Epoxide Ring Opening


Epoxides have considerable ring strain. For that reason, epoxides will undergo ring-opening reactions. Such reactions are exothermic. Epoxide ring opening may be effected by reaction with a strong nucleophile or with an acid.

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Ring Opening with an Acid

The Way Taught in CHEM 2127


As taught in CHEM 2127, acid-catalyzed epoxide opening follows an SN1 mechanism. The ring opens so as to form the most stable carbocation, and stereochemistry is lost at the carbocation, since the ion is trigonal planar.

The Textbook Mechanism


The textbook mechanism is a sort of hybrid of the SN1 and SN2 mechanisms. The reaction occurs by a backside attack of the nucleophile, but it is supposed that a transition state exists in which part of the SN1 bond-breaking has already happened, that is, in which partial charges have already appeared. The most stable partial carbocation is formed, but the nucleophilic attack on that partial ion is then a backside attack a la SN2. The key difference between this and the classroom mechanism is that in ths mechanism stereochemistry is inverted about the substitution carbon, rather than lost.

Ring Opening with a Strong Nucleophile

When an epoxide reacts with a strong nucleophile to open its ring, the reaction proceeds by an SN2 mechanism. The nucleophile attacks the least hindered of the ring carbons, and the ring oxygen serves as the leaving group. The reaction finishes with the protonation of the negatively charged oxygen. An example follows.


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