Preparation of ethers

Preparation of ethers:

1. Inter molecular dehydration of alcohol.

We have already learnt that when ethanol is treated with con.H₂ SO₄ at 443K, elimination takes place to form ethene. If the same reaction is carried out at 413K, substitution competes over elimination to form ethers.

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.

2. Williamsons synthesis:

When an alkyl halide is heated with an alcoholic solution of sodium alkoxide, the corresponding ethers are obtained. The reaction involves SN² mechanism.

CH₃ -ONa + Br-C₂H₅       ---------^∆→ CH₃ -O-C₂H₅ + NaBr

Mechanism:

We know that primary alkyl halides are more susceptible for SN² 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.

If tertiary butyl bromide and sodium methoxide are used, 2-methyl prop-1-ene is formed

Methylation of alcohol

Methyl ethers can be prepared by treating an alcohol with diazomethane in presence of catalyst, fluoroboric acid.

CH₃ -CH₂ -OH+CH₂N₂ ---_∆--^HBF4→ CH₃ -CH₂ -O-CH₃+N₂

Evaluate Yourself:

1. Which of the following reaction will give 1-methoxy-4-nitrobenzene.

a) 4-nitro-1-bromobenzene + sodium methoxide.

b) 4-nitrosodium phenoxide+bromomethane

2. Arrange the following compounds in the increasing order of their acid strength. propan-1-ol, 2,4,6-trinitrophenol, 3-nitrophenol, 3,5-dinitrophenol, phenol, 4-methylphenol.