1. By Dehydration of AlcoholsThis method involves heating of excess of primary alcohol with concentrated sulphuric acid at 413 K to get symmetrical ether.
The formation of reaction product, alkene or ether depends on the reaction conditions.In this reaction alcohol has to be used in excess and the temperature has to be maintained around 413 K. If alcohol is not used in excess or the temperature is higher, the alcohol will preferably undergo dehydration to yield alkene.
If ethanol is dehydrated to ethene in presence of sulphuric acid at
433 K but as 410 K ethoxyethane is the main product.The dehydration of secondary and tertiary alcohols to get corresponding ethers is unsuccessful as alkenes are formed easily in these reactions.
This reaction cannot be employed to prepare unsymmetrical ethers. It is because a mixture of products is likely to be obtained.
2. Williamson Synthesis of EthersIt is an important laboratory method for the preparation of symmetrical and unsymmetrical ethers.
The method involves the treatment of an alkyl halide with sodium or potassium salt of alcohol or phenol.
Ethers containing substituted alkyl groups (secondary or tertiary) may also be prepared by this method.The reaction involves a nucleophilic substitution of halide ion by an alkoxide ion.
Good results are obtained if the alkyl halide is primary. If a tertiary alkyl halide is used, an alkene is the only reaction product and no ether is formed. If tertiary alkyl halide is used, an alkene is the only reaction product and no ether is formed.
For e.g., the reaction of CH3ONa and (CH3)3C-Br gives exclusively 2-methyl propene.
This is because alkoxides are not only nucleophiles but also strong bases as well. They react with alkyl halides leading to elimination reaction.Phenols are also converted to ethers by this method.