Starting Heterogeneous Reactions with Metals

This practical chapter in a monograph on the Barbier reaction will deal with some experimental procedures and such special conditions-mainly with regard to the activation of the metal - required for the optimal execution of Barbier and Barbier-type reactions. 

Not only metallic zinc causes problems in its reactions with organo halides also magnesium and even more reactive metals such as lithium have their specific difficulties. 

Tissier and Grignard, in one of the earliest publications on Grignard chemistry [6], found that for the reaction of magnesium with bromobenzene, iodobenzene and analogous organo halides, the metal usually had to be activated by the addition of a crystal of iodine. 

Bayer and Villiger, shortly after Grignard and Tissier, reported [7] that reactions of bromo- and iododimethylaminobenzene with magnesium could not be brought about, even after boiling the reaction mixture for several days. 

Sachs and Ehrlich, in the same year [8], managed to get such reactions going by first etching the metal through preliminary reaction with bromo-ethane in diethyl ether after which the main portion of the ethereal solution was removed and the activated magnesium was used further. 

---

Metal oxidation is a heterogeneous solid state reaction and starts in the same way as other heterogeneous reactions with nucleation and initial growth. This was discussed in Chapter 6. A time-dependent nucleation rate may dominate the overall growth kinetics of thin Films. Even under an optical microscope (i.e., in macroscopic dimensions), preferential sites of growth can still be discerned [J. Benard (1971)). This indicates that lateral transport on the surface (e.g., at sites where screw dislocations emerge) can possibly be more important for the initial reactive growth than transport across thin oxide layers. 

Kinetics. If the kinetics and mechanism of the reaction in homogeneous solution are known, they can be translated at least approximately into the heterogenized system [30,37]. The principal difference between the homogeneous and heterogenized reactions is that the catalyst is distributed uniformly over the entire system in the former, but is concentrated in a part of it in the latter, a distinction of physics rather than chemistry. Provided the reaction is first order in the catalyst, as is usually true, the same amount of catalytically active species—say, hydrogen ions or complexed cobalt metal atoms—could be expected to produce approximately the same rate in both systems. This is a very crude, but useful hypothesis to start with. Complicating facets of the heterogenized reactions are ... 

As starting materials the metallopentachlorodisilanes 2a-c and the ferriodisilanes 3a,b (Fig. 1) have been used, which are easily accessible by heterogeneous reaction of the metalates Na[M(CO)2C5Rs] (la-c) with hexachlorodisilane followed by Cl/H substitution with LiAlHa [6]. 

---