When 1-propane reacts with HBr, the major product formed is isopropyl bromide and minor product is n-propyl bromide. I know that this is said to happen because the reaction mechanism favours the formation of a more stable product which is isopropyl bromide in this case which is again due to the formation of a stable reactive intermediate-secondary carbocation in this case. (For n propyl bromide,it is a primary carbocation that is formed as a reactive intermediate less stable and more reactive than isopropyl carbocation). This is based on markonikoff’s rule. I know this much but I have a question. My question is when a secondary carbocation is stable than primary, it means that secondary is less reactive than primary. So the major product should be n propyl bromide because it is formed from a primary carbocation which is more reactive than isopropyl carbocation. But that's not the case. Why? Kindly someone clarify my doubt. Suggestions for some good books in this topic are much appreciated.
Well yes, a primary carbocation is more reactive than a secondary carbocation. But both are still so extremely reactive that they have lifetimes as short as is physically possible. Any reactions with both are basically, for all intents and purposes, diffusion limited.
Furthermore, the fastest reaction a primary carbocation can perform is a 1,2-hydride shift to form the corresponding secondary carbocation. Since this is not a diffusion process but basically intramolecular tunneling, it is even faster. This is why it is said that primary carbocations do not even form in the first place.