Ozonolysis (oxidative work-up): practice and mechanism
Ozone cleaves the double bond. With oxidative workup (H₂O₂), aldehydes are further oxidised to carboxylic acids; ketones stay ketones.
Example
Ozone cleaves the double bond. With H₂O₂ the work-up is oxidative: where the alkene carbon carried an H, a carboxylic acid forms instead of the aldehyde (=CH₂ becomes CO₂). Without an H a ketone forms.
Mechanism step by step
- 1
Ozone adds to the double bond to give the primary ozonide (1,3-dipolar cycloaddition).
- 2
The primary ozonide falls apart and recombines to the secondary ozonide; the C=C bond is now fully cleaved.
- 3
H₂O₂ cleaves the ozonide oxidatively: CH fragments become carboxylic acids, carbons with two groups become ketones.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Background
Oxidative or reductive workup
Ozonolysis itself gives an ozonide. The workup decides what comes out: reductively (dimethyl sulfide or zinc/acetic acid) aldehydes stay aldehydes. Oxidatively with hydrogen peroxide (H₂O₂) every aldehyde fragment is oxidised further to a carboxylic acid. Ketones stay ketones either way.
How to read off the product
Mentally cut the C=C double bond and put an =O on each carbon. If a carbon still carries an H, the oxidative workup turns it into a COOH group; if it carries two carbon groups it becomes a ketone. A terminal =CH₂ is broken down oxidatively all the way to CO₂.
Typical mistakes
Often confused with: Ozonolysis (reductive work-up)
Frequently asked questions
When aldehyde, when acid?
Reductive workup (Zn or (CH₃)₂S) stops at the aldehyde. Oxidative workup (H₂O₂) oxidises on to the carboxylic acid.
What happens to a =CH₂ end?
It becomes formaldehyde, or with oxidative workup formic acid and CO₂.
What is the oxidative workup in ozonolysis?
The ozonide is cleaved with H₂O₂, which oxidises any aldehydes that form to carboxylic acids; ketones are unchanged. With a reductive workup (Me₂S or Zn/AcOH) the aldehydes are kept.