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Alkanes: Properties

Alkanes: Preparation

Alkanes can be synthesized by numerous methods:

Summary of alkane preparations
Catalytical hydrogenation of alkenes
Hydrogen is activated by the catalyst leading to metal-bound hydrogen on the surface of the catalyst. Additionally, the catalyst coordinates with the double bond of the alkene thereby facilitating the addition of hydrogen.
Reduction of alkyl halides
Radical reductions of alkyl halides can be carried out successfully using molecular hydrogen and certain metal hydrides, for example stannanes R3SnH, germanes R3GeH and silanes R3SiH. The reduction with metal hydrides proceeds via a radical chain mechanism.
Hydrolysis of alkyl lithium compounds
Reactions of of organometallic compounds with water yield alkanes and metal hydroxides. The metal must have a electronegativity of ca. 1.7 or less because more electropositive compounds hydrolyze faster.
Hydrolysis of Grignard compounds
Hydrolysis of alkyl aluminum compounds
Hydrolysis of carbides
Saltlike carbides are composed of electropositive metals and carbon and exist in the form of C4- (methanides), C22- (acetylides) and C34- (allylenides). Hydrolysis of methanides (Al4C3 , BeC, etc.) yields methane, the acetylides (Li2C2, MgC2, Al2(C2)3 , etc.) form ethene and allylenides (Li4C3 and Mg2C3) give propyne.
Wurtz coupling
Alkyl halides react with sodium alkyl derivatives in the Wurtz reaction. Electropositive derivatives, such as sodium, react with alkyl halides via carbanionic intermediates to give alkanes. Eliminations to form alkenes have been observed as side reactions.
Wurtz coupling to cycloalkanes
Kolbe electrolysis
Decarboxylation of carboxylic acids to alkanes is called Kolbe electrolysis. The electron of the caboxylate ion is removed by anionic oxidation. The carboxyl radical decomposes to form carbon dioxide and an alkyl radical. Subsequently, the alkyl radical dimerizes to yield the alkane.
Dimerization of radicals
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