Oxidation of Alkenes – HSC Chemistry
This is an extension of the HSC Chemistry course under the topic of Products of Reactions Involving Hydrocarbons.
HSC Chemistry Syllabus
This topic is not explicitly outlined in the HSC Chemistry syllabus and may be assessed as an extension topic.
Oxidation of Alkenes
What Does Oxidation of Alkenes Produce?
When an alkene reacts with KMnO₄, the double bond (C=C) undergoes an electrophilic addition reaction. This reaction forms a short-lived cyclic intermediate with the MnO₄⁻ ion, which then breaks down to yield different products based on the reaction conditions.
Cold, Dilute KMnO₄ (Neutral or Alkaline Condition): Produces diols (1,2-diols) through dihydroxylation, adding two hydroxyl (OH) groups to the same side of the alkene.
Hot, Concentrated KMnO₄ (Acidic condition): Results in dihydroxylation and oxidative cleavage of C=C bond. The cleavage of C=C bond produces two alcohol molecules which are further oxidised to produce:
- Carboxylic acid and carbon dioxide for terminal alkenes.
- Two carboxylic acids for internal alkenes.
- A ketone and a carboxylic acid if an alkyl group (like a methyl group) is attached to the double-bonded carbon.
Example 1: Oxidation of Ethene
Consider ethene oxidised with hot concentrated KMnO₄:
- The two =CH₂ parts form methanoic acid (formic acid), which is oxidised further to produce carbon dioxide and water.
Example 2: Oxidation of Propene
Consider propene oxidised with hot concentrated KMnO₄:
- The CH₃-CH= part becomes acetic acid
- The =CH₂ part forms formic acid, which is oxidised further to produce carbon dioxide and water.
Example 3: Oxidation of 2-methylpropene
Consider 2-methylpropene oxidised with hot concentrated KMnO₄:
- The CH₃-(C H₃)CH= part becomes propanone (acetone)
- The =CH₂ part forms formic acid, which is oxidised further to produce carbon dioxide and water.