The most reactive alkene towards dil. HCl is 2,3-Dimethyl-2-butene. Let's understand why.

Acid Catalyzed Hydrogenation Reaction

When an alkene reacts with a hydrogen halide like HCl in the presence of a catalyst like sulfuric acid, it leads to the formation of an alkyl halide. This reaction is known as acid-catalyzed hydrogenation.

Factors Affecting Reactivity

The reactivity of an alkene towards acid-catalyzed hydrogenation depends on the following factors:

- Degree of Substitution: Alkenes with higher degrees of substitution are less reactive towards acid-catalyzed hydrogenation.
- Stability of the Carbocation Intermediate: The stability of the carbocation intermediate formed during the reaction determines the reactivity of the alkene. The more stable the carbocation intermediate, the more reactive the alkene.

Comparing the Reactivity of Alkenes

Let's compare the reactivity of the given alkenes towards acid-catalyzed hydrogenation.

- Ethene: It is a terminal alkene with the highest degree of unsaturation. It is the most reactive towards acid-catalyzed hydrogenation.
- Propene: It is a mono-substituted alkene. It is less reactive than ethene but more reactive than butene and 2,3-dimethyl-2-butene.
- Butene: It is a di-substituted alkene. It is less reactive than propene but more reactive than 2,3-dimethyl-2-butene.
- 2,3-Dimethyl-2-butene: It is a tri-substituted alkene with the highest degree of substitution. It is the least reactive towards acid-catalyzed hydrogenation.

Therefore, the most reactive alkene towards dil. HCl is 2,3-Dimethyl-2-butene.