Question

Explain the following with an example.

Williamson ether synthesis

Answer 1

It is an important laboratory method for the preparation of symmetrical and unsymmetrical ethers. In this method, an alkyl halide is allowed to react with sodium alkoxide.

\[\ce{R - X + R' - \overset{\overline{\bullet\bullet}}{\underset{\bullet\bullet}{O}}N\overset{+}{a} -> R - \overset{\bullet\bullet}{\underset{\bullet\bullet}{O}} - R' + NaX}\]

Ethers containing substituted alkyl groups (secondary or tertiary) may also be prepared by this method. The reaction involves S_N2 attack of an alkoxide ion on primary alkyl halide.

Better results are obtained if the alkyl halide is primary. In case of secondary and tertiary alkyl halides, elimination competes over substitution. If a tertiary alkyl halide is used, an alkene is the only reaction product and no ether is formed. For example, the reaction of CH₃ONa with (CH₃)₃C–Br gives exclusively 2-methylpropene.

\[\begin{array}{cc}
\ce{CH3}\phantom{.................................................}\\
|\phantom{....................................................}\\
\ce{CH3 - C - Br + N\overset{+}{a} \overset{\overline{\bullet\bullet}}{\underset{\bullet\bullet}{O}} - CH3 -> CH3 - C = CH2 + NaBr + CH3OH}\\
\phantom{}|\phantom{.................................}|\phantom{...................}\\
\phantom{}\ce{CH3}\phantom{..........................}\ce{\underset{2-Methylpropene}{CH3}}\phantom{............}
\end{array}\]