Chapter 11.8: Synthesis of Epoxides
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Describe the mechanism of epoxidation of an alkene by RCO3H (peroxycarboxylic acids)
The bond-making and bond-breaking steps are thought to be concerted or nearly so The concerted formation of two C-O bonds of the epoxide ensures that the reaction is stereospecific (cis-alkenes give cis-epoxides and trans-alkenes give trans-epoxides)
How is ethylene oxide prepared?
o Ethylene oxide is prepared by passing a mixture of ethylene and air (or oxygen) over a silver catalyst
Describe synthesis of epoxides using internal nucleophilic substitution in halohydrins
o One method for the preparation of epoxides from alkenes involves 1) treating the alkene with chlorine or bromine in water to form a chlorohydrin (or bromohydrin) followed by 2) treating the halohydrin with a base to bring about intramolecular displacement of Cl- • An internal Sn2 reaction • Causes inversion of configuration at the site of substitution o Mechanism: Step 1: Take a proton away • Gives an antiperiplanar transition state Step 2: Make a bond in the same molecule
What is the stereochemistry of preparing an epoxide using internal nucleophilic substitution in halohydrins?
inversion