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Epoxides are three-membered cyclic ethers whose ring strain contributes to their electrophilic character. Their formation includes alkene epoxidation and intramolecular halohydrin cyclization. Ring-opening selectivity depends on conditions: acidic conditions favor attack at the more substituted carbon, while basic or neutral conditions favor attack at the less substituted carbon. These reactions support the synthesis of compounds such as 1,2-diols and amino-alcohols. Thiols are more acidic than alcohols and form nucleophilic thiolates; oxidation links thiols with disulfides and other sulfur oxidation states. Sulfides can be oxidized to sulfoxides and sulfones, while alkylation forms sulfonium ions. The topic also connects these reactions to protein disulfide bridges, glutathione redox buffering, and the potential toxicity of some epoxide intermediates.