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Oxidation of Alkenes Under Mild Oxidizing Conditions Examples
Mild oxidising conditions are present when the oxidising agent is a cold (room temperature), alkaline solution (OH-(aq)) of potassium permanganate (KMnO4)
The mild oxidation of alk-1-enes (1-alkenes) produces an alkane-1,2-diol (often just referred to as a diol).
Examples of the mild oxidation of alk-1-enes include:
- ethene (ethylene) to ethane-1,2-diol (ethylene glycol)
- but-1-ene (1-butene) to butane-1,2-diol (1,2-butanediol)
The mild oxidation of alk-2-enes (2-alkenes) produces an alkane-2,3-diol (2,3-alkanediol).
Examples of the mild oxidation of alk-2-enes includes:
- but-2-ene (2-butene) to butane-2,3-diol (2,3-butanediol)
- pent-2-ene (2-pentene) to pentane-2,3-diol (2,3-pentanediol)
Because the mild oxidation of alkenes results in two hydroxyl (OH) groups adding across the double bond, these reactions are also commonly referred to as hydroxylation reactions.
| alkene |
mild oxidation
→ |
alkanediol |
|
|
MnO4-/OH-(aq)
→
cold |
| |
H
| |
H
| |
|
| R− |
C− |
C− |
R′ |
| |
|
OH |
|
OH |
|
|
Example of Mild Oxidation of Alkene: Hydroxylation of ethene (ethylene)
Hydroxylation of ethene (ethylene), CH2=CH2, using cold, alkaline potassium permanganate solution produces ethane-1,2-diol (1,2-ethanediol or ethylene glycol).
This synthesis of ethane-1,2,-diol from ethene is shown below:
ethene
(ethylene) |
|
intermediate
(cyclic inorganic ester) |
|
ethane-1,2-diol
(ethylene glycol) |
manganese dioxide
(solid) |
|
|
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 |
Example of Mild Oxidation of Alkene: Hydroxylation of but-2-ene (2-butene)
Hydroxylation of but-2-ene (2-butene), CH3-CH=CH-CH3, using cold, alkaline potassium permanganate solution produces butane-2,3-diol (2,3-butanediol).
This synthesis of butane-2,3-diol from but-2-ene is shown below:
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 |
Oxidation of Alkenes Under Strong Oxidizing Conditions
Strong oxidising conditions are represented by the use of hot, concentrated potassium permangante solution (KMnO4(aq)).
Under these conditions, the double bond (C=C) in the alkene breaks.
The products of the strong oxidation of alkenes can include carbon dioxide, water, alkanones (ketones) and alkanoic acids (carboxylic acids) depending on the location of the double bond within the alkene molecule.
The examples below shown the strong oxidation of
- ethene (ethylene) to carbon dioxide (CO2) and water (H2O)
- propene (propylene) to acetic acid (ethanoic acid), carbon dioxide and water
- 2-methylpropene (2-methylpropylene) to acetone (propan-2-one), carbon dioxide and water
Example of Strong Oxidation of Alkene: Strong Oxidation of Ethene (ethylene)
Oxidation of ethene (ethylene), CH2=CH2, using hot, concentrated potassium permanganate solution produces carbon dioxide and water.
The synthesis of carbon dioxide and water from the strong oxidation of ethene is shown below:
ethene
(ethylene) |
|
intermediate
(2 × formic acid)
(2 × methanoic acid) |
|
intermediate
(2 × carbonic acid) |
|
carbon dioxide
+ water |
|
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 |
Example of Strong Oxidation of Alkene: Strong 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).
This synthesis of acetic acid (ethanoic acid) from propene (propylene) is shown below:
propene
(propylene) |
|
intermediate
(acetaldehye)
(ethanal) |
+ |
intermediate
(formaldehyde)
(methanal) |
| |
H
| |
|
H
| |
|
H
| |
|
| H− |
C |
− |
C |
= |
C |
−H |
| |
|
H |
|
|
|
|
|
|
conc. MnO4-
→
heat |
|
+ |
|
| |
|
MnO4-↓ |
|
↓MnO4- |
| |
|
product: acetic acid
(ethanoic acid) |
|
intermediate: formic acid
(methanoic acid) |
| |
|
|
|
|
| |
|
|
|
↓MnO4- |
| |
|
|
|
intermediate (carbonic acid) |
| |
|
|
|
|
| |
|
|
|
↓MnO4- |
| |
|
|
|
products (carbon dioxide and water)
CO2 + H2O |
Example of Strong Oxidation of Alkene: Strong 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.
This synthesis of acetone (propan-2-one) from 2-methylpropene is shown below:
| 2-methylpropene |
|
product: acetone
(propanone) |
+ |
intermediate: formaldehyde
(methanal) |
| |
H
| |
|
CH3
| |
H
| |
|
| H− |
C |
− |
C= |
C |
−H |
| |
|
H |
|
|
|
conc. MnO4-
→
heat |
|
+ |
|
| |
|
|
|
↓ MnO4- |
| |
|
acetone (propanone) can also
be written as: |
|
intermediate: formic acid
(methanoic acid) |
| |
|
|
|
|
| |
|
|
|
↓ MnO4- |
| |
|
|
|
intermediate (carbonic acid) |
| |
|
|
|
|
| |
|
|
|
↓ MnO4- |
| |
|
|
|
products: carbon dioxide and water
CO2 + H2O |
