Alkane from alkyl halides

Preparing Alkyl Halides from Alkanes Radical Halogenation 33B... 

Alkyl halides are important starting materials in the synthesis and manufacture of a variety of organic compounds. Free-radical halogenation (Scheme 4.12) is an important methodology for the synthesis of alkyl halides from alkanes. Brominations (X = Br) tend to be very selective because of the... 

Preparing Alkyl Halides from Alkanes Radical Halogenation 335 Table 10.1 A Comparison of the Halomethanes... 

This difference in selectivity between chlorination and bromination of alkanes needs to be kept in mind when one wishes to prepare an alkyl halide from an alkane... 

Chemical reactivity and functional group transformations involving the preparation of alkyl halides from alcohols and from alkanes are the mam themes of this chapter Although the conversions of an alcohol or an alkane to an alkyl halide are both classi tied as substitutions they proceed by very different mechanisms... 

As noted earlier, [HCr(CO)5] also converts alkyl halides to alkanes, but the reactivity of the acyl chloride is much higher, such that it was possible to selectively convert the acyl chloride to an aldehyde in one step, without interference from the alkyl bromide functionality. A second equivalent of [HCr(CO)5] further reduced the alkyl bromide (Eq. (27)). 

Sodium displaces halogens from alkyl halides forming alkanes (Wurtz reaction) ... 

Similar reactivity towards activated methyl compounds, especially methyl iodide, has been observed for Ni OEiBC [114, 115, 61, 116] and for Ni (tmtaa) (where tmtaa is the dianion of 6,8,15,17-tetramethyl-5,14-dihydro-dibenzo [b,i] [1, 4, 8, ll]tetraazacyclotetradecine) [112, 113]. Ni tmtaa), generated in situ by reduction of Nin(tmtaa) and observed by EPR, was proposed as the active catalyst in the reduction by NaBH4 of alkyl halides to alkanes. Recently, a different Ni(I) tetraazamacrocyclic complex, produced by irradiation of the Ni(II) parent compound with 5 MeV electrons, produced small amounts of methane from subsequently added CH3S-CoM [111]. The represented the first report of reactivity of any identifiably Ni(I) species with the natural substrate. 

The reactions to be described in the remainder of this chapter use either an alkane or an alcohol as the starting material for preparing an alkyl halide. By knowing how to prepare alkyl halides, we can better appreciate the material in later chapters, where alkyl halides figure prominently in key functional group transformations. Just as important, the preparation of alkyl halides will serve as our focal point as we examine the principles of reaction mechanisms. WeTl begin with the preparation of alkyl halides from alcohols by reaction with hydrogen halides. 

It is possible to consider that elimination of hydrogen halide (H-X X = F, Cl, Br, I) from an alkyl or alkenyl halide constitutes oxidation as the alkene (or alkyne) that results contains less one proton less (i.e., oxidation corresponding to the loss of hydrogen). However, while alkenes (or alkynes) are doubtlessly in a higher oxidation state than the corresponding alkanes (to which they can be reduced) they are, nonetheless, in the same oxidation state as the alkyl halide from which they were produced—as measured by electron loss and gain (Introduction to Part II). 

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... 

Sheppard N and De La Cruz C 1998 Vibrational spectra of hydrocarbons adsorbed on metals. Part II. Adsorbed acyclic alkynes and alkanes, cyclic hydrocarbons including aromatics and surface hydrocarbon groups derived from the decomposition of alkyl halides, etc Adv. Catal. 42 181-313... 

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