Introduction:
Haloalkenes and haloarenes are two types of organic compounds that contain halogens. Haloalkenes have a double bond between two carbon atoms, while haloarenes have a halogen atom attached to an aromatic ring. When it comes to nucleophilic substitution reactions, haloalkenes are generally more reactive than haloarenes. This can be explained by several factors.

Electron density:
One of the main reasons why haloalkenes are more reactive towards nucleophilic substitution reactions is their higher electron density. The presence of a double bond in haloalkenes leads to a higher electron density between the carbon atoms involved in the double bond. This increased electron density makes the carbon atom more susceptible to attack by a nucleophile.

Stability of carbocation intermediates:
During nucleophilic substitution reactions, a carbocation intermediate is often formed. Haloalkenes tend to form more stable carbocation intermediates compared to haloarenes. This is because the positive charge in a carbocation is delocalized over a larger number of carbon atoms in haloalkenes due to the presence of the double bond. In haloarenes, the positive charge in the carbocation is localized on the carbon atom directly bonded to the halogen, resulting in a less stable intermediate.

Resonance stabilization:
Haloarenes can exhibit resonance stabilization due to the presence of an aromatic ring. The delocalization of electrons in the aromatic ring creates a stabilizing effect, making the carbon-halogen bond stronger and less reactive towards nucleophilic substitution reactions. In contrast, haloalkenes lack this resonance stabilization, making their carbon-halogen bond weaker and more susceptible to nucleophilic attack.

Steric hindrance:
Steric hindrance refers to the presence of bulky groups that can hinder the approach of a nucleophile to the carbon atom bonded to the halogen. Haloarenes often have larger substituents attached to the aromatic ring, which can create significant steric hindrance. This hindrance makes it more difficult for a nucleophile to access the carbon atom, reducing the reactivity of haloarenes towards nucleophilic substitution reactions.

Conclusion:
In summary, haloalkenes are generally more reactive towards nucleophilic substitution reactions compared to haloarenes due to their higher electron density, the stability of their carbocation intermediates, the absence of resonance stabilization, and less steric hindrance. These factors influence the ease with which a nucleophile can attack the carbon atom bonded to the halogen, ultimately determining the reactivity of the compound in nucleophilic substitution reactions.