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Oxidation of Alkenes

Key Concepts

    • The active site on an alkene molecule is the double bond.

      R-C=C-R'
      Active site (double bond) shown in red
      R and R' represent hydrogen (H) or alkyl groups (alkane chains)

    • Mild oxidizing conditions will not break the alkene's double bond.
      Cold, alkaline solution of KMnO4 represents mild oxidizing conditions.

    • Mild oxidizing conditions produce diols, so these reactions can be referred to as hydroxylation reactions.
      H
      |
      H
      |
      R- C - C -R'
      |
      OH
      |
      OH

      1,2-diols are also known as glycols.

    • Strong oxidizing conditions will break the alkene's double bond.
      Hot, concentrated KMnO4(aq) represents strong oxidizing conditions.

    • Strong oxidizing conditions produce alkanones (ketones) and/or alkanoic acids (caboxylic acids),
      unless a methylene, CH2 group is to be oxidized, then the products will include carbon dioxide gas and water.

      Alkene to be oxidized
      (R, R', R", R'" represent any alkyl groups)
      Product 1 Product 2
      R-CH=CH2 R-COOH
      alkanoic acid
      CO2 + H2O
      R-CH=CH-R' R-COOH
      alkanoic acid
      R'-COOH
      alkanoic acid
      R-CR''=CH2 R-CO-R''
      alkanone
      CO2 + H2O
      R-CR'=CH-R" R-CO-R'
      alkanone
      R"-COOH
      alkanoic acid
      R-CR'=CR"-R'" R-CO-R'
      alkanone
      R"-CO-R'"
      alkanone

    Mild Oxidizing Conditions Examples

    1. Hydroxylation of ethene (ethylene)

    Hydroxylation of ethene (ethylene), CH2=CH2, using cold, alkaline potassium permanganate solution.

    ethene
    (ethylene)
    intermediate
    (cyclic inorganic ester)
    ethane-1,2-diol
    (ethylene glycol)
    manganese dioxide
    (solid)
    H
    |
    H
    |
    H- C = C -H
    MnO4-

    25oC
    H
    |
    H
    |
    H- C - C -H
    |
    O
    |
    O
    \ /
    Mn
    // \
    O O
    OH-

    H
    |
    H
    |
    H- C - C -H
    |
    OH
    |
    OH
    + MnO2(s)
    colourless
    reactant
    purple
    reagent
    colourless
    organic product
    brown
    by-product

    Hydroxylation of but-2-ene (2-butene)

    Hydroxylation of but-2-ene (2-butene), CH3-CH=CH-CH3, using cold, alkaline potassium permanganate solution.

    but-2-ene
    (2-butene)
    intermediate
    butane-2,3-diol
    (2,3-butanediol)
    manganese dioxide
    (solid)
    H
    |
    H
    |
    H
    |
    H
    |
    H- C - C = C - C -H
    |
    H
    |
    H
    MnO4-

    25oC
    H
    |
    H
    |
    H
    |
    H
    |
    H- C - C - C - C -H
    |
    H
    |
    O
    |
    O
    |
    H
    \ /
    Mn
    // \
    O O
    OH-

    H
    |
    H
    |
    H
    |
    H
    |
    H- C - C - C - C -H
    |
    H
    |
    OH
    |
    OH
    |
    H
    + MnO2(s)
    colourless
    reactant
    purple
    reagent
    colourless
    organic product
    brown
    by-product

    Strong Oxidizing Conditions Examples

    Oxidation of ethene (ethylene)

    Oxidation of ethene (ethylene), CH2=CH2, using hot, concentrated potassium permanganate solution produces carbon dioxide and water.

    ethene
    (ethylene)
    intermediate
    (2 x methanoic acid)
    intermediate
    (2 x carbonic acid)
    carbon dioxide
    + water
    H
    |
      H
    |
     
    H- C = C -H
             
    conc. MnO4-

    heat
      H
    |
      H
    |
     
    HO- C   +   C -OH
      ||
    O
      ||
    O
     
    conc. MnO4-

    heat
      OH
    |
      OH
    |
     
    HO- C   +   C -OH
      ||
    O
      ||
    O
     
    conc. MnO4-

    heat
    CO2 + H2O
    colourless
    reactant
    purple
    reagent
            colourless
    products

    Oxidation of propene (propylene)

    Oxidation of propene (propylene), CH2=CH-CH3, using hot concentrated potassium permangante solution produces carbon dioxide, water and acetic acid (ethanoic acid).

    propene
    (propylene)
    intermediate
    (ethanal)
    + intermediate
    (methanal)
      H
    |
      H
    |
      H
    |
     
    H- C - C = C -H
      |
    H
             
    conc. MnO4-

    heat
      H
    |
      H
    |
     
    H- C - C =O
      |
    H
         
    +
      H
    |
     
    O= C -H
         
        MnO4-   MnO4-
        product: acetic acid
    (ethanoic acid)
      intermediate: formic acid
    (methanoic acid)
       
      H
    |
      OH
    |
     
    H- C - C =O
      |
    H
         
     
      OH
    |
     
    O= C- H
         
            MnO4-
            intermediate (carbonic acid)
           
      OH
    |
     
    O= C- OH
         
            MnO4-
            products (carbon dioxide and water)
    CO2 + H2O

    Oxidation of 2-methylpropene

    Oxidation of 2-methylpropene, CH3-C(CH3)=CH2, using hot, concentrated potassium permanganate solution produces acetone (propan-2-one), carbon dioxide and water.

    2-methylpropene   product: acetone
    (propanone)
    + intermediate: formaldehyde
    (methanal)
      H
    |
      CH3
    |
    H
    |
     
    H- C - C= C -H
      |
    H
       
    conc. MnO4-

    heat
      H
    |
      CH3
    |
    H- C - C=O
      |
    H
       
    +
      H
    |
     
    O= C -H
         
            MnO4-
        acetone (propanone) can also
    be written as:
      intermediate: formic acid
    (methanoic acid)
       
      O
    ||
       
    CH3- C - CH3
           
     
      OH
    |
     
    O= C- H
         
            MnO4-
            intermediate (carbonic acid)
           
      OH
    |
     
    O= C- OH
         
            MnO4-
            products: carbon dioxide and water
    CO2 + H2O


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