Osmylation-OsO4 (Osmium Tetroxide)

Key Questions

  • Osmium tetroxide is an oxide of osmium.

    It has many uses, despite the fact that the abundance of Os in the earth's crust is only 1.5 ppb by mass.


    OsO₄ is colourless and has a chlorine-like odour. Most samples appear yellowish because of contamination by yellow-brown OsO₂.

    OsO₄ is tetrahedral and nonpolar.


    Some of its physical properties are:

    • melting point = 40°C
    • sublimes at room temperature
    • boiling point = 130°C
    • solubility in water = 6.2 g/100 mL
    • solubility in CCl₄ = 375 g/100 mL

    The most common use of OsO₄ in organic chemistry is to convert alkenes to vic-diols.

    The mechanism involves a concerted cis addition to form a cyclic osmate ester, which then hydrolyzes to form the diol.


    OsO₄ is usually used in small amounts as a catalyst. Reactants such as H₂O₂ are added to regenerate the OsO₄.

    OsO₄ is expensive (over $200/g) and highly toxic. The permissible exposure limit is only 2 µg/m³ over 8 h.

    OsO₄ can even diffuse through plastic, so it must be kept in glass containers and in a fume cupboard.

  • it is used to make alkenes in to 1,2-diols.

    Refer to this site:masterorganicchemistry

  • OsO₄ is a catalyst. MnO₄⁻ is a reactant.



    1. The OsO₄ is a catalyst. It reacts with the π electrons of the alkene in a syn addition to form a cyclic osmate ester.

    2. The OH⁻ hydrolyzes the ester. This forms the cis-diol and H₂OsO₄.

    3. The t-BuOOH oxidizes the H₂OsO₄ and regenerates the OsO₄ catalyst:

    t-BuOOH + H₂OsO₄ → t-BuOH + OsO₄ + H₂O

    #"KMnO"_4"/NaOH/0°C Reaction:"#


    As with OsO₄, the reaction goes through a cyclic ester to form a cis diol.

    In this case, the MnO₄⁻ is a reactant, not a catalyst, because the MnO₄⁻ is not regenerated.