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

Explanation

`underset("Isobutylene")(CH_(3)-overset(CH_(3))overset(|)(C)=CH_(2)) underset("Mark. Addn.")overset(HCl)to underset("2-Chloro-2-methylpropane")(CH_(3)-underset(CH_(3))underset(|)overset(CH_(3))overset(|)(C)-Cl)`
Mechanism. This reaction is an example of electrophilic addition reaction and occurs in two steps:
Step I) In the first step, a proton adds to form two possible carbocations (I and II). Since carbocations (I) is `3^(@)`, therefore, it is much more stable than carbocation (II) which is `1^(@)`.
`underset("Isobutylene")(CH_(3)-overset(CH_(3))overset(|)(C)=CH_(2)) underset("Slow")overset(H^(+))to underset("(I) "3^(@)" Carbocation (more stable)")(CH_(3)-underset(CH_(3))underset(|)overset(+)(C)-CH_(3))+underset("(II) "1^(@)" Carbocation (less stable)")(CH_(3)-underset(CH_(3))underset(|)(CH)-overset(+)(C)H_(2))`
Step II. In the second step, the `Cl^(-)` ion readily attacks the `3^(@)` carbocation (I) forming 2-chloro-2-methylpropane as the major product.