Difference Between Markovnikov and Anti Markovnikov Rule

Main Difference – Markovnikov vs Anti Markovnikov Rule

We use chemical reactions to synthesize chemical compounds. If we have the required amounts of reactants and catalysts, we can get the desired product by providing other conditions such as proper temperature. But sometimes, the chemical reaction may not give the desired compound or may give a mixture of products that is composed of the desired product as well as other products. This situation can be explained using the Markovnikov rule. The Markovnikov rule explains why a certain atom or a group is attached to a certain carbon atom instead of any other carbon atom in the same molecule. The Anti Markovnikov rule explains the opposite situation of the Markovnikov rule. The main difference between Markovnikov and Anti Markovnikov rule is that Markovnikov rule indicates that hydrogen atoms in an addition reaction are attached to the carbon atom with more hydrogen substituents whereas Anti Markovnikov rule indicates that hydrogen atoms are attached to the carbon atom with the least hydrogen substituents.

Key Areas Covered

1. What is Markovnikov Rule
      – Definition, Reaction Mechanism
2. What is Anti Markovnikov Rule
      – Definition, Reaction Mechanism
3. What is the Difference Between Markovnikov and Anti Markovnikov Rule
      – Comparison of Key Differences

Key Terms: Anti Markovnikov Rule, Catalysts, Markovnikov Rule, Reactants, Regioselectivity

What is Markovnikov Rule

The Markovnikov Rule explains that in addition reactions of alkenes or alkynes, the proton is added to the carbon atom that has the highest number of hydrogen atoms attached to it. This rule is very helpful in predicting the end product of a certain chemical reaction. Let us understand this rule with the help of an example.

Figure 1: Application of the Markovnikov Rule for a Chemical Reaction

As shown in the above example, proton or the hydrogen atom is attached to the carbon atom that already has the highest number of hydrogen atoms attached to it. The counter ion is attached to the other carbon atom. The end product is an alkane.

This happens due to the formation of the most stable carbocation as an intermediate for the reaction. The addition of the hydrogen atom to the carbon atom opens up the double bond. This gives the other vinyl carbon atom a positive charge. This carbocation should be a stable carbocation in order to form a stable product at the end. After the formation of the correct carbocation, the chloride ion attach to the positively charged carbon atom.

However, at the end of the reaction, we get a mixture of products; this mixture is composed of the product given by the stable carbocation and the unstable carbocation.

What is Anti Markovnikov Rule

Anti Markovnikov Rule explains that in addition reactions of alkenes or alkynes, the proton is added to the carbon atom that has the least number of hydrogen atoms attached to it. The end product obtained from this reaction is called Anti Markovnikov product. This mechanism does not involve the formation of a carbocation intermediate. Chemical reactions can be made into reactions that give the Anti Markovnikov product by adding a peroxide such as HOOH to the reaction mixture.

Figure 2: All Possible Products of Addition of HNNR’

Here, the peroxide effect takes place. A peroxide can change the regioselectivity of an addition reaction. Regioselcetivity is the formation of a bond at a particular atom over all other possible atoms. Therefore, the peroxide act as a catalyst.

Difference Between Markovnikov and Anti Markovnikov Rule

Definition

Markovnikov Rule: Markovnikov Rule explains that in addition reactions of alkenes or alkynes, the proton is added to the carbon atom that has the greatest number of hydrogen atoms attached to it.

Anti Markovnikov Rule: Anti Markovnikov Rule explains that in addition reactions of alkenes or alkynes, the proton is added to the carbon atom that has the least number of hydrogen atoms attached to it.

Carbon Atom

Markovnikov Rule: According to Markovnikov rule, the hydrogen atom is attached to the carbon atom with the highest number of hydrogen substituents.

Anti Markovnikov Rule: According to Anti Markovnikov rule, the hydrogen atom is attached to the carbon atom with the least number of hydrogen substituents.

Addendum

Markovnikov Rule: The negative part of addendum (that is X¯ or Cl¯/Br¯) goes to the carbon which has less number of hydrogen atoms attached to it.

Anti Markovnikov Rule: The negative part of addendum goes to the carbon which has more number of hydrogen atoms attached to it.

Conclusion

Markovnikov and Anti Markovnikov rules are very important in predicting the end products of a chemical reaction. The main difference between Markovnikov and Anti Markovnikov rule is that Markovnikov rule indicates that hydrogen atoms in an addition reaction are attached to the carbon atom with more hydrogen substitutes whereas Anti Markovnikov rule indicates that hydrogen atoms are attached to the carbon atom with the least hydrogen substitutes.

References:

1. “Regioselectivity.” Illustrated Glossary of Organic Chemistry – Regioselective; Regiochemistry, Available here. Accessed 12 Sept. 2017.
2. “Markovnikov’s rule.” Wikipedia, Wikimedia Foundation, 31 Aug. 2017, Available here. Accessed 12 Sept. 2017.

Image Courtesy:

1. “Markovnikov rule” (CC BY-SA 3.0) via Commons Wikimedia
2. “Markovnikov and Anti-Mark Addition” By 5402013SD – Own work (CC BY-SA 3.0) via Commons Wikimedia