Question

Predict the major product formed when HCl is added to isobutylene. Explain the mechanism involved.

Answer 1

The major product is 2-chloro-2-methypropane

\[\begin{array}{cc}
\phantom{............................}\ce{Cl}\phantom{}\\
\phantom{...........................}|\phantom{}\\
\ce{CH3 - C = CH2 + HCl -> CH3 - C - CH3}\\
\phantom{.}|\phantom{...........................}|\phantom{.}\\
\phantom{.........}\ce{\underset{(Isobutylene)}{CH3}}\phantom{..............}\ce{\underset{(2-Chloro-2-methylpropane)}{CH3}\phantom{.}}
\end{array}\]

The mechanism involved in this reaction is:

Step I

\[\begin{array}{cc}
\ce{CH3 - C = CH2 ->[H+] CH3 - \overset{+}{C} - CH3 + CH3 - CH - \overset{+}{C}H3}\\
\phantom{........}|\phantom{....................}|\phantom{..................}|\phantom{..........}\\
\phantom{..}\ce{\underset{Isobutylene}{CH3}}\phantom{............}\ce{\underset{\underset{(more stable)}{3°carbocation}}{CH3}}\phantom{.........}\ce{\underset{\underset{(less stable)}{1°carbocation}}{CH3}}\phantom{.}
\end{array}\]

Step II

\[\begin{array}{cc}
\phantom{......................}\ce{Cl}\phantom{}\\
\phantom{.....................}|\phantom{}\\
\phantom{}\ce{CH3 - \overset{+}{C} = CH3 ->[Cl-] CH3 - C = CH3}\phantom{}\\
\phantom{..........}|\phantom{.....................}|\phantom{..........}\\
\phantom{...}\ce{CH3}\phantom{.................}\ce{CH3}\phantom{}
\end{array}\]