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In this video lecture, we will discuss one of the most important topics of organic chemistry hydroxylation of alkenes

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September 6, 2020

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Hydroxylation of Alkenes


Reaction Type : Electrophilic Addition


Summary

  • Typical regants :  OsO4 / tBuOOH / HO-  or  KMnO4 / NaOH / 0 oC
  • With osmium tetroxide, OsO4, is used catalyically and the peroxide is a co-oxidant.
  • Potassium permanganate, KMnO4 which turns from purple to colourless during the reaction can be used as a simple functional group test.
  • During these reactions, the two oxygen atoms are transferred from the same species at the same time.
  • Both sets of reagents form 5 membered cyclic ester intermediates.
  • As a result, the stereochemistry is syn


MECHANISM FOR REACTION OF ALKENES WITH OSMIUM TETROXIDE


Step 1:
The p electrons in the alkene act as a nucleophile forming a favourable 5 membered ring as a cyclic osmate ester as an intermediate. Note the origin of the cis stereochemistry.


Step 2:
The hydroxide liberates the cis-diol and the reduced osmium species.... this would then be reoxidised by the peroxide co-oxidant.


Dihydroxylation of alkenes with cold, dilute KMnO4 to give vicinal diols

Description: Treatment of alkenes with cold, dilute basic KMnO4 leads to 1,2-diols (vicinal diols).


Notes:

  • The reaction proceeds with “syn” stereochemistry of the alkene, meaning that the two alcohols end up on the same side of the alkene
  • Also note that osmium tetroxide (OsO4) does exactly the same reaction.
  • The purpose of the NaOH is to assist in breaking up the intermediate manganate ester that forms after dihydroxylation. If this manganate ester is allowed to sit around, oxidative cleavage of the diol may occur. This is also why the temperature is kept cold.


Notes: The reaction works well so long as it is kept cold. If higher temperatures are used, cleavage of the diol to give carbonyl compounds is observed. Note that the reaction does not occur with alkynes.


“Good yields of cis-l,2-cyclohexanediol were formed by the reaction of cyclohexene and aqueous potassium permanganate under turbulent stirring conditions only in the presence of low concentrations of sodium hydroxide. Larger amounts had no further benefit or were deleterious.”



Also, apparently under acidic or neutral conditions, more highly oxidized products such as alpha-hydroxy ketones can be formed “without going through the glycols”

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