Anti-Markovnikov's Rule | Peroxide Effect | Kharasch Effect
Anti-Markovnikov rule explains the regiochemistry where the substituent is bonded to a less substituted carbon, rather than the more substituted carbon. This rule is also known as the peroxide effect or the Kharasch effect.
According to this rule, when a reagent is added to unsymmetrical alkene in the presence of peroxide or radical initiator, the negative part of the reagent attach to that double bonded carbon which has the largest number of hydrogen atoms.
Note: This reaction follows radical mechanism, so saying positive and negatice part of the reagent is not correct but use here, just for understanding.
1st carbon has two hydrogen atoms and 2nd carbon atom has one hydrogen atom in the example given below, so according to Anti- Markovnikov's rule, Negative part will attach at 1st carbon and positive part will attach at 2nd carbon.
According to Anti-Markovnikov's rule, the hydrogen (H) from HBr attach to the double bonded carbon with fewer hydrogen atoms (2nd carbon) and the bromine (Br) attach to the terminal double bonded carbon (1st carbon) and forms 1-bromopropane.
Mechanism of Anti-Markovnikov's Rule
Mechanism of Anti-Markovnikov's rule takes place in three steps mentioned below-
Chain Initiation: Homolytic cleavage of the weak peroxide (-O–O-) bond in the presence of heat or sunlight gives two free radicals. These radicals attack HBr and take hydrogen and create a bromine radical. Hydrogen radicals do not form as they tend to be extremely unstable with only one electron. Thus, the bromine radical, which is more stable, forms.
Chain Propagation: Bromine radical attacks the less substituted carbon of the alkene because, after the bromine radical attacks the alkene, a carbon radical will be formed. A carbon radical is more stable when it is at a more substituted carbon due to inductive effect and hyperconjugation. Thus, the radical is formed in the more substituted carbon, while the bromine is bonded to the less substituted carbon. After a carbon radical is formed, it attacks the hydrogen of an HBr, and a bromine radical is formed again.
Chain Termination: There are also termination steps, but we are generally not concerned about the termination steps as they are just radicals combined to create by-products.
Important Note: Hydrogen chloride and hydrogen iodide do not give anti-Markovnikov addition products with alkenes; in both cases, one of the propagating steps is endothermic and consequently so slow that the chain reaction terminates. As a result, HBr is the only hydrogen halide that adds to an alkene under radical conditions to give antiMarkovnikov products. Additions of HCl and HI proceed only by ionic mechanisms to give normal Markovnikov products regardless of the presence or absence of radicals. Other reagents such as thiols, however, do undergo radical additions to alkenes.
