The most convenient method for the preparation of haloalkanes is from alcohols. Haloalkanes can be prepared from alcohols in the following ways :
We get haloalkanes as main product when an alcohol derivative organic compound reacts with halogen acid.
Primary and secondary alcohols react with hydrochloric acid gas in the presence of catalyst like anhydrous ZnCl2 to form haloalkanes. Tertiary alcohols do not need catalyst because they are more reactive and react readily by conc. HCl.
Example : The following three reactions illustrate this method of preparation of chloroalkanes :
Notice that anhydrous ZnCl2 is not used in the above reaction because tertiary alcohol does not need any catalyst.
Bromoalkanes are prepared by reacting hydrogen bromide (HBr) with alcohols. HBr needed for this purpose can be generated using sodium or potassium bromide with sulphuric acid (H2SO4).
Note : NaBr and H2SO4 react to produce HBr which reacts with alcohols.
Example : In the following reaction, bromoethane is prepared from ethanol :
Hydroiodic acid (HI) needed for the preparation of iodoalkanes can be generated in situ by the action of 95% phosphoric acid on potassium or soidum iodides (KI or NaI).
Chloroalkanes, bromoalkanes and iodoalkanes are prepared by this method. The general reaction is :
Example : Ethanol reacts with PCl3 to form chloroethane :
PBr3 and PI3 are not stable compounds and are usually prepared freshly by reaction of red phosphorus with bromine or iodine.
Chloroalkanes can alternatively be prepared by reacting alcohols with phosphorus pentachloride.
Example : Ethanol reacts with PCl5 to give chloroethane.
Chloroalkanes are prepared by using thionyl chloride (SOCl2 ).
Preparation of chloroalkanes using thionyl chloride is preferred because both the by-products are gases and escape out leaving behind the pure product.