Preparation of ethers: . Inter molecular dehydration of alcohol. We have already learnt that when ethanol is treated with con.H SO at 443K, elimination takes place to form ethene. If the same reaction is carried out at 413K, substitution competes over elimination to form ethers.
2CH -CH -OH CH -CH -O-CH -CH ethanol H SO 413K diethylether Mechanism: This method is useful for the preparation of simple ethers and not suitable for preparing mixed ethers. If a mixture of two different alcohols is used, mixture of different ethers will be formed and they are difficult to separate. XII U11-Hydroxy XII U11-Hydroxy - - - - CH CH O .. ..
SO H CH CH CH CH CH CH O .. CH CH CH CH H - O + H O + + . Williamsons synthesis: When an alkyl halide is heated with an alcoholic solution of sodium alkoxide, the corresponding ethers are obtained. The reaction involves SN mechanism.
CH -ONa + Br-C H CH -O-C H + NaBr D → Mechanism: O - Na + + CH Br -NaBr methoxyethane We know that primary alkyl halides are more susceptible for SN reaction. Hence for the preparation of mixed ether having primary and tertiary alkyl group, primary alkyl halide and tertiary alkoxide are used. On the other hand, if we use tertiary alkyl halide and primary alkoxide, elimination dominates and succeeds over substitution to form an an alkene. H C O Na + CH Br bromomethane -NaBr H C O ..
.. - methyl - - methoxypropane If tertiary butyl bromide and sodium methoxide are used, -methyl prop- -ene is formed O - Na + + α Br β -elimination CH OH + NaBr -methyl prop- -ene β H Methylation of alcohol Methyl ethers can be prepared by treating an alcohol with diazomethane in presence of catalyst, fluoroboric acid. CH -CH -OH+CH N CH