Question
Discuss the mechanism of addition hydrogen acids to symmetrical alkenes. Justify the order of reactivity of halogen acids HI > HBr > HCl.
Solution
Alkenes react with halogen acids to form mono-haloalkanes called alkyl halides.
The order of reactivity of the halogen acids is HI > HBr > HCl.
The above order is justified on the basis of bond dissociation energies of the halogen acids.
HI (300 kJ mo-1) < HBr (365 kJ mol-1) < HCl (430 kJ mor-1) For example,
Mechanism: It is electrophilic addition reaction and consists of the following steps:
(i) Electromeric effect and electrophilic attack:
(ii) Attack of nucleophile: The nucleophile released in the slow step combines with carbocation to give chloroethane. 
