Question

why is reactivity of halogen acids towards electrophilicsubstitution to ethylene is reverse of the order of reactivity of halogen molecles?

Answer 1

Electrophilic substitution reaction happens in many of the reaction of compound containing benzene ring.

and ethylene gives electrophilic addition reaction with halogen acid not electrophilic substitution.

The reactivity of halogen molecule is decreases moving from top to bottom because dissociation energy of X-X bond is increases from top to bottom and the very strong bond which are formed.

Reactivity order of halogen molecule is:

F₂> Cl₂>Br₂>I₂

Size of halogen also increases moving from top to bottom so, halogen acid effective overlapping is decreases that's why HI easily dissociate into H⁺ and I^-. and give electrophilic addition reaction.

So , reactivity of halogen acid is just reverse of halogen molecule.

HI>HBr>HCl>HF