Preparation of Haloalkanes
The most convenient method for the preparation of haloalkanes is from alcohols. Haloalkanes can be prepared from alcohols in the following ways :
By Reacting Alcohols with Halogen Acid
We get haloalkanes as main product when an alcohol derivative organic compound reacts with halogen acid.
Preparation of Chloroalkanes
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.
Preparation of Bromoalkanes
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).
Example : In the following reaction, bromoethane is prepared from ethanol :
Preparation of Iodoalkanes
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).
Predict the product
By Reacting Alcohols with Phosphorus Trihalides
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.
P4 + 6Br2 → 4PBr3
P4 + 6I2 → 4PI3
By Reacting Alcohols with Phosphorus Pentachlorides
Chloroalkanes can alternatively be prepared by reacting alcohols with phosphorus pentachloride.
Example : Ethanol reacts with PCl5 to give chloroethane.
By Reacting Alcohols with Thionyl Chloride
Chloroalkanes are prepared by using thionyl chloride (SOCl2 ).
Question : Why is sulphuric acid not used during the reaction of alcohols with KI?
Answer : Sulphuric acid (H2SO4) is an oxidising agent. It oxidises HI produced during the reaction to I2.
2KI + H2SO4 → 2KHSO4 + 2HI
2HI + H2SO4 → H2O + I2 + SO2
To prepare an alkyl iodide, a non oxidising agent such as H3PO4 is used instead of sulphuric acid.
Question : Why don't we need anhydrous ZnCl2 as catalyst when the alcohol is tertiary?
Answer : Since the stability of carbocation follows the order : 3° > 2° > 1°, the cleavage of C-O bond in tertiary alcohols occurs easily.
Question : Complete the reaction.
Question : The ease with which an alcohol can be halogenated by treating it with HX in the presence of anhydrous ZnCl2 is: