Page 228 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements
P. 228

GROUP 16 ELEMENTS: THE CHALCOGENS
                208
                An intramolecular attack by the enolate so produced on the hydroperoxide unit then pro-
                duces the final epoxide product.


                                                            O
                              O   O           O                      O    −
                                             −                         +  HO       (6.19)
                            H


                                                                      4+
                                                              −
                  The oxidation of H O by strong oxidants (Cl ,MnO , and Ce , among others) has
                                 2  2                  2     4
                been studied using labeled H  18 O . Analysis of the evolved oxygen proved that both oxy-
                                       2   2
                gen atoms in the O arise from H O and not from water. The same conclusion has also
                               2           2  2
                                                                        −
                been reached for catalytic decomposition of H O , for example, the I -catalyzed process
                                                     2  2
                described above. Let us consider the mechanism for H O oxidation with Cl as the oxidant:
                                                          2
                                                                           2
                                                            2
                                                              +      −
                                 H O + Cl + 2H O → O + 2H O + 2Cl                  (6.20)
                                  2
                                         2
                                     2
                                                            3
                                                      2
                                                2
                Chlorine in water exists in equilibrium with hypochlorous acid, as shown below:
                                  H                     H
                                                               +   −
                                     O +  Cl  Cl          O +  H +  Cl             (6.21)
                                   H                    Cl
                Hydrogen peroxide could then do a nucleophilic attack on either Cl or HOCl to produce
                                                                      2
                HOOCl:
                                      H                           H
                                                                         −
                             18 O  18 O  Cl   Cl          18 O  18 O +  +  Cl
                             H                           H        Cl
                                              H
                                                                                   (6.22)
                                            O
                                       H              +
                                              H  − H 3 O   18 O  18 O
                               18   18 O +
                                O
                                                          H        Cl
                              H        Cl
                Water, acting as the base, might then be expected to decompose the highly unstable inter-
                mediate HOOCl via an E2 elimination:
                                               Cl
                                                     +
                                       18  18                         −
                                H       O    O   − H 3 O
                                                          18 O  18 O  +  Cl
                                      H                                            (6.23)
                                  O
                                H
                In this reaction, the dioxygen is initially produced in the singlet state, which rapidly decays
                to the triplet ground state. It’s worth noting that only the singlet state can be conveniently
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