Glycols (also called 1,2 diols) are most commonly prepared by the reaction of alkenes with osmium tetroxide (OsO4) in the presence of aqueous sodium bisulfite (NaHSO3), hydrogen peroxide (H2O2) or tert-butylhydroperoxide ((CH3)3COOH). OsO4 is expensive and toxic, so variations of the reaction requiring only catalytic amounts of this reagent are commonly used. The OsO4 delivers both oxygen atoms to the same face of the alkene (syn addition), so a cis-glycol results (Section 6.4).
Hydrolysis of an epoxide affords a trans-glycol. In this reaction, a molecule of water attacks the back side of a carbon-oxygen bond of an epoxide, affording a trans glycol. Acid catalysis is necesary to open the epoxide. If considered as a two-step sequence from alkene to glycol (alkene to epoxide to glycol), this is an anti addition (Section 8.7).
