Question

Illustrate with examples the limitations of Williamson synthesis for the preparation of certain types of ethers.

Answer 1

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 S_N2.