Chapter 15: Alcohols, Diols and Thiols

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. This is shown by the following example:

QUESTIONS
What other reaction is commonly used as a functional group test for alkenes ? ANSWER
What is the oxidation state of the osmium in osmium tetroxide ? ANSWER
Is the osmium oxidised or reduced during the formation of the osmate ester ? ANSWER
What is the oxidation state of the potassium permanganate ? ANSWER

Related reactions

• Addition reactions of alkenes (chapter 6)
• Hydrolysis of Epoxides

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.

© Dr. Ian Hunt, Department of Chemistry