Page 159 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements
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5A.6 DIAZONIUM SALTS  139
                                                        H 2 O, Δ
                                                                 Ar   OH
                                                         H PO 2
                                                          3
                                             +                   Ar   H
                               HNO 2
                        ArNH 2         Ar   N   N                                (5A.28)
                                                           Kl
                                                                 Ar   I
                                                         Ar′H
                                                                 Ar   Ar′
                                                         NaOH
                  The structure of a diazonium cation probably provides the most valuable clue to
               the mechanism of its formation. A N–N bond has to form, and for that ArNH is a
                                                                                  2
                                        +
               plausible nucleophile and NO the likely electrophile; there are really not many other
               choices.
                       Ar      +             Ar             +  Ar
                               N   O             +       − H
                          N                     N   N             N   N          (5A.29)
                       H                     H
                         H                     H      O         H       O

               Successive protonation of the nitroso (N=O) oxygen and elimination of water then leads to
               the formation of a diazonium cation, as shown below:

                                          Ar
                       Ar           +        N   N         +  Ar
                                   H                +  − H 3 O
                         N   N                                   N   N
                                           H       OH
                       H       O    H                                  OH
                                        O
                                                                                 (5A.30)
                                         H
                            Ar
                                        +  − HOH         +
                               N   N    H           Ar   N   N
                                     OH


                  The great synthetic utility of diazonium ions stems from the fact that they readily lose N ,
                                                                                      2
               producing highly reactive aryl cations, which can be captured by a variety of nucleophiles
                   –
               (Nu ):
                                    +       − N 2     +  Nu −
                               Ar   N   N           Ar           Ar  Nu          (5A.31)

               Equation 5A.31 provides several examples of this type of transformation.
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