#Markovnikovsrule

#Chemistry

#AdditionReaction

Introduction

• Markovnikov's rule states that when an asymmetrical reagent is added to an asymmetrical alkene, the negative part of the reagent will attach to the carbon that already has the most hydrogen atoms.

• The halide component of HX bonds preferentially at the more highly substituted carbon, whereas the hydrogen prefers the carbon which already contains more hydrogens.

• Markovnikov’s rule is an empirical rule used to predict regioselectivity of electrophilic addition reactions of alkenes and alkynes. It states that, in hydrohalogenation of an unsymmetrical alkene, the hydrogen atom in the hydrogen halide forms a bond with the doubly bonded carbon atom in the alkene, bearing the greater number of hydrogen atoms.

• Markovnikov's rule is an organic chemistry rule that predicts the regiochemistry of electrophilic addition reactions to unsymmetrical alkenes and alkynes.

• The term “regiochemistry” refers to the area of a reaction where changes occur.

• The rule states that:

  • The electron-rich part of the reagent adds to the carbon atom with fewer hydrogen atoms.

  • The electron-deficient part of the reagent adds to the carbon atom with more hydrogen atoms. 

• However, the addition reaction is not random. One of the products is the major product (being produced in higher abundance) while the other product is the minor product.

Reasoning

• Carbocation formation - When a hydrogen ion is added to an alkene, a positive charge is created on the other carbon atom, forming a carbocation intermediate. 

• Carbocation stability - The most stable carbocation is the one where the carbon with the most alkyl substituents holds the positive charge. 

• Product formation - The halide ion nucleophile attacks the carbocation, resulting in the formation of an alkyl halide. The majority of the product will have the halide added to the carbon with the most alkyl groups. 

Limitations

• Markovnikov’s rule is followed by unsymmetrical alkenes.

• Markovnikov's rule only applies when the carbons in the double bond of an alkene have a different number of hydrogen atoms.

• If both carbons have the same number of hydrogen atoms, the rule is not useful for determining the major product. In this case, a mixture of both possible isomers is produced. 

• Electrophilic addition reactions of aromatic alkenes and alkynes follow Markovnikov’s rule.

• When a reagent reacts with an unsymmetrical alkene in the presence of peroxide, the addition takes place against Markovnikov's rule. This is called the peroxide effect or anti-Markovnikov's rule.

Mechanism

• We are explaining the mechanism of Markovnikov’s rule for the reaction of propene with hydrobromic acid. The mechanism can be explained by the following steps :

• Step 1. Protonation or addition of acidic hydrogen ion - Hydrobromic acid (HBr) breaks into H+ and Br-. H+ (Electrophile) gets attached to the carbon which has a greater number of hydrogen substituents.

• Step 2. Addition of bromide anion - In this step nucleophile or bromide ion attacks on carbocation and forms major product 2-bromo propene. Bromide ion gets attached to the carbon which has a greater number of alkyl substituents.