1. By dehydration of alcohols

• Alcohols undergo dehydration in the presence of protic acids (H2SO4 , H3PO4 ).

• The formation of the reaction product, alkene or ether depends on the reaction conditions. For example, ethanol is dehydrated to ethene in the presence of sulphuric acid at 443 K.

• At 413 K, ethoxyethane is the main product

• The formation of ether is a nucleophilic bimolecular reaction (SN2) involving the attack of alcohol molecule on a protonated alcohol, as indicated below:

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• Acidic dehydration of alcohols, to give an alkene is also associated with substitution reaction to give an ether.

• The method is suitable for the preparation of ethers having primary alkyl groups only.

• The alkyl group should be unhindered and the temperature be kept low.

• Otherwise the reaction favours the formation of alkene.

• The reaction follows SN1 pathway when the alcohol is secondary or tertiary about which you will learn in higher classes.

• However, the dehydration of secondary and tertiary alcohols to give corresponding ethers is unsuccessful as elimination competes over substitution and as a consequence, alkenes are easily formed.

• 2. Williamson synthesis

• It is an important laboratory method for the preparation of symmetrical and unsymmetrical ethers.

• In this method, an alkyl halide is allowed to react with sodium alkoxide.

• Ethers containing substituted alkyl groups (secondary or tertiary) may also be prepared by this method.

• The reaction involves SN2 attack of an alkoxide ion on primary alkyl halide.

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• Better results are obtained if the alkyl halide is primary.

• In case of secondary and tertiary alkyl halides, elimination competes over substitution.

• If a tertiary alkyl halide is used, an alkene is the only reaction product and no ether is formed.

• For example, the reaction of CH3ONa with (CH3)3C–Br gives exclusively 2-methylpropene.-

• It is because alkoxides are not only nucleophiles but strong bases as well.

• They react with alkyl halides leading to elimination reactions.

Phenols are also converted to ethers by this method. In this, phenol is used as the phenoxide moiety