General Methods of Metal Activation

In recent years, three major approaches have been developed to increase the reactivity of metal powders metal vaporization, use of ultrasound, and reduction of metal salts. An in-depth review of each approach is beyond the scope of this book, and only the reduction of metal salts is treated here in detail. 

The use of ultrasound to accelerate heterogeneous reactions is growing rapidly [22-25]. This technique has generated a field of study commonly labeled sonochemistry. Much of the rapid rise in popularity of sonochemistry stems from the fact that ultrasound can be delivered safely, conveniently, and with inexpensive equipment. Ultrasound can affect chemical reactions at a metal-solution interface when the sound waves induce cavitation or the rapid growth and sudden collapse of bubbles at the metal-solution interface. 

The process involves the initial formation of an organolithium reagent. Thus, this procedure precludes the presence of most functional groups in the organic halide. 

Although our initial entry into this area of study involved the reduction of MgCl2 with potassium biphenylide, our early work concentrated on reductions without the use of electron carriers. In this approach, reductions are conveniently carried out with an alkali metal and a solvent whose boiling point exceeds the melting point of the alkali metal. The metal salt to be reduced must also be partially soluble in the solvent, and the reductions are carried out under an argon 

The electron carrier is normally used in less than stoichiometric proportions, generally 5-10% by mole based on the metal salt being reduced. This procedure allows reductions to be carried out at ambient temperatures or at least at lower temperatures compared to the previous approach, which requires refluxing. A convenient reducing metal is lithium. Not only is the procedure much safer when lithium is used rather than sodium or potassium, but also in many cases the reactivity of the metal powders is greater. 

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