Question

Explain the following with an example.

Williamson ether synthesis

Answer 1

It is an important laboratory method for preparing symmetrical and unsymmetrical ethers. In this method, an alkyl halide reacts with a 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 an S_N2 attack of an alkoxide ion on a primary alkyl halide.

Better results are obtained if the alkyl halide is primary. In the case of secondary and tertiary alkyl halides, elimination competes with 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.

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