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Epoxides are three-membered cyclic ethers. Epoxides can be synthesized by the oxidation of corresponding alkenes with peroxy acid.

Some Reactions of Epoxides
Epoxides can undergo acid or base-catalyzed ring-opening reactions. If the epoxide used in the reaction is symmetric, the product will be the same under both conditions. But the products will be different, if the epoxide is unsymmetric.
Under Acid-Catalyzed Conditions
Under acidic conditions, the nucleophilic attack is directed to the protonated epoxide ring. The nucleophile adds to the more highly substituted carbon in the epoxide ring. This is because the in the protonated epoxide, the positive charge is shared by the ring-carbon atoms attached to the oxygen. The positive charge is more stabilized when it is on the more highly substituted carbon (stronger electrophile), and thus the attack of the nucleophile is preferentially on the more substituted carbon under acidic conditions.
Under Basics Conditions
In a base-catalyzed ring opening of an epoxide, the nucleophilic attack is on the less substituted (less hindered) carbon atom in the epoxide ring. The mechanism is shown below.

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