Markovnikov's Rule
Markovnikov's rule is an empirical rule used to predict regioselectivity of electrophilic addition reactions of alkenes and alkynes. This rule was given by Vladimir Markovnikov in 1869.
According to this rule, when a protic acid or reagent is added to unsymmetrical alkene, the positive part of the reagent or hydrogen attached to that double bonded carbon which has largest number of hydrogen atoms.
1st carbon has two hydrogen atoms and 2nd carbon atom has one hydrogen atom in the example given below, so according to Markovnikov's rule, electrophile (E) (i.e. positive part) will attach at 1st carbon and nucleophile (Nu) (i.e. negative part) will attach at 2nd carbon.
Example: Addition of HBr to Propene
According to Markovnikov's Rule, the hydrogen (H) from HBr attach to the terminal double bonded carbon (1st carbon) and the bromine (Br) attach to the double bonded carbon with fewer hydrogen atoms (2nd carbon) and forms 2-bromopropane.
Mechanism of Markovnikov's Rule
Mechanism of Markovnikov's rule takes place in three steps mentioned below-
Electrophilic Attack: The π-electrons of the double bond attack the hydrogen atom or positive part of the reagent, leading to the formation of a carbocation intermediate.
Carbocation Stability: The more stable carbocation (stability order: tertiary > secondary > primary. see inductive effect and hyperconjugation) is formed rather than the less stable.
Nucleophilic Attack: The nucleophile or negative part of the reagent then attacks the carbocation, leading to the final product.
Examples of Markovnikov's Rule
Exception to Markovnikov's Rule
In the presence of peroxides, the addition occurs in the reverse way that means the positive part of the reagent attach at the that double bonded carbon which has least number of hydrogen atom. This is called Anti-Markovnikov rule. This reaction follows free radical mechanism.
Example: Addition of HBr to propene in the presence of peroxides results in 1-bromopropane instead of 2-bromopropane.
