Page 160 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements
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NITROGEN
                140

                  REVIEW PROBLEM 5A.8
                  Diazonium salt chemistry provides a classic synthetic route to fluorinated arenes, the
                  Balz–Schiemann reaction:

                                                     +
                                                                    Ar   F
                                       HNO 2    Ar   N   N    Δ
                              Ar  NH 2               +                +
                                        HBF 4       −       − N 2
                                                    BF 4             BF 3



                  Write out the mechanism for the last step.




                5A.7  AZO COMPOUNDS AND DIAZENE

                                                             2
                                                    1
                                                                            2
                                                                      1
                Azo compounds have the general formula R –N=N–R , where R and R are alkyl or
                aryl groups. Aromatic azo compounds are generally strongly colored and many are easily
                prepared via a so-called diazo coupling reaction, in which a diazonium cation is coupled
                with a relatively “active” (i.e., highly nucleophilic) arene such as a phenol or aniline. The
                dye called aniline yellow, for example, is prepared as follows:
                                                                                 NH 2
                          −  +
                                N                    +
                         Cl  N               NH 2  − H
                                 +                               N
                                                     −               N
                                                  − Cl

                                     Aniline                      Aniline yellow
                                                                                 (5A.32)



                  REVIEW PROBLEM 5A.9

                  Suggest a mechanism for the above reaction.



                  Aliphatic azo compounds are also important, most notably as initiators of radical reac-
                tions, including radical polymerizations. The most common reagent used for this purpose
                is azobisisobutyronitrile (AIBN), which decomposes on gentle heating to N and a pair of
                                                                            2
                2-cyanoprop-2-yl radicals:
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