Dioxygen

    Dioxygen is obtained industrially by the liquefaction and fractional distillation of air, and is stored and transported as a liquid. Convenient laboratory preparations of O₂ are the electrolysis of aqueous alkali using Ni electrodes, and decomposition of H₂O₂ (equation 1.1). A mixture of KClO₃ and MnO₂ used to be sold as ‘oxygen mixture’ (equation 1.2) and the thermal decompositions of many other oxo salts (e.g. KNO₃, KMnO₄ and K₂S₂O₈) produce O₂.

         )         1.1(

                      (1.2(

    Dioxygen is a colourless gas, but condenses to a pale blue liquid or solid. and 1.13. In all phases, it is paramagnetic with a triplet ground state, i.e. the two unpaired electrons have the same spin, with the valence electron configuration being:

    In this state, O₂ is a powerful oxidizing agent  but, fortunately, the kinetic barrier is often high; if it were not, almost all organic chemistry would have to carried out in closed systems. However, asinglet state, O^2- , with a valence electron configuration of:

  lies only 95 kJ mol^-1 above the ground state. This excited state can be generated photochemically by irradiation of O₂ in the presence of an organic dye as sensitizer, or nonphotochemically by reactions such as 1.3.

                               (1.3)

    Singlet O₂ is short-lived, but extremely reactive, combining with many organic compounds, e.g. in  reaction 1.4, O^2- acts as a dienophile in a Diels–Alder reaction.

                                           (1.4)

      At high temperatures, O₂ combines with most elements, exceptions being the halogens and noble gases, and N₂ unless under special conditions. Reactions with the group 1 metals are of particular interest, oxides, peroxides, superoxides and suboxides being possible products. Bond lengths in O₂, [O₂]^-  and [O₂]^2-  are 121, 134 and 149pm , consistent with a weakening of the bond caused by increased occupation of the  π^* MOs.   The first ionization energy of O₂ is 1168 kJ mol^-1 and it may be oxidized by very powerful oxidizing agents such as  PtF6 (equation 1.5). The bond distance of 112pm in [O₂]⁺‏ is in keeping with the trend for O₂, [O₂]^- and [O₂]^2-. Other salts include [O₂]⁺‏‏[SbF₆]^- (made from irradiation of O₂ and F₂ in the presence of SbF₅, or from O₂F₂ and SbF₅) and [O₂]‏⁺‏[BF₄]^- (equation 1.6).

                                    (1.5)

             (1.6)

   The chemistry of O₂ is an enormous topic, and examples of its reactions can be found throughout this book.