids.
Substrate: CC\C(C)=C(/C)C(C)C
Fully substituted alkene (no vinylic hydrogens).
Reagents: O3, H2O2
O3; H2O2, H2O
Products: two ketones
Both vinylic carbons are quaternary → two ketones.


Multiple Alkenes & Selectivity

  • More electron-rich or less hindered alkenes are cleaved first. Cool temperatures and slow ozone delivery help target a single C=C.
  • Oxidative workups are unforgiving—any pendant aldehyde will continue on to a carboxylic acid.
  • Plan for CO₂ loss when terminal vinyl groups are present; capture the carbon fragment accordingly in syntheses.


Practical Tips & Pitfalls

  • Handle ozone safely. Use an ozone generator with proper destruct, keep runs cold, and flush apparatus thoroughly.
  • Quench oxidants. After the peroxide workup, reduce residual peroxides (e.g., sulfite/bisulfite) before concentration.
  • Beware over-oxidation. Sensitive functionality (allylic alcohols, sulfides) can be hit by ozone or H₂O₂.
  • Compare to reductive workup. Swapping H₂O₂ for reductive conditions (Me₂S, Zn/HOAc) stops at aldehydes—know which the problem expects.


Exam-Style Summary

O₃ cleaves the C=C via molozonide → ozonide chemistry. An oxidative H₂O₂/H₂O workup converts vinylic hydrogens into carboxylic acids while fully substituted carbons become ketones; terminal CH₂ units oxidize further to CO₂. The mechanism is closed-shell—no rearrangements—and the electron flow can be tracked step-by-step using the nine illustrated frames above.


Interactive Toolbox

  • Run custom substrates through the Mechanism Solver and export the nine-step sequence for your own notes.
  • Compare oxidative (H₂O₂) vs reductive (DMS) ozonolysis in the Reaction Solver.


FAQ / Exam Notes

  • Why acids vs ketones? Any vinylic hydrogen can be abstracted by water in Step 8, enabling carboxylate formation before the OOH leaves. Without that proton, the fragment stops at a ketone.
  • Where does CO₂ come from? Terminal vinyl CH₂ groups lose both hydrogens, and the carbon is oxidized completely by the peroxide.
  • Can I stop at aldehydes? Only under reductive workup (e.g., Me₂S). H₂O₂ pushes aldehydes further to acids.
  • Do I always get both fragments? Yes—the carbon skeleton splits at the original C=C. Rings open; alkenes embedded in complex frameworks fragment accordingly.

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