When is Markovnikov’s rule valid?

When Markovnikov's Rule IS valid, the fundamental idea behind it is that the carbocation intermediate needs to be as stable as possible, regardless of whether this is most stable following the initial proton addition, a 1,2-hydride shift, or a 1,2-alkyl shift. To ensure this, you should always ask yourself,

"is this carbocation stable enough, or is there a possible shift or conjugation to account for to achieve a more stable carbocation?"

Maybe it would be more appropriate to ask, "When is Markovnikov's Rule NOT valid?"

When the aforementioned 1,2-hydride shift or 1,2-alkyl shift occurs, the "rule" that states that the carbon with more protons gets another one is not a complete description of the reaction because the second atom in a diatomic molecule that is involved in the addition reaction is not always added to the carbon that is directly adjacent to the carbon with the most protons.

Such an exception with the carbocation intermediate arises, for instance, when an alkyl group or proton is two carbons away ("carbon C", for example) from the carbon ("carbon A", for example) that has the greatest number of protons, or even when there is a double bond between the carbon next to carbon A and carbon C (resonance stabilization changes the pi bond).

You can learn more about Markovnikov's Rule by reading this link: https://tutor.hix.ai or by continuing reading about it in your book.

Markovnikov's rule is valid for addition reactions of unsymmetrical alkenes with protic acids, such as HCl or HBr, in the presence of polar solvents.