Ozonolysis of Alkenes: Reductive vs Oxidative
Ozonolysis is an organic reaction where ozone ($O_3$) is used to break the carbon-carbon double bond ($C=C$) of an alkene. The reaction proceeds through a cyclic intermediate called an Ozonide. The final product depends entirely on the "workup" conditions: Reductive or Oxidative.
1. General Mechanism
The reaction occurs in two steps:
- Ozonide Formation: Ozone adds to the alkene to form a Molozonide (unstable), which rearranges to a stable Ozonide (1,2,4-trioxolane).
- Cleavage (Workup): The ozonide is cleaved by a reducing or oxidizing agent.
2. Reductive Ozonolysis
This is the most common form. It cleaves the double bond to form Aldehydes and/or Ketones.
Reagents for Workup
- Zinc dust and Water ($Zn/H_2O$)
- Dimethyl Sulfide ($Me_2S$ or DMS)
These reagents act as reducing agents to scavenge the extra oxygen atom, preventing the oxidation of aldehydes.
The "Trick":
Simply break the double bond and add an $=O$ to each carbon.
$$ R-CH=C(R')_2 \xrightarrow{1. O_3, 2. Zn/H_2O} R-CHO + O=C(R')_2 $$
Examples (Reductive):
- 2-Butene: $CH_3-CH=CH-CH_3 \to 2 \times CH_3CHO$ (Acetaldehyde)
- 2-Methyl-2-butene: $CH_3-C(CH_3)=CH-CH_3 \to CH_3-CO-CH_3$ (Acetone) + $CH_3CHO$ (Acetaldehyde)
3. Oxidative Ozonolysis
In this variation, the ozonide is cleaved in the presence of an oxidizing agent.
Reagents for Workup
- Hydrogen Peroxide ($H_2O_2$)
- Water ($H_2O$) alone (Less common, usually implies oxidative)
Product Rules (Oxidative)
- Ketones remain Ketones: The $C=O$ bond is stable to $H_2O_2$.
- Aldehydes oxidize to Carboxylic Acids: The $H$ attached to the carbonyl carbon is oxidized to $-OH$.
- Terminal Alkenes ($=CH_2$) become $CO_2 + H_2O$: The intermediate Formaldehyde/Formic acid oxidizes further to Carbon Dioxide.
Examples (Oxidative):
- 2-Butene: $CH_3-CH=CH-CH_3 \to 2 \times CH_3COOH$ (Acetic Acid)
- 2-Methyl-2-butene: $CH_3-C(CH_3)=CH-CH_3 \to CH_3-CO-CH_3$ (Acetone) + $CH_3COOH$ (Acetic Acid)
- 1-Butene: $CH_3-CH_2-CH=CH_2 \to CH_3-CH_2-COOH$ (Propanoic Acid) + $CO_2$
4. Summary Comparison
| Alkene Structure | Reductive ($Zn$) | Oxidative ($H_2O_2$) |
|---|---|---|
| $R-CH=CH-R$ | 2 Aldehydes | 2 Carboxylic Acids |
| $R_2C=CH-R$ | Ketone + Aldehyde | Ketone + Carboxylic Acid |
| $R_2C=CR_2$ | 2 Ketones | 2 Ketones |
| $R-CH=CH_2$ | Aldehyde + HCHO | Acid + $CO_2$ |
5. Reverse Ozonolysis
To find the structure of the original alkene from the products:
1. Draw the structures of the products.
2. Remove the oxygen atoms.
3. Join the carbonyl carbons with a double bond.
Ozonolysis Quiz
Test your concepts on Alkene cleavage. 10 MCQs with explanations.
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