Hydrogen peroxide, H₂O₂

The oldest method for the preparation of H₂O₂ is reaction 1.1. The hydrolysis of peroxodisulfate (produced by electrolytic oxidation of [HSO₄]^- at high current densities using Pt electrodes) has also been an important route to H₂O₂ (equation 1.2).

                                                                       (1.1)

        (1.2)

     Nowadays, H₂O₂ is manufactured by the oxidation of 2- ethylanthraquinol (or a related alkyl derivative). The H₂O₂ formed is extracted into water and the organic product is reduced back to starting material; the process is summarized in the catalytic cycle in Figure 1.1.

Fig. 1.1 The catalytic cycle used in the industrial manufacture of hydrogen peroxide; O₂ is converted to H₂O₂ during the oxidation of the organic alkylanthraquinol. The organic product is reduced by H₂ in a Pd- or Ni-catalysed reaction.

    Some physical properties of H₂O₂ ; like water, it is strongly hydrogen-bonded. Pure or strongly concentrated aqueous solutions of H₂O₂ readily decompose (equation 1.3) in the presence of alkali, heavy metal ions or heterogeneous catalysts (e.g. Pt or MnO₂), and traces of complexing agents  or adsorbing materials (e.g. sodium stannate, Na₂[Sn(OH)₆]) are often added as stabilizers.

      (1.3)

(1.1)

Mixtures of H₂O₂ and organic or other readily oxidized materials are dangerously explosive; H₂O₂ mixed with hydrazine has been used as a rocket propellant. A major application of H₂O₂ is in the paper and pulp industry where it is replacing chlorine as a bleaching agent. Other uses are as an antiseptic, in water pollution control and for the manufacture of sodium peroxoborate and peroxocarbonates. Figure 1.1 shows the gas-phase structure of H₂O₂ and bond parameters are listed in Table 1.1.

Table 1.1 Selected properties of H₂O₂.

    The internal dihedral angle is sensitive to the surroundings (i.e. the extent of hydrogen bonding) being 518 in the gas phase, 908 in the solid state and 1808 in the adduct Na₂C₂O₄.H₂O₂. In this

last example, H₂O₂ has a trans-planar conformation and the O lone pairs appear to interact with the Na‏ ions. Values of the dihedral angle in organic peroxides, ROOR, show wide variations (≈ 808–858).  In aqueous solution, H₂O₂ is partially ionized (equation 1.4), and in alkaline solution, is present as the [HO₂]^- ion.

         )1.4(

Fig. 1.1 The gas-phase structure of H2O2 showing the oxygen atom lone pairs. The angle shown as 518 is the internal dihedral angle, the angle between the planes containing each OOH-unit; see Table 1.1 for other bond parameters.

Hydrogen peroxide is a powerful oxidizing agent as is seen from the standard reduction potential (at pH = 0) in equation 1.5; e.g. it oxidizes I^- to I₂, SO₂ to H₂SO₄ and (in alkaline solution) Cr(III) to Cr(VI). Powerful oxidants such as [MnO₄]^- and Cl₂ will oxidize H₂O₂ (equations

1.6–1.8), and in alkaline solution, H₂O₂ is a good reducing agent (half-equation 1.9).

                (1.5(

            (1.6(

           (1.7(

                                                          (1.8)

      (1.9)

Tracer studies using ¹⁸O show that in these redox reactions H₂(¹⁸O)₂ is converted to(¹⁸O)₂, confirming that no oxygen from the solvent (which is not labelled) is incorporated and the O^_O bond is not broken.

Deprotonation of H₂O₂ gives [OOH]^- and loss of a second proton yields the peroxide ion, [O₂]^2- . In addition to peroxide salts such as those of the alkali metals, many peroxo complexes are known. Figure 1.2 shows two such complexes, one of which also contains the [OOH]^- ion in a bridging mode; typical O^_O bond distances

Fig. 1.2 The structures (X-ray diffraction) of (a) [V(O₂(₂ )O) bpy)]^- in the hydrated ammonium salt [H. Szentivanyi et al. (1983) Acta Chem. Scand., Ser. A, vol. 37, p. 553] and (b) [Mo₂)O₂(₄)O(₂)µ-OOH(₂]^2- in the pyridinium salt [J.-M.  Le Carpentier et al. (1972) Acta Crystallogr., Sect. B, vol. 28, p. 688]. The H atoms in the second structure were not located but have been added here for clarity. Colour code: V, yellow; Mo, dark blue; O, red; N, light blue; C, grey; H white.