Question

Write the chemical equation for Williamson's synthesis of 2-ethoxy-2-methyl pentane starting from ethanol and 2-methyl pentan-2-ol.

Answer 1

Step 1:

\[\ce{\underset{(Ethanol)}{CH3CH2OH} ->[HBr/\Delta][(or) PBr3] \underset{(Ethyl bromide)}{CH3 - CH2 - Br} + H2O}\]

Step 2:

\[\begin{array}{cc}
\phantom{.}\ce{OH}\phantom{.....................................}\ce{ONa}\phantom{..................}\\
\phantom{.......}|\phantom{........................................}|\phantom{...........................}\\
\ce{2CH3 - C - CH2 - CH2 - CH3 ->[Na - metal] 2CH3 - C - CH2 - CH2 - CH3 + H2 ^}\\
\phantom{.}|\phantom{........................................}|\phantom{.....................}\\
\phantom{..}\ce{\underset{(2-methyl-pentan-2-ol)}{CH3}}\phantom{..........................}\ce{\underset{(Sodium alkoxide)}{CH3}}\phantom{.....................}
\end{array}\]

Step 3:

\[\begin{array}{cc}
\phantom{......................}\ce{O - Na}\phantom{....................................................}\ce{O - CH2 - CH3}\phantom{..............................}\\
\phantom{....................}|\phantom{...........................................................}|\phantom{..........................................}\\
\ce{CH3 - C - CH2 - CH2 - CH3 + CH3 - CH2 - CH2 - Br ->[\Delta] CH3 - C - CH2 - CH2 - CH3 + NaBr}\\
\phantom{....................}|\phantom{...........................................................}|\phantom{..........................................}\\
\phantom{}\ce{\underset{(2-methyl-pentan-2-ol)}{CH3}}\phantom{.........................................}\ce{\underset{(2-ethoxy-2-methyl pentane)}{CH3}}\phantom{.................}
\end{array}\]

Williamson's synthesis occurs by SN² mechanism and primary alkyl halides are more reactive in SN² reactions. Therefore ethanol is converted into ethyl bromide.