Salinic carbides

Carbon is the only Group 14/IV element that forms both monatomic and polyatomic anions. There are three classes of carbides saline carbides (saltlike carbides), covalent carbides, and interstitial carbides. The heavier elements in Group 14/IV form polyatomic anions, such as Si44 and Sn52, in which the atoms form a tetrahedron and trigonal bipyramid, respectively. 

The saline carbides are formed most commonly from the metals of Groups 1 and 2, aluminum, and a few other metals. The s-block metals form saline carbides when their oxides are heated with carbon. The anions present in saline carbides are either C>2 or C4. All the C4 carbides, which are called methides, produce methane and the corresponding hydroxide in water ... 

This reaction shows that the methide ion is a very strong Bmnsted base. The species C22 is the acetylide ion, and the carbides that contain it are called acetylides. The acetylide ion is also a strong Bronsted base, and acctylides react with water to produce ethyne (acetylene) and the corresponding hydroxide. Calcium carbide, CaC2, is the most common saline carbide. 

There are three classes of carbides saline carbides (or saltlike carbides), covalent carbides, and interstitial carbides. 

Examples of saline carbides are Be2C (see Section 11.4 and equation 11.14) and AI4C3, both made by heating the constituent elements at high temperatures. Although their solid state structures contain isolated C centres which are converted to CH4 on reaction with H2O, it is unlikely that the ion is present since the interelectronic repulsion energy would be enormous. 

The salt-like (or salinic) carbides are formed with carbon and the most electropositive elements, found in Groups I, II and III to the left of the Periodic Table (Table 2.1.). These elements have an electronegativity difference of about two or more which corresponds to an atomic bond that is at least S0% ionic. 

Saline carbides are most commonly formed from Group 1 and 2 metals, Al, and a few other metals. S -Block metals form saline carbides when their oxides are heated with C. 

In acetylene engineering - and especially in the generation of acetylene from carbide as well as in supply engineering - the possibihty of generating acetylide is an important factor when selecting the material for the equipment. The formation of acetylides is not only possible in saline solutions in laboratory experiments but it also occurs when moist crude acetylene comes into contact with metallic silver or copper surfaces. Corrosion products encourage acetylide formation on copper surfaces. 

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