Magnesium reactivity

Rieke RD, Bales SE (1973) Activated metals. The effect of added metal salts on magnesium reactivity. J Chem Soc, Chem Commun 879-880... 

Chemically, carbon dioxide is not very reactive, and it is often used as an inactive gas to replace air when the latter might interact with a substance, for example in the preparation of chromium II) salts (p. 383). Very reactive metals, for example the alkali metals and magnesium can, however, continue to bum in carbon dioxide if heated sufficiently, for example... 

Alkynyl anions are more stable = 22) than the more saturated alkyl or alkenyl anions (p/Tj = 40-45). They may be obtained directly from terminal acetylenes by treatment with strong base, e.g. sodium amide (pA, of NH 35). Frequently magnesium acetylides are made in proton-metal exchange reactions with more reactive Grignard reagents. Copper and mercury acetylides are formed directly from the corresponding metal acetates and acetylenes under neutral conditions (G.E. Coates, 1977 R.P. Houghton, 1979). 

The reaction proceeds by formation of the Grignard reagent from o bromofluorobenzene Because the order of reactivity of magnesium with aryl halides is Arl > ArBr > ArCl > ArF the Gngnard reagent has the structure shown and forms benzyne by loss of the salt FMgBr... 

Properties. Lithium fluoride [7789-24-4] LiF, is a white nonhygroscopic crystaUine material that does not form a hydrate. The properties of lithium fluoride are similar to the aLkaline-earth fluorides. The solubility in water is quite low and chemical reactivity is low, similar to that of calcium fluoride and magnesium fluoride. Several chemical and physical properties of lithium fluoride are listed in Table 1. At high temperatures, lithium fluoride hydroly2es to hydrogen fluoride when heated in the presence of moisture. A bifluoride [12159-92-17, LiF HF, which forms on reaction of LiF with hydrofluoric acid, is unstable to loss of HF in the solid form. 

When prepared by direct reaction of the elements, magnesium hydride is stable in air and only mildly reactive with water. 

Acylation. Acylation is the most rehable means of introducing a 3-substituent on the indole ring. Because 3-acyl substituents can be easily reduced to 3-aLkyl groups, a two-step acylation—reduction sequence is often an attractive alternative to direct 3-aLkylation. Several kinds of conditions have been employed for acylation. Very reactive acyl haUdes, such as oxalyl chloride, can effect substitution directiy without any catalyst. Normal acid chlorides are usually allowed to react with the magnesium (15) or 2inc (16) salts. The Vilsmeier-Haack conditions involving an amide and phosphoms oxychloride, in which a chloroiminium ion is the active electrophile, frequentiy give excellent yields of 3-acylindoles. 

Thermal Reduction. Magnesium metal can also be formed by the thermal reduction of magnesium oxide with a reactive metal, such as siUcon [7440-21-3] which forms a stable oxide. 

Upon exposure to the atmosphere, magnesium hydroxide absorbs moisture and carbon dioxide. Reactive grades are converted to the basic carbonate 5MgO 4CO2 XH20 over a period of several years. Grades that resist carbonization at high temperature and humidity have been reported (71). 

Chemica.1 Properties. The reactivity of magnesium hydroxide is measured primarily by specific surface area in units of /g and median particle size in p.m. Reactivity ranges from low, 1-2 /g, 5 p.m, eg, Kyowa s product to high, 60-80 /g, 5—25 pm, eg, Barcroft s CPS and CPS-UF... 

Magnesium hydroxide can also be produced by slaking or pressure hydrating various reactive grades of magnesium oxide. The reaction is highly exothermic (AH gg = —40.86 kJ/mol (—9.77 kcal/mol)) to produce crystalline form at stoichiometric water addition ... 

AH gg = —43.03 kJ/mol ( — 10.28 kcal/mol) including heat of solution, at standard state m = V) and may require a heat sink to prevent boiling of the reaction mixture. A 30% by weight suspension of MgO in 20°C water boils in the absence of any heat sink. The time to reach boiling is dependent on the reactivity of the MgO raw material, and this time can be only several hours for the more reactive grades of MgO. Investigations of the kinetics of formation of magnesium hydroxide by hydration of MgO have been reported (79). 

Dead Seas Periclase Ltd., on the Dead Sea in Israel, uses yet another process to produce magnesium oxide. A concentrated magnesium chloride brine processed from the Dead Sea is sprayed into a reactor at about 1700°C (127,128). The brine is thermally decomposed into magnesium oxide and hydrochloric acid. To further process the magnesia, the product is slaked to form magnesium hydroxide which is then washed, filtered, and calcined under controlled conditions to produce a variety of MgO reactivity grades. A summary of MgO purities, for the various processes is given in Table 20. 

Very reactive metals, eg, titanium or 2irconium, which in the Hquid state react with all the refractory materials available to contain them, also require reduction to soHd metal. Titanium is produced by metallothermic reduction of its chloride using Hquid magnesium at 750°C (KroU process). 

Heating ammonia with a reactive metal, such as magnesium, gives the nitride. 

---