From Alcohols or other Halides

The triphenylphosphine-CCl4 route for alcohol to chloride conversions has been shown to be strongly solvent dependent. Thus chlorides are formed when CCI4 is the solvent whereas dehydration often occurs in acetonitrile. 

Primary chlorides can be converted into bromides by the action of ethyl bromide and catalytic quantities of metal bromides, and a quaternary ammonium ion-exchange resin, in its fluoride form, has been used in the formation of alkyl fluorides from other alkyl halides (or methane sulphonates) as in equation (9). Both of these methods can be generalized to other halide interconversions. 

A novel route to tertiary alkyl iodides in good yields involves reaction of iodide ion with the tertiary chlorides under the influence of Lewis acid catalysts (such as ZnCl2), and seems to be dependent on electrophilic catalysis with in situ trapping of a carbenium species. 

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Preparation.—From Alcohols or other Halides. Factors affecting the formation of isomerically and optically pure alkyl halides from saturated aliphatic alcohols have been discussed by Hudson and co-workers. - Reactions with thionyl chloride give reduced amounts of rearrangement products if pyridine hydrochloride is added, and isomerically pure chlorides RCl from almost all alcohols ROH if HMPT or DMF is the solvent. In the latter medium (58) is a presumed intermediate, as is the case in the reaction of alcohols with Vilsmeier reagents (59, X = Cl or Br). Such species produce halides via inversion of configuration from secondary alcohols, presumably according to Scheme 24. In the related reaction of PCI3 with unhindered primary alcohols in DMF to produce alkyl... 

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