Page 227 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements
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6.2 THE DIVALENT STATE: HYDROGEN PEROXIDE  207

                                     I
                                               − H 2 O  −
                                      O   O             I  +  O  O
                                                                                  (6.14)
                                            H     −
                                                  OH

                  This reaction is the basis of the so-called elephant toothpaste demonstration. In a typical
               experiment, a 2 M KI solution (say, 100 ml) is poured into a graduated cylinder containing
               30% H O (20 ml), dishwashing liquid (about 5 ml), and some food coloring, the latter two
                     2  2
               only for dramatic effect. The O produced gets trapped in the colored, soapy water, creating
                                        2
               a foamy column—the “elephant toothpaste”—that quickly fills the graduated cylinder and
               spills out. (Caution. Thirty percent H O is severely corrosive to the skin, respiratory tract,
                                             2  2
               and eyes. Please therefore consult a full set of instructions before attempting this experiment
               yourself; this brief description is not intended as a laboratory procedure!)
                  Some molecular iodine is also produced in this experiment, presumably through the
                         −
               reaction of I and HOI:
                                           O                     −
                                   −                                              (6.15)
                                        I      H         I    I  +  OH
                                   I
                                                              −
               Excess iodide converts the I produced to yellow-brown I :
                                      2                      3
                                                             −
                                     −     I   I         I   I   I                (6.16)
                                      I

                  Alkaline hydrogen peroxide is widely used for the epoxidation of   ,  -unsaturated car-
               bonyl compounds, as shown by the following example:



                                                         O
                                          O   H 2 O 2             O   −
                                                                   +  HO          (6.17)
                                              NaOH



               In the first step of the mechanism, H O , or rather its deprotonated form, the hydroperoxide
                                            2
                                              2
                         −
               anion, HOO , acts as the nucleophile in a Michael addition (for a reminder, see Sections
               1.14 and 1.15) to the   ,  -unsaturated carbonyl compound, as shown below:
                                           −
                                     H                      −
                                          OH
                                              − HOH    O   O
                               O   O
                                                     H
                             H
                                                                                  (6.18)
                                               O                         −
                              −                         O   O           O
                          O   O
                                                      H
                        H
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