Alkyl halides alkane formation from

Several functional group exchange reactions produce alkyl halides. The most general one is the formation of an alkyl halide (R-X) from an alcohol using various reagents. Alkyl halides are also formed from alkenes, as shown, and from alkanes via radical halogenation. 

This technique was quickly adopted by others and it was soon found by F.O. Rice and co-workers that the pyrolysis of many organic compounds at 800 to 1000°C removed metallic mirrors, implicating the formation of free radicals. The cleavage of larger free radicals into smaller radicals and olefins under these conditions, was also proposed (equation 22), as well as chain reactions in which radicals abstract hydrogen from alkanes. Reactions of alkyl halides with metal atoms in the gas phase were also found by M. Polanyi and co-workers to yield alkyl radicals (equation 23). 

Alkanes can also be prepared from alkyl halides by reduction, directly with Zn and acetic acid (AcOH) (see Section 5.7.14) or via the Grignard reagent formation followed by hydrolytic work-up (see Section 5.7.15). The coupling reaction of alkyl halides with Gilman reagent (R 2CuLi, lithium organocuprates) also produces alkanes (see Section 5.5.2). 

Alternatively alkyl halides undergo coupling reactions with lithium organ-ocuprates (which are prepared from alkyl halides) to give alkanes by carbon-carbon bond formation. Other metals can be used to promote the same kind of coupling, but die use of cuprates is the most efficient and general. 

The reductive elimination product from alkyl halide complexes L M(X)yR2 is usually the alkane R—R. In some cases, however, C—X bond formation may compete with C—C elimination, as in (dppe)Pt(I)Me3 101... 

In an important report, Janowicz and Bergman have described the photochemical incorporation of an alkyl group from an alkane into an iridium complex. The process does not appear to involve alkyl radicals, but is thought to occur via two successive concerted steps. This report may point the way to new catalytic processes for the functionalization of alkanes. The photoaddition of alkyl halides to alkenes catalysed by Cu complexes represents a novel process for formation of C-C bonds, and may have useful synthetic applications (Mitani et ai). The possibility of analogous intramolecular reactions comes readily to mind. 

Alcohols react with HX to form alkyl halides, but the reaction works well only for tertiary alcohols, R,COH. Primaiy and secondary alkyl halides are normally prepared from alcohols using either SOClj or PBr ). Alkyl halides react with magnesium in ether solution to form organomagnesium halides, or Grignard ret ents (RM O- Since Grignard reagents are both nucleophilic and basic, they react with acids to yield hydrocarbons. The overall result of Grignard formation and protonation is the conversion of an alkyl halide into an alkane (RX— RM RH). 

Kochi JK (1965b) Formation of alkyl halides from acids by decarboxylation with lead(IV) acetate and halide salts. J Org Chem 22A 3265-3271 Kosicki GW, Kipovac SN (1964) The pH and pD dependence of the spontaneous and magnesium-ion-catalyzed decarboxylation of oxalacetic acid. Can J Chem 42 403-415 Kraeutler B, Bard AJ (1978) Heterogeneous photocatalytic decomposition of saturated carboxylic acids on Ti02 powder. Decarboxylative route to alkanes. J Am Chem Soc 100 5985-5992... 

Depending on the solvent, supporting electrolyte, electrode material, and potential, it is possible to electrogenerate either alkyl radicals or carbanions, which can lead to the formation of dimers (R2), alkanes (RH), and olefins [R(-H)]. In addition, the solvent (SH) can act as a hydrogen atom donor or a proton donor. Also olefins can arise from the base-promoted dehydrohalogenation of RX, and other products (RB) can be formed via displacement of halide from RX by a base (B"). When cathode materials such as... 

Other similar methods which involve cyclization of iV-substituted diaminoalkenes or -alkanes include the preparation of 2-alkyl- and 2-acyl-l-methylimidazole-4-carboxylates from methyl (Z)-j -dimethylamino-a-isocyanoacrylate (191) in reaction with an alkyl or acyl halide <82LA2093>, the preparation of perfluorinated 3-imidazolines (192) <83S169>, the high yield synthesis of 2-imidazolines by decay of phosphaza compounds (193) in a version of the intramolecular Staudinger reaction <85T793>, and the formation of 1,3-diaroylimidazolidines (34-71%) when bis-alkylidene-or -arylidene-ethylenediamines react with acid chlorides in polar solvents (Scheme 129)... 

Organolithium and Grignard reagents are prepared from alkyl (or aryl) halides. The reactions are carried out under anhydrous conditions because reaction with water leads to the formation of alkanes. 

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