Chapter 11.8: Synthesis of Epoxides

Question 1

Describe the mechanism of epoxidation of an alkene by RCO3H (peroxycarboxylic acids)

Answer 1

 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)

Question 2

How is ethylene oxide prepared?

Answer 2

o Ethylene oxide is prepared by passing a mixture of ethylene and air (or oxygen) over a silver catalyst

Question 3

Describe synthesis of epoxides using internal nucleophilic substitution in halohydrins

Answer 3

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

Question 4

What is the stereochemistry of preparing an epoxide using internal nucleophilic substitution in halohydrins?

Answer 4

inversion