Markownikoffs And Peroxide Effect

Markownikoffs and Peroxide effect

All chemical compounds are grouped into two categories, namely organic and inorganic. Inorganic chemistry deals with the concepts of inorganic compounds such as metals, minerals, and organometallic compounds and their behaviour and properties. Organic chemistry is associated with everything related to carbon-containing chemical compounds. This includes studying the structures, properties, compositions, reactions, and preparations of these types of compounds. The study material notes on Markownikoff’s rule and Peroxide effect involve the unsaturated hydrocarbons known as alkenes and their mechanisms. The peroxide effect is the reverse effect of Markownikoff’s rule and involves the addition of peroxide as a catalyst.

• Alkanes 

Alkanes are defined as the simplest type of organic compounds made of only carbon and hydrogen. They are saturated and can be expressed by the formula CnH2n+2, where n can represent any number such as 1,2,3, etc.

• Alkenes

Alkenes can be defined as the type of hydrocarbons made up of carbon-carbon double bonds(C=C) in their molecule. These compounds are unsaturated and contain two hydrogen atoms less than alkanes. They are represented by the general formula CnH2n, where n represents the number of carbon atoms present in the molecule.

• Alkynes

Alkynes are defined as highly unsaturated hydrocarbons containing carbon atoms linked together with triple bonds(C≡C) in their molecule. They are represented by the general formula CnH2n-2, where n represents the number of carbon atoms present in the molecule.

Markownikoff’s rule

This rule is regarded as one of the most important roles in studying organic chemistry. It is based on additional reactions that involve the double bond unsaturated compounds known as alkenes. This rule was framed and put forward by the Russian chemist Vladimir Markovnikov in 1869.

Statement

When an unsymmetrical reagent adds to an unsymmetrical alkene, the positive part of the reagent becomes attached to the double-bonded carbon that bears the greatest number of hydrogen atoms.

Mechanism

Markownikoff’s rule can be easily explained by a simple mechanism. Let us consider a simple reaction, such as adding hydrogen bromide (HBr) to propene.

Step 1: HBr breaks down to produce a proton and a bromide ion. The proton is the electrophile, and the bromide ion is the nucleophile.

H-Br → H+ + Br–

Step 2: This proton attacks the double bond to form a more stable carbonium ion.

CH3-CH=CH2 + H+ → CH3-CH+-CH3 + CH3-CH2-CH2+

Step 3: The bromide ion attacks the more stable secondary carbonium ion to give the major product.

CH3-CH+-CH3 + Br– → CH3-CHBr-CH3

Essential features to keep in mind about Markownikoff’s rule:

• It involves the addition of hydrogen halides to alkenes
• It occurs in unsymmetrical alkenes
• It is followed by all electrophilic addition reactions of aromatic alkenes and alkynes
• It produces stable carbocations as the final major product
• The protonation step acts as the initial rate-determining step

Peroxide Effect

The peroxide effect can also be referred to as the Anti-Markovnikov’s rule. This is based on the concept of a free radical mechanism instead of electrophilic addition. Peroxide is essentially required as a catalyst that can be involved in breaking HBr into hydrogen and bromine free radicals.

Three steps can further explain the process of the peroxide effect:

• Initiation
• Propagation
• Termination
  
  

• Initiation

Hydrogen peroxide is typically unstable and easily breaks down into hydroxyl (OH) free radicals when exposed to sunlight. These newly formed OH free radicals approach and attack HBr, leading to the formation of hydrogen and bromine radicals. Bromine radicals are highly stable, while hydrogen radicals are said to be highly unstable.

• Propagation

This bromine radical further attacks an alkene molecule at the less substituted carbon region. As a result, a carbon radical is formed, which happens to be highly stable due to hyperconjugation and induction. Thus, the free radical gets established at the more substituted carbon, and the bromine gets bonded to the less substituted carbon.

• Termination

The final steps involve the formation of a whole molecule from the addition of free radicals. For example, a free radical of chlorine reacts with another free radical of chlorine to produce a complete chlorine molecule. Likewise, a free radical of bromine reacts with another free radical of bromine to produce a complete bromine molecule.

Statement

The addition reactions of alkenes or alkynes in which the proton gets added to the carbon atom that contains the least number of hydrogen atoms attached to it.

Mechanism

The peroxide effect follows a simple mechanism that can be explained with a suitable example.

Let us consider the reaction of propylene with HBr in the presence of a peroxide.

Step 1: The peroxide initially dissociates to give alkoxy free radicals.

R-O-O-R → 2R-O∙

Step 2: The alkoxy free radical attacks HBr to form a bromine atom.

R-O∙ + H:Br → R-OH + Br∙

Step 3: The bromine atom attacks propylene to give a primary free radical and a secondary free radical.

Br∙ + CH3-CH-CH2 → CH3-CHBr-CH3 or CH3-CH-CH2Br

Step 4: More stable secondary free radical attacks the HBr molecule to form the anti-Markovnikov’s product along with a bromine atom.

Conclusion

Organic chemistry is based on various organic sources obtained from plants and animals. Various such organic compounds obtained are carbohydrates, fats, oils, proteins, etc., that we use in our daily lives. The processes in which atoms or groups of atoms are added to other chemical compounds containing double and triple bonds are known as addition reactions. Both Markownikoff’s rule and the Peroxide effect play an important part in the study of organic chemistry and explain the stability of carbonium ions.