Question

As F2 has the smallest size and greatest electron density it should be least reactive slso HF should also be least reactive and I2 due to larger size will be easily reacted HI should also be easily reacted... but it is not the case... then what is correct regarding the size and reactivity?
HydroHalogenation is in order of
HI>HBr>HCl>HI
same order should be for halogenation
But it is not so..

Answer 1

The rate of hydrohalogenation is decided by the bond strength of hydrogen halide to be added. During the addtion reaction the bond between H-X should be broken so that the addition takes place. But in case of HF the bond is very strong due to same size of atoms and effective overlapping. In case of HI the bond is very weak due to different size of atoms.
So the correct order of hydrohalogenation of alkenes is HF< HCl < HBr < HI
But for simple halogenation (addition of halogens to alkenes) the rate is not the same case. This is because in the reaction between alkene and halogen, the halogen molecule should get polarisation and the addition product should be free from steric hindrance.
Now F2 cannot be polarised as partial positive or partial negative. So addition F2 to alkene will not give addition product.
I2 cannot be added eventhough it can be polarised to great extend. This is becasue we cannot keep to iodine atoms in the adjacent carbon atoms. Due to this reason we are restricting to addition of chlorine and bromine only.
Addition of Br2 will be easier than Cl2 becasue Br2 can be polarised by the alkene pi electrons.