Simulation · Chemistry · Class 12
Williamson synthesis: which pairing gives the ether?
From the lesson Preparing ethers in Alcohols, Phenols and Ethers. Change the values and watch what happens.
Williamson synthesis: which pairing gives the ether?Chemistry · Class 12
The idea behind it
NCERT §7.6.1
- Protic acids (H₂SO₄, H₃PO₄) dehydrate alcohols, and the conditions decide the product: ethanol with sulphuric acid gives ethene at 443 K but mainly ethoxyethane at 413 K.
- Ether formation from an alcohol is an SN2 reaction: one alcohol molecule attacks a protonated alcohol, water leaves, and the protonated ether loses H⁺.
- This route suits only primary alcohols with unhindered alkyl groups at low temperature. With secondary and tertiary alcohols, elimination to the alkene wins over substitution.
- Williamson synthesis: an alkyl halide reacts with a sodium alkoxide to give an ether, R–X + R′–O⁻Na⁺ → R–O–R′ + NaX. It makes both symmetrical and unsymmetrical ethers.
- The alkoxide attacks the alkyl halide by SN2, so primary alkyl halides work well. With secondary and tertiary halides elimination competes, and a tertiary halide gives only the alkene.
- Example: sodium methoxide with (CH₃)₃C–Br gives only 2-methylpropene, so tert-butyl methyl ether must be made from the tertiary alkoxide and a methyl halide instead.
- Phenols are converted to ethers the same way: the phenoxide ion is the nucleophile that attacks the alkyl halide.
- Diethyl ether was once widely used as an inhalation anaesthetic, but its slow action and unpleasant recovery led to its replacement by other compounds.