Due to angle strain, the bonds in three‐ and four‐membered carbon rings are weak. Because of these weak bonds, cyclopropane and cyclobutane undergo reactions that are atypical of alkanes. For example, cyclopropane reacts with halogens dissolved in carbon tetra‐chloride to form dihaloalkanes.

Under acidic conditions, epoxides open in an “SN1 like” fashion with the nucleophile attacking the more substituted end.

The mechanism is concerted, so the original cis stereochemistry is not changed. This leads to “two” epoxides. However, these two mirror images are actually identical due to the mirror plane of the cis geometry. It is a meso compound, so the final result is a single stereoisomer, but not a single enantiomer.

The Stereochemistry of Epoxide Reactions Both strong and weak nucleophiles open the epoxide ring by an opposite-side nucleophilic attack. This puts the nucleophile and the alkoxy group of opposite sides and as a result, trans or anti-products are always formed.

Epoxides are much more reactive than simple ethers due to ring strain. Nucleophiles attack the electrophilic C of the C-O bond causing it to break, resulting in ring opening. Opening the ring relieves the ring strain. The products are typically 2-substituted alcohols.

Like in other SN2 reactions, nucleophilic attack takes place from the backside, resulting in inversion at the electrophilic carbon. Probably the best way to depict the acid-catalyzed epoxide ring-opening reaction is as a hybrid, or cross, between an SN2 and SN1 mechanism.

The carbons in an epoxide group are very reactive electrophiles, due in large part to the fact that substantial ring strain is relieved when the ring opens upon nucleophilic attack. Both in the laboratory and in the cell, epoxides are usually formed by the oxidation of an alkene.