When an alkene does not have the same substituents on its sp 2 , it can form two types of products in electrophilic substitution reactions. The majority product is the one obtained by adding the proton to the sp 2 that generates the most stable carbocation.
For example, when propene reacts with HBr, the proton can be attached to carbon 1 , forming the carbocation on carbon 2 . This carbocation is secondary and hyperconjugation gives it significant stability.
If the proton is attached to carbon 2 , a primary carbocation is obtained on carbon 1 . The stability of the primary carbocation is much lower than that of the secondary and it will form more slowly, giving rise to a small percentage of 1-bromopropane.
rule markovnikov01
[1] Majority product, its formation takes place through the most stable carbocation.
[2] Minority product
1-bromopropane and 2-bromopropane are structural isomers, also called regioisomers. When a reaction generates one structural isomer, in greater proportion than the other, it is said to be regioselective.
There are different degrees of regioselectivity, depending on the proportion in which the final products are obtained. A reaction that gives similar proportions of both products is poorly regioselective. A reaction that generates almost 100% of a regioisomer will have high regioselectivity.
The minimum percentages obtained from 1-bromopropane make the previous reaction highly regioselective.
The addition of HI to 2-pentene has a very low regioselectivity, since both products are obtained in a similar proportion.
markovnikov rule02
[3] 2-Pentene
[4] 3-Iodopentane
[5] 2-iodopentane
Both the formation of 3-iodopentane and 2-iodopentane take place through secondary carbocations, with a very close stability, which explains a similar performance in both products.

In 1865, the Russian chemist Vladimir Markovnikov published a paper in which he predicted the position occupied by the proton when added to asymmetric alkenes. The merit of Markovnikov lies in predicting by means of a simple rule, the regioselectivity of electrophilic additions, at a time when the existence of carbocations was not yet known.

Markovnikov's rule: "Hydrogen adds to the sp 2 that has the greatest number of hydrogens."

Reactions that add hydrogen to carbon with fewer hydrogens are called anti-Markovnikov.