Question

Normally RO– is not a good leaving group. Why are epoxides so reactive? What pKa would you quote to indicate that you know RO– is a bad leaving group.

Answer 1

Epoxides are so reactive due to the ring strain imposed by three membered cyclic C-O-C bond angles. This is relieved when the ring opens upon nucleophilic attack, and the resulting molecule is at a lower energy state. Moreover protonating the epoxide with very strong acid allows the oxygen atom to create a nucleophilic center.

good leaving group is always the conjugate base of a strong acid. ROH is not a strong acid, hence its conjugate base is a bad leaving group. For example Alcohols are weak acids, even weaker than water. Ethanol has a pKa of 15.9 compared to water's pKa of 15.7. Hence a pKa of 16 to indicate that RO– is a bad leaving group.

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