Arrange the following hydrogen halides in order of their decreasing reactivity with propene.

Reactivity of the hydrogen halides depends on their bond strength which further depends on the size of halogen attached to the alkyl. Bigger the size of halogen less will be the bond strength and thus the reactivity will be more. We know that I has the biggest size followed by Br and then Cl. Thus the order of their decreasing reactivity with propene is- (iii) HI > HBr > HCl