Anti-Markovnikov Rule vs. Markovnikov Rule

Attribute	Anti-Markovnikov Rule	Markovnikov Rule
Definition	Electrophile adds to the less substituted carbon of a double bond or triple bond.	Electrophile adds to the more substituted carbon of a double bond or triple bond.
Reaction Outcome	Forms a more substituted product.	Forms a less substituted product.
Regioselectivity	Regioselective towards the less substituted carbon.	Regioselective towards the more substituted carbon.
Examples	Hydroboration of alkenes.	Hydration of alkenes.
Exceptions	Radical additions and peroxide reactions.	Carbocation rearrangements.

Introduction

In organic chemistry, the Markovnikov rule and the Anti-Markovnikov rule are two fundamental concepts that help predict the regioselectivity of addition reactions to unsymmetrical alkenes. These rules provide insights into the preferred orientation of the addition of electrophiles to double bonds. While the Markovnikov rule predicts that the electrophile adds to the carbon atom with the most hydrogen atoms, the Anti-Markovnikov rule suggests that the electrophile adds to the carbon atom with fewer hydrogen atoms. Let's explore the attributes of these two rules in more detail.

Markovnikov Rule

The Markovnikov rule, formulated by Russian chemist Vladimir Markovnikov in 1870, states that in the addition of a protic acid (HX) to an unsymmetrical alkene, the hydrogen atom of the acid attaches to the carbon atom with the most hydrogen atoms, while the halogen atom attaches to the other carbon atom. This rule is based on the principle that the electrophile (H+) is attracted to the carbon atom with more electron density, which is provided by the greater number of hydrogen atoms.

The Markovnikov rule is widely applicable to various addition reactions, including the addition of water (hydration), hydrogen halides, and other protic acids to alkenes. It helps chemists predict the major product formed in these reactions and understand the mechanism behind them. The rule is particularly useful in the synthesis of organic compounds, as it allows chemists to control the regioselectivity of the reaction and obtain the desired product.

For example, when HBr is added to propene according to the Markovnikov rule, the hydrogen atom attaches to the carbon atom with two hydrogen atoms, resulting in 2-bromopropane as the major product. This rule is consistent with experimental observations and has been validated by numerous studies over the years.

Anti-Markovnikov Rule

The Anti-Markovnikov rule, also known as the Kharasch rule, was proposed by chemists Morris S. Kharasch and Reinhard H. Heydt in the 1930s. It states that in the addition of hydrogen halides to alkenes in the presence of peroxides, the hydrogen atom attaches to the carbon atom with fewer hydrogen atoms, while the halogen atom attaches to the other carbon atom. This rule contradicts the Markovnikov rule and is observed under specific reaction conditions.

The Anti-Markovnikov rule is based on the radical mechanism of the reaction, where the peroxide initiates the formation of a radical intermediate. The radical intermediate reacts with the hydrogen halide, leading to the observed regioselectivity. This rule is commonly observed in the addition of HBr or HI to alkenes in the presence of peroxides, such as benzoyl peroxide (BPO).

For instance, when propene is treated with HBr in the presence of peroxides, the hydrogen atom attaches to the carbon atom with one hydrogen atom, resulting in 1-bromopropane as the major product. This outcome is contrary to the Markovnikov rule but can be explained by the radical mechanism proposed by the Anti-Markovnikov rule.

Comparison of Attributes

While the Markovnikov rule and the Anti-Markovnikov rule provide contrasting predictions for the regioselectivity of addition reactions, they share some common attributes:

• Both rules are based on the concept of electrophilic addition, where an electrophile adds to a double bond to form a new bond.
• Both rules are empirical observations derived from experimental evidence and have been validated by numerous studies.
• Both rules are applicable to a wide range of addition reactions involving unsymmetrical alkenes.
• Both rules help chemists predict the major product formed in these reactions and understand the underlying mechanism.
• Both rules have significant implications in organic synthesis, allowing chemists to control the regioselectivity and obtain desired products.

However, there are also distinct attributes that set the Markovnikov rule and the Anti-Markovnikov rule apart:

• The Markovnikov rule is generally applicable to addition reactions of protic acids, while the Anti-Markovnikov rule is specific to addition reactions of hydrogen halides in the presence of peroxides.
• The Markovnikov rule is based on the principle that the electrophile is attracted to the carbon atom with more electron density, provided by the greater number of hydrogen atoms. In contrast, the Anti-Markovnikov rule is based on the radical mechanism, where the peroxide initiates the formation of a radical intermediate.
• The Markovnikov rule is a more commonly observed regioselectivity pattern in organic reactions, while the Anti-Markovnikov rule is observed under specific reaction conditions.
• The Markovnikov rule has a longer history and was formulated earlier than the Anti-Markovnikov rule.

Conclusion

The Markovnikov rule and the Anti-Markovnikov rule are two important concepts in organic chemistry that help predict the regioselectivity of addition reactions to unsymmetrical alkenes. While the Markovnikov rule predicts that the electrophile adds to the carbon atom with the most hydrogen atoms, the Anti-Markovnikov rule suggests that the electrophile adds to the carbon atom with fewer hydrogen atoms. Both rules have their own set of attributes and applications, providing chemists with valuable tools to control the outcome of addition reactions and synthesize desired organic compounds. Understanding these rules is crucial for students and researchers in the field of organic chemistry.