Ch_322a_11.019 - Reactions of Epoxides The three-membered...

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Reactions of Epoxides The three-membered ring of epoxides is highly strained leading to much greater reactivity than observed with open-chain or unstrained cyclic ethers. Two general types of ring-opening reactions are observed. Acid-Catalyzed Hydrolysis of Epoxides C C O: : + H-O-H H + : C C O H : + H 2 O + fast C C O H : + + O H H : : water as nucleophile enhanced reactivity OH 2 C C anti ring-opening HO + OH C C HO -H +
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Base-Catalyzed Ring Opening of Epoxides Strong nucleophiles, such as alkoxides and hydroxide, open the strained epoxide in an S N 2 manner: C C O : : + O-R : : : OR - C O anti ring-opening : : : C - OR C C O : : : - RO-H C C HO OR + + R-O : : : -
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Regiochemistry of the Ring-Opening Reactions Nucleophilic Ring-Opening In reactions where there is little or no electrophilic assistance to the ring-opening reaction, as in the base-catalyzed reactions, the nucleophile attacks at the less substituted carbon . This reactivity is in accord with an S N 2 reaction.
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