Anti-Markovnikov's Rule (Addition of HBr to an unsymmetrical alkene/alkyne)

 
Few reagents such as Hydrogen Halides (HX), Sulfuric Acid (H2SO4), Alcohols (R-OH), Water (H2O) follow Markovnikov's Rule for the addition across the double bond of an unsymmetrical alkene. But HBr in the presence of a peroxide shows a behavior opposite to the Markovnikov's rule, thereby termed as The Anti-Markovnikov's Rule. It was studied extensively by Morris Kharash was also known as the Kharasch effect or the Peroxide Effect. This behavior is shown only by HBr and not HF, HCl, HI even in the presence of the peroxide. In the absence of the peroxide, HBr adds as per the Markovnikov's Rule. 
 
Anti Markownikoff rule HBr Addition
 
In Anti-Markovnikov addition, the bromide attaches itself to the less substituted carbon (the carbon carrying more Hydrogen atoms) of the double bond/triple bond. There is a definite difference in the reaction route taken by HBr in the presence of peroxide and its absence. In the absence of the peroxide, HBr adds to the double bond by electrophilic addition mechanism (Markonikov's Rule) but in the presence of peroxide, the addition of the HBr is via free radical chain reaction. The free radical reaction mechanism can be classified in 3 short steps - Initiation, Propagation and Termination.
 
A) Initiation
The initiation step requires any one of the radical generator - Hydrogen peroxide (H2O2), Organic Peroxides (R-O-O-R), Benzoyl peroxide (C6H5-CO-O-O-CO-C6H5), Organic Hydroperoxides (R-O-O-H), Azobisisobutyronitrile (AIBN), the most common being the peroxides.
 
The alkyl peroxides (R-O-O-R) when exposed to UV light or heat undergoes homolytic cleavage to generate two alkyloxy radicals (2R-O.). 
 
Initiation Step free radical reaction anti markonikov
 
The alkoxy radical picks up the Hydrogen from H-Br to give Bromine Radical that is now ready to react with an unsymmetrical alkene (example, 1-propene) in the next step.  
 
B) Propagation
The Bromine radical can attach itself from either side of the double bond but it chooses to add from the side that will generate a stable 2o radical species versus the less stable 1o radical. As the radical is considered to be an electron deficient species, it follows the stability order similar to the carbocations- 30> 2o> 1o> CH3
 
The stable 2o radical now picks up the H from the HBr to form a bromoalkane (1-bromopropane). In the processs it also generates a new Bromine radical that will propagate the chain reaction by continuing these two steps continue till the reaction reaches the termination stage. 
Propogation Step free radical anti markonikov
 
Note that here the addition of Hydrogen is the second step in contrast to the first step in Markovnikov's Rule
 
C) Termination
The reaction discontinues when the free radical species (bromo and/or alkyl) are exhausted by any one following steps-  
Termination step free radical reaction anti markovnikov
The radical termination reactions also gives an insight on the nature of the byproducts formed which later can be separated from the desired products by employing various purification methods. 
 
In summary, Anti Markovnikov's addition of HBr on an alkene gives a bromoalkane with the halogen attached to the least substituted carbon. In both the Markovkinov and Anti-Markovnikov's Rule, it is the stability of the carbocation and the radical that decides on the regiochemistry of the product. 
 
Apart from the addition of HBr to an unsymmetrical alkene/alkyne, the addition of water across the double bond also follows Anti Markovnikov's Rule by the Hydroboration and Oxidation Method to generate an alcohol. The Hydroboration reagent is Diborane (B2H6) in solvent THF and for oxidation a peroxide reagent is used.   
 
 
 

 

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