Markovnikov’s Rule and the Peroxide Effect

Introduction

In the field of Chemistry, Markovnikov’s Rule is one of the important topics and basics in organic Chemistry. This is a rule that demonstrates the result of some additional reactions. This rule was given and stated by a Russian Scientist Vladimir Markovnikov. 

The Markovnikov’s Rule states the sum of a Protic Acid or any other reagent that is polar to an alkene that is asymmetric in nature. In easier words, Markovnikov’s rule is an empirical method used to predict the active selectivity of alkene and alkyne electrophilic addition processes. It claims that during the hydrohalogenation of an asymmetric alkene, a hydrogen atom in the hydrogen halide establishes a bond with the doubly bound carbon atom in the alkene that has the most hydrogen atoms.

Markovnikov’s Rule

Markovnikov’s rule states that in the process of addition of a reagent in an unsymmetrical alkene, the negative part of the added reagent moves towards the carbon that bears the double bond and that has the least number of Hydrogen (H) atoms (keeping in the note that the Halide will attack the lesser electropositive carbon).

In simple words, in the addition of hydrogen halide to the double bonded carbon of an alkene, the hydrogen atom links itself to the carbon atom of the double bond that already holds a greater number of hydrogens (keeping in a note that the hydrogen will only attack a more electropositive carbon atom).

Working of Markovnikov’s Rule

To fully understand the working of Markovnikov’s rule, we must consider the reaction of the addition of hydrobromic acid and propane to comprehend this process further. The mechanism of Markovnikov’s rule may be broken down into two steps, which are given below.

Step 1 – The alkene gains a proton and then gives more stable carbocation.

Step 2 – The carbocation is now attacked by the nucleophile of a known halide ion. 

The alkyl halide is formed as a result of this reaction. Because the formation of a stable carbocation is favoured, the main product of this reaction is 2-bromopropane.

Markovnikov’s rule was created expressly for use in the reaction of the addition of certain hydrogen halides with the known specific alkenes. Based on the region of selectivity of the reaction, the opposite of ‘Markovnikov’ addition reactions may be defined as anti-Markovnikov.

Peroxide Effect/Anti-Markovnikov’s rule

Hydrogen peroxide is a colourless liquid at room temperature with a bitter taste.

Hydrogen peroxide is available in several households at (3%-9%)  low concentrations for medical purposes and as hair and clothes bleach. In the chemical factories, hydrogen peroxide in a higher concentration is used to bleach textiles and papers, used as rocket fuels, and produce organic chemicals and foam rubber.

When a reagent is react with an unsymmetrical alkene with a peroxide’s presence, an addition is to take place against Markovnikov’s rule, the negative part of the reagent is also added to that specific carbon or carbon-carbon double bond carries several numbers of Hydrogen atoms. This is called a peroxide effect or Markovnikov’s rule.

For example, when isobutylene is to be treated with HBr in the presence of peroxide, addition takes place to form isobutyl bromide (1-bromo-2-methyl propane).

The addition of HBr to propene with the peroxide’s presence does not occur as Markovnikov’s rule as peroxide effect or kharash effect.

It is a free radical addition reaction and proceeds through more stable Carbon-free radicals.

Mechanism of the Peroxide Effect

Hydrogen halides (hydrogen bromide, hydrogen chloride, and so on) typically react with alkenes by the electrophilic addition process. However, hydrogen bromide is added via a different method with organic peroxides.

Examples of Markovnikov’s Addition Rule

When hydrocarbons are exposed to certain aqueous acids (commonly sulfuric acid), the consequent electrophilic addition process produces alcohol. Markovnikov’s rule can predict the activity and selectivity of such reactions. As a result, these reactions are known as Markovnikov reactions. The H+ ion works as an electrophile in the hydration of alkenes, attacking the alkene to produce a carbon formation. Following the nucleophilic assault by the carbon formation by water molecules, an oxygen-ammonium ion is formed, making it extremely difficult to provide the necessary alcohol product.

When a hydrocarbon is treated with Borane (BH3) in hydrogen gas or sodium hydroxide, the result is alcohol. The boron atom functions as an electrophile in this electrophilic addition process. This response does not follow Markovnikov’s rule and qualifies as an anti-Markovnikov reaction.

Conclusion

Markovnikov’s rule and peroxide effect study makes it easy for an individual to learn how various hydrocarbons act and behave in nature.

By the study of the module, we know that the addition of hydrogen halide to the carbon to carbon double bond of an alkene, the hydrogen atom links itself to the carbon atom of the double bond that already holds a greater number of hydrogens (keeping in a note that the hydrogen will only attack a more electropositive carbon atom).

Markovnikov’s rule works in two periodic steps that occur one after the other.