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Question
Illustrate with examples the limitations of Williamson synthesis for the preparation of certain types of ethers.
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.
\[\begin{array}{cc}
\ce{CH3}\phantom{..........................}\ce{CH2}\phantom{.........}\\
\phantom{.}|\phantom{..............................}||\phantom{............}\\
\ce{CH3 - C - Br + NaOCH3 -> CH3 - C + NaBr + CH3OH}\\
|\phantom{..............................}|\phantom{............}\\
\ce{\underset{tert-Butylbromide}{CH3}\phantom{..................}\ce{\underset{2-Methylpropene}{CH3}}}\phantom{..........}\
\end{array}\]
This is due to the formation of alkene and the dominance of the competing elimination reaction over SN2.
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