While hydrogen peroxide is a ubiquitous chemical routinely used in oxidative chemistry, it is purchased as an aqueous solution.[1] This is due to the inherent dangers associated with its concentrated form. In addition, introduction of large amounts of water as part of the introduction of aqueous hydrogen peroxide solution into organic reactions can pose polarity issues leading to the formation of unreactive biphasic mixtures, as well as adventitious side-reactions. Furthermore, hydrogen peroxide solutions degrade over time, introducing the need for titrimetric methods to determine reagent concentrations before use. Crystalline peroxosolvates have gained attention as more practical sources of hydrogen peroxide and are thus worth noting.
The first and most common solvates, sodium percarbonate and H2O2-urea, were reported by Tanatar in 1899 and 1906, respectively.[2] Since then, nearly 100 examples of crystalline peroxosolvates have been added into the Cambridge Structure Database.[3] A series of H2O2-tertiary phosphine oxide adducts were reported in 2012.[4a] These are noteworthy due to their mechanical and thermal stabilities, as well as solubilities in organic solvents, all of which make them ideal candidates as practical reagents for the laboratory.
Tertiary phosphine oxide peroxosolvates fall under two categories depending on the nature of the peroxide: Hilliard adducts of the form (R3PO-H2O2)2 or Ahn adducts of the form R3PO-(HOO)2CR’R". The procedure below is given for the triphenylphosphine oxide analog,[4] but the methodology is applicable to other alkyl and aryl derivatives.
Procedure:[4] Although the following compounds were shown to react non-explosively, excess caution should always be taken when working with hydrogen peroxide and its derivatives!
Synthesis of Hilliard adduct, (Ph3PO-H2O2)2:[4a] A Schlenk flask with stir bar was charged with triphenylphosphine (5g, 19.1mmol), anhydrous dichloromethane (150mL), and the system was placed under argon. At room temperature, aqueous 35% H2O2 (18.5mL, 191mmol) was added dropwise via syringe, and the mixture was stirred overnight. The bottom DCM layer was then transferred into a separate round-bottom flask open to air via metal cannula. The aqueous layer was extracted with DCM (3 x 20mL) and the combined organic portions were combined and evaporated in vacuo at room temperature. Product was obtained as a white crystalline solid. Product should be stored in the freezer.
Synthesis of Ahn adduct, Ph3PO-(HOO)2CMe2:[4b] A Schlenk flask with stir bar was charged with triphenylphosphine (0.200g, 0.762mmol), anhydrous acetone (100mL), and the system was placed under argon. At room temperature, aqueous 35% H2O2 (1mL, 10mmol) was added dropwise via syringe, and the solution stirred for 3h. The solvent was concentrated to ca. 5mL in vacuo, and the product was allowed to crystallize upon standing in air. Product was obtained as a white crystalline solid after filtration under suction. Product should be stored in the freezer.
Notes:
- Aqueous H2O2 decomposes over time upon contact with glass, which explains why vendors sell it in plastic bottles. Silica also decomposes H2O2.
- Curious note: According to Ullmann’s Encyclopedia, 100% pure H2O2 can be obtained via fractional distillation from its highly-concentrated aqueous solutions. It is reported to be stable at room temperature, but forms explosive vapor mixtures upon heating. A lot of the dangers of concentrated hydrogen peroxide stem in part due to its ability to form explosive peroxides or rapidly decompose when in contact with organics, alkaline substances, and metals.[1]
- H2O2-urea is a popular, commercially available peroxosolvate that has found use in organic reactions, but has low solubility in organic solvents.[5] It is hygroscopic and should be stored in the fridge to minimize its degradation long-term. The compound is easily synthesized by recrystallizing urea from aqueous H2O2.[6]
- There is an account of a hydrogen peroxide - urea explosion after long-term storage, but the details are lacking. “The contents of a screw-capped brown glass bottle spontaneously erupted after 4 years’ storage at ambient temperature.” The reference also describes explosive outcomes for potassium citrate tri(hydrogen peroxide), and DABCO-hydrogen peroxide.[7]
- Treatment of tertiary phosphine oxide peroxosolvates with molecular sieves destroys the H2O2, returning the unbound phosphine oxide.
- The Hilliard adducts synthesized via the current procedure are reported to be sensitive to vacuum, gradually losing the H2O2 moiety upon prolonged exposure.
- Me3PO and nBu3PO are hygroscopic, Cy3PO and Ph3PO are not.
- (2Ph3PO-H2O2)2 is soluble in dioxane, chloroform, acetone, DMF, methanol. It is sparingly soluble in benzene, diethyl ether, chlorobenzene. It is insoluble in water, cyclohexane, pentane.[8]
- Ph3PO-(HOO)2CMe2 is soluble in chloroform, DCM, polar organic solvents, aryl hydrocarbons, ethanol, and insoluble in water, and hexanes.
[1] Goor, G.; Glenneberg, J.; Jacobi, S.; Dadabhoy, J.; Candido, E. Hydrogen Peroxide. Ullmann’s Encyclopedia of Industrial Chemistry. 2019, 1-31.
[2] a) Tanatar, S. J. Percarbonate. Ber. Dtsch. Chem. Ges. 1899, 32, 1544– 1546. b) Tanatar, S. J. Russ. Phys. Chem. Soc. 1906, 40L, 376– 380.
[3] Chernyshov, I. Y.; Vener, M. V.; Prikhodchenko, P. V.; Medvedev, A. G.; Lev, O.; Churalov, A. V. Cryst. Growth Des. 2017, 17, 1, 214-220.
[4] a) Hilliard, C. R.; Bhuvanesh, N.; Gladysz, J. A.; Blümel, J. Synthesis, purification, and characterization of phosphine oxides and their hydrogen peroxide adducts. Dalton Trans. 2012, 41, 1742-1754. b) Ahn, S. H.; Cluff, K. J.; Bhuvanesh, N.; Blümel, J. Hydrogen Peroxide and Di(hydropexory)propane Adducts of Phosphine Oxides as Stoichiometric and Soluble Oxidizing Agents. Angew. Chem. Int. Ed. 2015, 54, 13341-13345.
[5] Heaney, H.; Cardona, F.; Goti, A.; Frederick, A. L. Hydrogen Peroxide - Urea. EROS, 2013. DOI: 10.1002/047084289x.rh047.pub3.
[6] Lu, C.-S.; Hughes, E. W.; Giguere, P. A. J. Am. Chem. Soc. 1941, 63, 1507-1513.
[7] Bretherick’s Handbook of Reactive Chemical Hazards : An Indexed Guide to Published Data. 7th ed. Elsevier Ltd. 2007, 213, 814.
[8] Temple, R. D.; Tsuno, Y.; Leffler, J. E. Triphenylphosphine Oxide - Hydrogen Peroxide Adduct. J. Org. Chem. 1963, 28, 9, 2495.