${\mathrm{CH}}_3\mathrm{I} + {\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{O}}^{-}{\mathrm{Na}}^{+} \to \mathrm{A} \underset{△}{\overset{\mathrm{Conc}. \mathrm{HI}}{\to }} \mathrm{B}+\mathrm{C}.$  ‘B’ and ‘C’ in this reaction are

$$\begin{gathered} \mathrm{A} = {\mathrm{CH}}_3-\mathrm{O}-{\mathrm{C}}_2{\mathrm{H}}_5 ........ \mathrm{Williamson} \mathrm{synthesis} \\ \mathrm{B} = {\mathrm{CH}}_3\mathrm{I} \\ \mathrm{C} = {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH} \end{gathered}$$

When a mixed ether (containing 1^o and 2^o alkyl groups) is heated with conc. HI, the product contains alkyl iodide in which the halo group goes with smaller alkyl group.

The mechanism of the reaction is S_N2.