Question 15.3: Predicting the Products of hemiacetal and Acetal Formation W...
Predicting the Products of hemiacetal and Acetal Formation
Write the structure of the intermediate hemiacetal and the acetal final product formed in the following reaction:
\begin{matrix}\\\qquad \,\,\,\,\,\,\,\, O &&\begin{matrix}\text{Acid}\\\text{catalyst}\end{matrix} \\ CH_3CH_2\overset{||}{C}H &+2 CH_3OH &\longrightarrow ? \end{matrix}
ANALYSiS First, rewrite the structure showing only a single bond between C and O, along with partial bonds to both C and O.
\begin{matrix}\qquad \,\,\,\,\,\,\,\, O \\ CH_3CH_2—\overset{||}{C}—H \end{matrix} is rewritten as \begin{matrix}\qquad \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, O— \\ CH_3CH_2—\overset{|}{\underset{|}{C} }—\\\,\,\,\,\,\,\,\,\,\,\,\,\,\qquad H \end{matrix}
Next, add 1 molecule of the alcohol (CH_3OH in this case) by attaching —H to the oxygen partial bond and —OCH_3 to the carbon partial bond. This yields the hemiacetal intermediate.
\begin{matrix}\qquad \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, O— \\ CH_3CH_2—\overset{|}{\underset{|}{C} }—\\\,\,\,\,\,\,\,\,\,\,\,\,\,\qquad H \end{matrix}+ CH_3OH \longrightarrow \underset{\text{Hemiacetal}}{\begin{matrix}\qquad \,\,\,\,\,\,\,\,\,\,\,\,\,\,\, O—H \\ CH_3CH_2—\overset{|}{\underset{|}{C} }—CH_3\\\,\,\qquad H \end{matrix}}
Finally, replace the —OH group of the hemiacetal with an —OCH_3 from a second molecule of alcohol.
The reaction produces acetal and water.
\begin{matrix}\qquad \,\, O—H \\ CH_3CH_2—\overset{|}{\underset{|}{C} }—O—CH_3\\ H \end{matrix} + CH_3OH\longrightarrow\underset{\text{Acetal}}{ \begin{matrix}\qquad \,\,\,\,\,\,\,\, O—CH_3 \\ CH_3CH_2—\overset{|}{\underset{|}{C} }—O—CH_3\\ H \end{matrix}+H_2O}