Question

Write the mechanism of acid-catalysed dehydration of ethanol to yield ethene.

Answer 1

The mechanism of acid dehydration of ethanol to yield ethene involves the following three steps:

Step 1: Protonation of ethanol to form ethyl oxonium ion:

\[\begin{array}{cc}
\phantom{}\ce{H}\phantom{...}\ce{H}\phantom{.........................}\ce{H}\phantom{...}\ce{H}\phantom{...}\ce{H}\phantom{..}\\
|\phantom{....}|\phantom{..........................}|\phantom{....}|\phantom{....}|\phantom{..}\\
\ce{H - C - C - \overset{\bullet\bullet}{\underset{\bullet\bullet}{O}} - H + H^+ ⇌[Fast] H - C - C - \underset{\bullet\bullet\phantom{...}}{O^+} - H}\\
\phantom{....}|\phantom{....}|\phantom{..........................}|\phantom{....}|\phantom{..........}\\
\ce{\underset{Ethanol}{H\phantom{...}H}}\phantom{.....................}\ce{\underset{(Ethyl oxonium ion)}{\underset{Protonated alcohol}{H\phantom{...}H}}}\phantom{..}
\end{array}\]

Step 2: Formation of carbocation (rate determining step): It is the slowest step and hence the rate-determining step of the reaction.

\[\begin{array}{cc}
\ce{H}\phantom{...}\ce{H}\phantom{...}\ce{H}\phantom{...............}\ce{H}\phantom{....}\ce{H}\phantom{....}\\
|\phantom{....}|\phantom{....}|\phantom{................}|\phantom{.....}|\phantom{....}\\
\ce{H - C - C - \underset{\bullet\bullet\phantom{...}}{O^+} - H <=>[Slow] H - C - C^+ + H2O}\\
|\phantom{....}|\phantom{.....................}|\phantom{....}|\phantom{....}\\
\ce{H}\phantom{...}\ce{H}\phantom{....................}\ce{H}\phantom{...}\ce{H}\phantom{....}
\end{array}\]

Step 3: Elimination of a proton to form ethene:

\[\begin{array}{cc}
\phantom{..}\ce{H}\phantom{...}\ce{H}\phantom{....}\ce{H}\phantom{........}\ce{H}\phantom{.}\\
|\phantom{....}|\phantom{......}\backslash\phantom{......}/\\
\ce{H - C - C^+ <=> C = C + H^+}\\
|\phantom{....}|\phantom{......}/\phantom{......}\backslash\\
\phantom{..}\ce{H}\phantom{...}\ce{H}\phantom{....}\ce{\underset{Ethene}{H\phantom{........}H}}\phantom{.}
\end{array}\]

The acid consumed in step 1 is released in Step 3. After the formation of ethene, it is removed to shift the equilibrium in a forward direction.