Markownikoffs and Peroxide Effect

Organic chemistry is a field full of fascinating rules and effects that govern how molecules interact. Two of the most fundamental concepts in this domain are Markovnikov’s rule and the Peroxide Effect. These principles are essential for predicting the outcomes of addition reactions involving unsaturated hydrocarbons and other compounds. But how do they work, and what sets them apart? Let’s dive deep into their chemistry to understand their significance and applications.

Markownikoffs and Peroxide Effect Overview

Markownikoffs and Peroxide Effect: When a protic acid (HX) is introduced to an asymmetrical alkene, its acidic hydrogen attaches to the carbon atom with more hydrogen substituents, whereas the halide group binds to the carbon chain with one of the most alkyl substituents.

“Hydrogen is added to a carbon with the most hydrogens, and the halide is placed to the carbon with the least hydrogens,” to simplify the rule. The introduction of hydrobromic acid (HBr) to propene is an example of a reaction that follows Markovnikov’s rule.

Markownikoffs Effect

Markownikoff effect: Consider the same example as before, namely the addition reaction of hydrobromic acid with propene, to understand the mechanism behind Markovnikov’s Rule. Markovnikov’s rule mechanism can be split down into the two steps below.

Step 1: The protonation of the alkene produces a more stable carbocation. The protonation of the alkene can result in the formation of two types of carbocations: a primary carbocation and a secondary carbocation. The production of a secondary carbocation is significantly more stable than the synthesis of a primary carbocation.

Step 2:The carbocation is now attacked by the halide ion nucleophile. The alkyl halide is formed on the basis of this reaction. Because the production of the secondary carbocation is favoured, 2-bromopropane would be the main result of this reaction.

Also Check: 

• Halogenation of Alkanes
• Bohr's Theory of Hydrogen Atoms
• Uses of Hydrogen Peroxide

Peroxide Effect

Peroxide effect: Alkanes are unsaturated hydrocarbons, meaning that each molecule of an alkane has at least one double bond. Alkenes exhibit anti-addiction Markovnikov’s reactions, in which the electrophile strikes the carbon-carbon double bond to produce extra products, due to the presence of ‘pi’ electrons. When any polar molecule is introduced to an unsymmetrical alkene in the form of organic peroxide, the negative half of the molecule is attached to the carbon atom that has more Hydrogen atoms bonded to that than the other unsaturated carbon. The peroxide effect is what causes this.

The Chemistry Behind Markovnikov's Rule

• Regioselectivity: This principle determines the position of substituents on a molecule. In alkenes, the electrophile adds to the carbon with fewer substituents, ensuring stability through carbocation formation.
• Examples:
  • When HBr is added to propene, the major product is 2-bromopropane, as the bromine attaches to the more substituted carbon.
• Mechanism:
  1. Electrophile (H+) attacks the double bond.
  2. A carbocation intermediate forms, stabilizing on the more substituted carbon.
  3. Nucleophile (Br⁻) adds to the carbocation.

Understanding the Peroxide Effect

The Peroxide Effect is an exception to Markovnikov's rule, observed specifically in the addition of hydrogen bromide (HBr) to alkenes in the presence of peroxides. It leads to the anti-Markovnikov product, where the bromine atom attaches to the carbon with more hydrogen atoms.

• Discovery: This phenomenon was identified through experiments highlighting the role of radicals in chemical reactions.
• Key Difference: Unlike Markovnikov’s rule, the Peroxide Effect is driven by a free radical mechanism rather than an electrophilic addition.

Mechanism of the Peroxide Effect

The Peroxide Effect operates through a free radical chain reaction, consisting of three main steps:

1. Initiation:
   • Peroxides decompose under heat or light to form free radicals (R•).
   • Example: ROOR → 2RO•
2. Propagation:
   • A bromine radical forms and attacks the alkene, creating a carbon-centered radical.
   • This radical reacts with HBr to regenerate the bromine radical, continuing the chain.
3. Termination:
   • Radicals combine to form stable molecules, ending the reaction.

• Why Only HBr?
  The Peroxide Effect does not occur with HCl or HI because their bond dissociation energies differ, making radical formation unfavorable.

Comparing Markovnikov’s Rule and the Peroxide Effect

• Mechanism: Markovnikov’s rule follows an electrophilic addition mechanism, while the Peroxide Effect relies on free radicals.
• Products: Markovnikov's rule produces the more substituted product, while the Peroxide Effect yields the less substituted (anti-Markovnikov) product.
• Conditions: Peroxides are necessary for the Peroxide Effect but irrelevant for Markovnikov’s rule.

Applications in Organic Chemistry

• Industrial Use:
  • Synthesis of alcohols and halogenated compounds.
  • Polymer production (e.g., polyethylene).
• Research: These principles help design new reactions and predict product outcomes.

Markovnikov vs Anti-Markovnikov Products

To determine whether a product follows Markovnikov’s or anti-Markovnikov's rule, examine where the halogen or other substituents end up.

• Examples:
  • Markovnikov: HBr + Propene → 2-Bromopropane
  • Anti-Markovnikov: HBr + Propene (with peroxide) → 1-Bromopropane

Conclusion

Markovnikov’s rule and the Peroxide Effect are cornerstones of organic chemistry. While one predicts regioselective addition, the other provides a fascinating exception through radical mechanisms. Together, they highlight the dynamic and intricate nature of chemical reactions, offering chemists tools for innovation and discovery.