Other Methods for Converting Alcohols to Alkyl Halides

14 Other Methods for Converting Alcohols to Alkyl Halides 

Alkyl halides are such useful starting materials for preparing other functional group types that chemists have developed several different methods for converting alcohols to alkyl halides. Two methods, based on the inorganic reagents thionyl chloride and phosphorus tribromide, bear special mention. 

Thionyl chloride reacts with alcohols to give alkyl chlorides. The reaction is typically carried out in the presence of pyridine, which acts both as solvent and a base that reacts with the HCl that is produced. 

Because tertiary alcohols are so readily converted to chlorides with hydrogen chloride, thionyl chloride is used mainly to prepare primary and secondary alkyl chlorides. 

Mechanism 4.3 shows the conversion of a primary alcohol to the alkyl chloride using thionyl chloride. The reaction involves the formation of an alkyl chlorosulfite that is attacked by the nucleophilic chloride ion. 

14 OTHER METHODS FOR CONVERTING ALCOHOLS TO ALKYL HALIDES 

Phosphorus tribromide reacts with alcohols to give alkyl bromides and phosphorous acid. 

Because tertiary alcohols are so readily converted to chlorides with hydrogen chloride, thionyl chloride is used mainly to prepare primary and secondary alkyl chlorides. An early step in the mechanism of this reaction is the conversion of the alcohol to a chlorosulfite, which then reacts with chloride ion to yield the alkyl chloride. 

Chloride ion Alkyl chlorosulfite Alkyl chloride Sulfur dioxide Chloride ion 

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Alkyl Halides. The classical method for converting alcohols to alkyl iodides (31) involves heating the alcohol with iodine in the presence of phosphorus (Reaction XXIII). Like other iodi-... 

As esters the alkyl halides are hydrolysed by alkalis to alcohols and salts of halogen acids. They are converted by nascent hydrogen into hydrocarbons, by ammonia into amines, by alkoxides into ethers, by alkali hydrogen sulphides into mercaptans, by potassium cyanide into nitriles, and by sodium acetate into acetic esters. (Formulate these reactions.) The alkyl halides are practically insoluble in water but are, on the other hand, miscible with organic solvents. As a consequence of the great affinity of iodine for silver, the alkyl iodides are almost instantaneously decomposed by aqueous-alcoholic silver nitrate solution, and so yield silver iodide and alcohol. The important method of Ziesel for the quantitative determination of alkyl groups combined in the form of ethers, depends on this property (cf. p. 80). 

Because radical chlorination of an alkane can yield several different monosubstitution products as well as products that contain more than one chlorine atom, it is not the best method for synthesizing an alkyl halide. Addition of a hydrogen halide to an alkene (Section 4.1) or conversion of an alcohol to an alkyl halide (a reaction we will study in Chapter 12) is a much better way to make an alkyl halide. Radical halogenation of an alkane is nevertheless still a useful reaction because it is the only way to convert an inert alkane into a reactive compound. In Chapter 10, we will see that once the halogen is introduced into the alkane, it can be replaced by a variety of other substituents. 

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