I believe this is the full text to the method described in Post 455585
(hermanroempp: "Marbles - grignard activation with glass splinters", Chemistry Discourse)Hendrik van der WorpNone
(
http://www.ub.rug.nl/eldoc/dis/science/h.v.d.worp)
Methods to prepare magnesium surfacesThere are many ways to activate Mg, and the chemist who tries - without much success - to initiate Grignard reagent formation can reckon on well-intentioned colleagues suggesting a myriad of techniques, from the sublime to the strange; from adding a crystal elementary iodine, to the use of slightly wet solvent or the addition of some saliva.
To prepare magnesium surfaces reproducibly is notoriously difficult. Surface oxides, adsorbed insulating layers and crystal lattice orientation can affect the heterogeneous reaction rates. As a result, not only the initiation but also the reproduction of the Grignard reaction is difficult. The following survey of magnesium surface pretreatments is indicative, and does not claim to be complete; they can be divided in two types: mechanical and chemical.
Generally, magnesium turnings are sufficiently prepared for reactive halides after removal of surface oxides and contaminations by hand with pestle and mortar. It is suggested that bending magnesium strips would cause crystal lattice dislocations, making them appropriate for Grignard reactions. Magnesium turnings can be activated by
[3,4i] sonication; this method appears to be critically dependent on the water content of the ethereal solution. It is suggested that the function of the ultrasound is to disperse surfacebound water from the magnesium.
[16] Sometimes, sonication is the preferred technique for effecting the Barbier variation of the Grignard reaction, where magnesium, organohalide and carbonyl compound are introduced concomitantly and the Grignard reagent is intercepted as fast as it is formed.
[17]Instead of mechanical activation, chemical activation with a crystal of elementary iodine is satisfactory in a fair number of the reactions. Among the numerous other chemical activators are bromine, iron trichloride, or a readily reactive alkyl halide (e.g. methyl iodide). A small portion of a preformed Grignard reagent (preferentially a left-over of a former experiment), is often successful.
A well known activation procedure is Rieke's method, where magnesium halides are reduced in situ by metallic potassium, yielding a finely divided black powder of metallic magnesium.
[18] An alternative method for the synthesis of finely divided magnesium requires the evaporative sublimation of high purity metal in vacuo with condensation into a solvent slurry at –19°C.
[19]Magnesium amalgam can be formed by dissolving magnesium powder in mercury, which appears to react slowly, but uniformly over the entire surface. Reduction of mercuric halides by magnesium furnishes an amalgam as well,
[20] though contaminated with halides, a disadvantage, as we will see. In the next section, we discuss the influence of the composition of Grignard solutions on ee in asymmetric couplings.
Experimental
Magnesium turnings were purchased from Fluka and from Janssen (now Acros Chimica). The magnesium turnings obtained from Janssen were nearly flat pieces [probably offcuts of a magnesium ribbon instead of turnings], and were activated more readily than the more wrinkled magnesium turnings that were purchased from Fluka.
Magnesium turnings (16 g, 660 mmol) were transferred to a 250-mL roundbottomed three necked B24 flask, equipped with a 250 mL pressure-equalized dropping funnel. Glass splinters, 1 cm in size approximately (1-2 Pasteur pipettes, carefully granulated by gloved hand), and a Teflon stirring egg (40 mm) were added. The system was purged by N2. The stirring speed was adjusted in such a manner that excessive vortex formation was avoided, which could have led to less interaction between the Teflon stirring egg and the mixture of glass splinters and magnesium turnings. This mechanical activation was carried out overnight. After this time, the magnesium particle size was reduced and a charcoal-grey powder covered the Mg-particles, glass splinters and the wall of the flask. A small amount of Et2O was added, sufficient to moisten the magnesium. The mixture was cooled to 0°C and a solution of alkyl halide (100 mmol) in 150 mL Et2O was added slowly (~1 drop/sec). It should be emphasized that no additional activation or initiation is performed. After addition, the solution was stirred for another 3 hours.References3. Kharasch, M.S.; Reinmuth, O. "
Grignard reactions of nonmetallic substances", Prentice-Hill Inc. New York; 1954.
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Stripping Analysis: Principles, Instrumentation, and Applications VCH Publishers: Deerfield Beach, FL, 1985