During monochlorination of propane why is chloropropane the major product formed. As far as I know using free radical mechanism 2-chloropropane is the supposed to be formed as the secondary free radical is more stable than a primary free radical. But the calculations involving reactivity order of chlorination tell otherwise. Why is this so!?
When a Cl· atom is knocking a propane molecule, it may touch it at the end of the molecule, where there are 6 H atoms. Or it may touch it by the middle, where there are are only 2 H atoms. The probability that the collision happens with the end carbon is at least 3 times higher than the other possibility. So the secondary fragment issues from this collision may be more stable than the primary fragment. It is much less frequent. Statistically speaking, more 1-choropropane than 2-chloropropane will be produced.
The 2-radical might well be more stable, but that doesn't mean that it will react faster than the 1-radical. If the reaction rate constant for the 1-radical with chlorine is much faster then the 1-chloro- product will predominate.
More likely, though is that it is that the 1-radical is formed in preference over the 2-radical in step 1. The halogen can attack at one of three positions, so statistically speaking you'd expect more 1-radical over 2-radical intermediates. If this species then reacts immediately in step 2 without rearrangement then you'd get a predominance of the 1-product formed. (Thanks to Karl for pointing this out).