Peroxide Effect Anti Markovnikov S Addition — the NEET Chemistry reaction: mechanism, reagents, conditions, structures and exam traps.
Peroxide Effect (Anti-Markovnikov's Addition) The Peroxide Effect, also known as Kharasch effect or anti-Markovnikov's addition, describes the free radical addition of hydrogen bromide (HBr) to an unsymmetrical alkene in the presence of organic peroxides. In contrast to Markovnikov's rule, the hydrogen atom adds to the carbon atom with fewer hydrogen substituents, and the bromine atom adds to the carbon atom with more hydrogen substituents (the less substituted carbon), leading to the anti-Markovnikov product. Typically, no significant visible change (e.g., color, precipitate) is observed during the reaction, as reactants and products are often colorless. The presence of peroxides does not usually lead to a distinct visual indicator. Generally an exothermic reaction due to the formation of stronger C-Br and C-H bonds from weaker C=C and H-Br bonds. Initiation: Organic peroxides (ROOR) undergo homolytic cleavage to form alkoxy radicals (RO•). These radicals then abstract a hydrogen atom from HBr to generate a bromine radical (Br•). (RO-OR -> 2 RO•; RO• + H-Br -> ROH + Br•) Propagation (Step 1): The bromine radical (Br•) attacks the alkene's double bond at the less substituted carbon atom (the carbon with more hydrogen atoms), forming the more stable alkyl radical (e.g., a secondary or tertiary radical). This step determines the anti-Markovnikov regioselectivity. Propagation (Step 2): The alkyl radical then abstracts a hydrogen atom from another HBr molecule, forming the final bromoalkane product and regenerating a bromine radical (Br•) to continue the chain reaction. Termination: Two radicals combine to form a stable molecule, ending the chain. (e.g., Br• + Br• -> Br2; Alkyl radical + Br• -> Product; Alkyl radical + Alkyl radical -> Dimer) Applying the peroxide effect to HCl or HI; it is only effective for HBr due to bond strengths and kinetics. Forgetting the requirement of peroxide; without peroxides, HBr adds according to Markovnikov's rule. Confusing the regioselectivity with other addition reactions like hydration or typical hydrohalogenation. Incorrectly placing the bromine atom on the more substituted carbon, which would be the Markovnikov product.