Formation of Alkyl Halides

In general, inorganic acids react with alcohols with loss of water and the formation of alkyl halide (either through direct nucleophilic substitution or by an elimination-addition process). The facility with which this substitution occurs varies 

Primary alcohols reaction slowly or they do not react at all. 

To the extent that the transition state for Sn2 substitution of 4-t-butylcyclohexyl systems requires the substituents at C-1 adapt coplanarity and is the same when approached from either direction, it should be clear that the approach of a nucleophile is generally less encumbered when an axial substituent 

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Evidence for this mechanism is as follows The addition of pyridine to the mixture of alcohol and thionyl chloride results in the formation of alkyl halide with inverted configuration. Inversion results because the pyridine reacts with ROSOCl to give ROSONC5H5 before anything further can take place. The Cl freed in this process now attacks from the rear. The reaction between alcohols and thionyl chloride is second order, which is predicted by this mechanism, but the decomposition by simple heating of ROSOCl is first order. ... 

Formation of Alkyl Halides from Esters of Sulfuric and Sulfonic Acids... 

Evidence for this mechanism is as follows the addition of pyridine to the mixture of alcohol and thionyl chloride results in the formation of alkyl halide with inverted configu-... 

A major development since the previous review is the discovery that some orga-nohalogen compounds can form in soils by a purely abiotic mechanism involving a Fenton oxidation pathway and the concomitant reduction of Fe(III) to Fe(H) (2384-2386). The formation of alkyl halides by this mechanism is shown in Scheme 4.6 (2387). The rates of production from soils decreased in this order ... 

The condensation of saturated hydrocarbons with haloolefins in the presence of anhydrous aluminum chloride also results in the formation of alkyl halides, as in the preparation of l-chloro-3,4-dimethylpentane from isobutane and allyl chloride (40%). Under the same conditions, alkyl halides react with olefinic halides to give dihaloalkanes. unsym-Heptachloropropane is synthesized from tetrachloroethylene and chloroform (93%). ... 

Formation of Alkyl Halides from Alcohols Halo-de-hydroxy lation... 

Recently Blum reported that chlorotris(triphenylphosphine) rhodium (XI) is an active catalyst for the decarbonylation of aroyl halides and showed several examples (2). But in this case too, the real catalyst seems to be chlorocarbonylbis(triphenylphosphine)rhodium (XII), which is formed in situ from XI by the stoichiometric reaction with acyl halides. Formation of alkyl halides by decarbonylation of acyl halides can be carried out by the Hunsdiecker reaction, but the reaction is unsatisfactory when applied to aroyl halides. Therefore, the decarbonylation reaction of aroyl halides by the rhodium complex is a new and useful means of introducing halogen onto the aromatic ring. 

The original procedure has been extensively modified, including the preparation of different xanthates, the application of different reduction systems, and the use of a catalytic amount of tributyltin hydride. Other modifications are collected in the literatureincluding the photolysis of a carbazole in the presence of Mg(C104)2 or benzoates and the formation of alkyl halide using carbon tetrachloride or bromotrichloromethane as the solvent instead of the tin hydride. 

This rearrangement from isopropyl to n-propyl Grignard implies that any Grignard may be used to provide the intermediate subvalent titanium compound, and that addition of externally generated olefin would lead to the synthesis of organomagnesium compounds directly from olefins, without the intermediate formation of alkyl halide. 

Metal-carbon bonds are not only cleaved by protons but by halogens or mercuric halides as the electrophile. These reactions typically lead to the formation of alkyl halides or alkylmercury products (Equation 12.6). In contrast to the more common reactions with protons, these reactions generate functionalized organic products. 

FIGURE 7.44 Formations of alkyl halides with phosphorus reagents. 

It certainly demands quite extreme conditions, under which sodium hahdes, or still better, lithium halides or cyanides in a highly polar nonprotic medium such as DMSO or phospholine oxide [214,215,908,909] cleave the ester 130 under formation of alkyl halides. Alternatively, it succeeds in the presence of tertiary amines [910], in quinoline [911] or just in water in the presence of copper salts [912]. On the other hand boiling with 25% sulfuric acid [913] accomplished the same. 

Preparation.—Halogenonium ion oxidation of alcohol carbazates (193) affords a new method for the formation of alkyl halides from alcohols under neutral conditions this procedure (Scheme 119) provides an efficient means of conversion of bridgehead alcohols, such as adamantan-l-ol, into the corresponding halides. 

Tertiary alcohols two layers are apparent almost at once owing to the great ease of formation of alkyl halides from these alcohols. 

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