Different reagents are used for alkene oxidation to 1,2-diol or vicinal diol (commonly called glycol) either in cis-form or trans-form. Following mechanisms will be discussed under this section;
- For cis-1,2-diol;
1. Osmium tetraoxide2. Potassium permanganate (cold dilute)
3. Iodine-silver acetate (wet)
- For trans-1,2-diol;
4. Iodine-silver acetate (dry), Prevost reaction5. Through epoxide
- Self assessment;
1. Osmium Tetraoxide
- An expensive and quite toxic reagent.
- More selective as compared to reactivity; according to general rule,
Reactivity is inversely proportional to the selectivity.
- Use in laboratory.
Mechanism:
(a) Reaction with OsO4 takes place in the presence of neutral conditions.
(b) Ether as solvent.
(c) Pyridine as catalyst and complexes osmium in the ester.
(d) Steps;
(I) Formation of osmate ester
(II) Hydrolysis with aqueous sodium sulfite i.e., Na2SO3 + H2O
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| OsO4 oxidation to 1,2 diol |
(e) Syn-stereosoecific
(f) Regioselective for more electron rich double bond, when more than one double bond is present.
(g) Attacks from less hindered side in rigid cyclic systems, thereby yielding the more stable of the two possible cis-diols.
Because of both its expense and toxicity, it is best to use osmium tetraoxide in catalytic quantities by carrying out the reaction in the presence of a;
- tertiary amine oxide
OR
- alkaline t-butyl hydroperoxide (hydrogen peroxide in t-butyl alcohol)
OR
- N-methylmorpholine-N-oxide (NMO)
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| OsO4 oxidation to diol |
2. Potassium Permanganate
- KMnO4 is cheaper (as compared to OsO4).
- Less hazardous to use
- More reactive and less selective. It also oxidizes other bonds in addition to the C-C double bond.
- Use in industry.
Mechanism:
(a) Reaction takes place with potassium permanganate in presence of basic conditions (alkaline KMnO4).
(b) In alkaline conditions, reaction stops at diol level and further oxidation (formation of hydroxyketone) is declined.
(c) Potassium permanganate acts similarly to osmium tetraoxide;
Steps;
(I) Formation of manganate ester
(II) Hydrolysis
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| Mechanism of KMnO4 oxidation to diol |
(d) This reaction is used as a test for the presence of a double bond (unsaturation) in an organic molecule and is known as Baeyer's test.
3. Iodine-Silver Acetate (wet)
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| wet method of iodine-silver acetate |
Following steps are involved in the mechanism;
- Formation of iodonium ion by reaction of iodine with double bond.
- Iodonium ion undergoes displacement by acetate in the SN2 manner, giving a trans-iodoacetate.
- Formation of cyclic acetoxonium ion.
- Reaction with water to give a cis-hydroxyacetate.
- Final hydrolysis gives the cis-diol.
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| Mechanism of wet method for diol formation |
4. Iodine-Silver Acetate (dry) OR Prevost Reaction
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| dry method (prevost reaction) of iodine-silver acetate |
The same above reaction carried out in the absence of water. In forth step, acetate ion reacts instead of water and gives trans ester (trans-1,2-diacetate). Reaction stops at this point then we work up by adding H+/H2O (hydrolysis). Hydrolysis gives the final trans-diol.
Steps can be arranged in following way;
- Formation of iodonium ion by reaction of iodine with double bond.
- Iodonium ion undergoes displacement by acetate in the SN2 manner, giving a trans-iodoacetate.
- Formation of cyclic acetoxonium ion.
- Reaction with acetate ion to form trans-ester.
- Final hydrolysis gives the trans-diol.
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| Mechanism of dry method (prevost reaction ) for diol formation |
5. Through Epoxide
When a nucleophile opens an epoxide, it generates an alcohol. If the nucleophile is water, the product is the diol. The two hydroxyl groups end up on opposite sides of the six membered ring; the product is anti-diol. The epoxide opening reaction can be done in acid or base.
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| diol formation from epoxide |
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Self Assessment:
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| Practice problem for alkene to 1,2 diol |
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