Magnesium oxide, cation reactions

We have learned that Group 2 metals form 2+ cations and fhaf Group 6 nonmetals form 2- anions, so we can predicf fhaf magnesium oxide contains Mg " " and 0 ions. This means that in the reaction given, each Mg loses two electrons to form Mg + and so is oxidized. Also each O gains two electrons to form 0 and so is reduced. 

Binary ionic compounds Thousands of compounds contain ionic bonds. Many ionic compounds are binary, which means that they contain only two different elements. Binary ionic compounds contain a metallic cation and a nonmetallic anion. Sodium chloride (NaCl) is a binary compound because it contains two different elements, sodium and chlorine. Magnesium oxide (MgO), the reaction product shown in Figure 7.5b, is also a binary ionic compound. 

The concentration of [Oo] in very high in the magnesium oxide and so is regarded to be constant, such as water molecules for ionic reactions in aqueous solution. This oxygen belongs to the solvent as well as the magnesium ions in normal cationic positions and the empty interstitial positions. 

The loss of electrons by magnesium atoms to form Mg cations indicates that this reaction between magnesium metal and hydronium ions involves oxidation and reduction. An atom of magnesium is oxidized, losing two electrons to form a Mg + cation. Because electrons must be conserved in every chemical process, electrons lost by Mg must be gained by some other species. In this example the electrons lost by Mg are gained by H3 O , which is reduced to form H2 and H2 O. 

Magnesium cations and oxide anions attract each other strongly, forming the ionic solid, MgO. Notice that in the balanced redox reaction there is no net change in the number of electrons two Mg atoms lose four electrons, and one O2 molecule gains four electrons. 

The magnesium cation is essential in these oxidation reactions. Several plausible mechanisms which provide a role for the magnesium cation and which account for the absence of guanidinium ion 129 are possible. One of them proposed by Wuest and collaborators (82) is the direct transfer of a proton and two electrons within intermediates similar to the complexes 138 and 139. 

Sulfate is principally derived from the weathering of CaS04 minerals (gypsum and anhydrite) in sedimentary rocks. Some sulfate in rivers, however, comes from the weathering of magnesium sulfate salts in sedimentary rocks and from oxidation of sulfides (primarily FeS2. pyrite) in sedimentary and crystalline rocks. The latter process also liberates small amounts of the cations Ca2+, Mg2+, Na+, and K+ by reactions like ... 

Magnesium ion-mediated nitrile oxide 1,3-DC reactions to allylic alcohols have been reviewed <02SL1371>. New examples have been recently reported, in particular, cycloadditions of aromatic and aliphatic nitrile oxides with optically active a-silylallyl alcohols in the presence of magnesium cations. The substituted isoxazolines, which were obtained with high diastereo- and enantioselectivity, were smoothly converted to [1,2]-oxazine derivatives by treatment with TBAF. For example, oxazin-3-one (S)-58 was obtained in 81% ee starting from dipolarophile (S)-55 <02T9613>. 

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