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Question
Illustrate with examples the limitations of Williamson synthesis for the preparation of certain types of ethers.
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Solution
The main limitation of Williamson’s ether synthesis is that it cannot be used to prepare unsymmetrical ethers where the compound has tertiary or secondary alkyl groups. For example, a reaction between tert-butyl bromide and sodium methoxide yields an alkene.
\[\begin{array}{cc}
\ce{CH3}\phantom{........................................}\ce{CH2}\phantom{..............}\\
|\phantom{...............................................}||\phantom{.................}\\
\ce{CH3 - C - Br + NaOCH3 -> CH3 - C + NaBr + CH3OH}\\
|\phantom{...............................................}|\phantom{..................}\\
\ce{\underset{tert-Butylbromide}{CH3}\phantom{............................}\ce{\underset{2-Methylpropene}{CH3}}}\phantom{..............}\
\end{array}\]
This is due to the formation of an alkene and the dominance of the competing elimination reaction over SN2.
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