In organic chemistry, Markovnikov’s rule is used in some addition reactions. For example, the addition of hydrogen halide to the alkenes follows this rule. It is given by Vladimir Markovnikov(Russian chemist) in 1870.
What is Markovnikov’s rule?
In the case of symmetrical alkene, the positive part of the attacking reagent attaches itself to either side of the double bond. Whereas, the negative part to the other side. However, in the case of unsymmetrical alkenes follows Markovnikov’s rule. Which is defined as:
“When unsymmetrical reagents are added to the unsymmetrical alkenes. The negative part of the adding reagents attaches itself to that carbon. Which constituting less number of hydrogen.”
Examples of Markovnikoves rule
Addition of hydrogen halide to alkenes
The addition of hydrogen halide to the alkenes is a highly regioselective reaction. In this reaction, two isomeric alkyl halides are formed. However, one is formed in preference to the other.
These observations were collected by the Vladimir Markovnikov at the University of Kazan(Russia) in 1870. According to Markovniko’s rule:
“When hydrogen halide is added to the unsymmetrical alkene. The hydrogen is added to that carbon which has a greater number of hydrogen. Whereas, halogen adds to the carbon with fewer hydrogens.”
The mechanism for the addition of hydrogen bromide to the 2-methylpropene
In the above mechanism, the first step is the rate-determining step. The regioselectivity occurs in the second step. The stabilities of two carbocation control this.
- addition according to Markovnikov’s rule
- Addition opposite to the Markovnikov’s rule
The addition of water to the unsymmetrical alkenes follows Markownikoff’s rule. According to this rule, the hydroxyl group of water attaches to the carbon of alkenes. Which has fewer hydrogens. Whereas hydrogen adds to the carbon with greater hydrogens.
The reaction of hydrogen halide to alkynes follows Markovnikov’s rule. This reaction is catalyzed by metallic halides.
Why Does “Markovnikov’s Rule” Work?
This rule works on the basis of the formation of the most stable carbonium ion. However, it does not work in the presence of unsymmetrical alkene.
What are some exceptions to Markownikov’s rule?
Anti-Markovnikov is the exception to Markovnikov’s rule. These are the reactions following the free radical mechanism. Instead of the electrophilic addition as suggested by Markovnikov.
What is the peroxide effect?
The addition of HBr to the unsymmetrical alkenes against Markovnikov’s rule is known as the peroxide effect or Anti Markovnikov’s rule. It follows a free radical mechanism.
Is Markownioff’s rule applicable when HBr is added to pent-2-ene?
The pent-2-ene is an unsymmetriocal alkene. Therefore Markovnikov’s is applicable to this reaction.
What is the product formed when pent 1 ene is reacted with HBr in the presence of H2O2?
When HBr is reacted with pent-1-ene in the presence of H2O2 1-bromopentane is formed.
- Organic Chemistry tenth edition by Francis A. Carey (University of Virginia) and Robert M. Guiliano ( University of Villanova)
- A textbook of Organic Chemistry by M.Younas (University of Punjab)