Question

Preparation of ethers by acid dehydration of secondary or tertiary alcohols is not a suitable method. Give reason.

Answer 1

Acid dehydration of primary alcohols to ethers occurs by S_N2 mechanism in which the nucleophilic attack of the alcohol molecule takes place on the protonated alcohol molecule.

\[\ce{CH3CH2CH2\overset{\bullet\bullet}{\underset{\bullet\bullet}{O}}H + CH3CH2CH2 - \overset{+}{\underset{\bullet\bullet}{O}}\overset{+}{H} ->[S_{N}2][-H^+, -H2O] CH3CH2CH2 - O - CH2CH2CH3}\]

Under these conditions, secondary and tertiary alcohols give alkenes instead of ethers. There is no nucleophilic attack of the alcohol molecule on the protonated alcohol molecule. Instead protonated secondary and tertiary alcohols lose a molecule of water to form stable 2° and 3° carbocations. These carbocations preferentially lose H+ to form alkenes.

\[\begin{array}{cc}
\phantom{.......}\ce{CH3}\phantom{.................}\ce{CH3}\phantom{....................}\ce{CH3}\phantom{.}\\\
\phantom{.....}|\phantom{.....................}|\phantom{........................}|\phantom{..}\\
\ce{\underset{\underset{(2^\circ alcohol)}{Propan-2-ol}}{CH3 - CH - OH} ->[H^-] \underset{Protonated 2^\circ alcohol}{CH3 - CH - \overset{+}{O}H2} ->[][-H2O] \underset{2^\circ Carbocation}{CH3 - CH^+}}
\end{array}\]

 

\[\begin{array}{cc}
\phantom{}\ce{CH3}\phantom{.......}\ce{CH3}\phantom{.........................}\ce{CH3}\phantom{..................}\\
\phantom{}|\phantom{..........}|\phantom{.............................}|\phantom{....................}\\
\ce{\underset{2-propoxy-2-propane}{CH3 - CH - O - CH - CH3} ->[CH3CHOCH3][-H^+] CH3 - CH^+ ->[][-H^+] \underset{Propene}{CH3 - CH = CH2}}
\end{array}\]

 

Similarly, 3° alcohols give alkenes instead of ethers.

\[\ce{\underset{\underset{(3^\circ alcohol)}{2-Methylpropan-2-ol}}{(CH3)3C - OH} ->[H^+] \underset{\underset{(3^\circ alcohol)}{Protonated 2-methylpropan-2-ol}}{(CH3)3C - \overset{+}{O}H2} ->[][-H2O] \underset{3^\circ butyl carbocation}{(CH3)3C^+}}\]

 

\[\begin{array}{cc}
\phantom{...}\ce{CH3}\phantom{.....}\ce{CH3}\phantom{.......................}\ce{CH3}\phantom{............}\ce{CH3}\phantom{...}\\
\phantom{.......}|\phantom{.........}|\phantom{..........................}|\phantom{................}|\phantom{.........}\\
\ce{CH3 - C - O - C - CH3 ->[(CH3)3COH][-H^+] CH3 - C^+ ->[][-H^+] \underset{2-Methylprop-1-ene}{CH3 - CH = CH2}}\\
\phantom{.....}|\phantom{.........}|\phantom{..........................}|\phantom{........................}\\
\phantom{}\ce{\underset{2-Methyl-2-(2-methyl-2-propoxy)propane}{\phantom{..}CH3\phantom{......}CH3}}\phantom{..........}\ce{\underset{3^\circ butyl carbocation}{CH3}}\phantom{....................}
\end{array}\]