Peroxide Effect

Alkenes are unsaturated hydrocarbons, meaning that each alkene molecule has at least one double bond. They demonstrate additional reactions in which an electrophile attacks the carbon-carbon double bond to create extra byproducts attributed to the prevalence of pi electrons.

Alkanes are unsaturated hydrocarbons, which means that each molecule of an alkane has at least one double bond. Alkenes exhibit anti-addiction Markovnikov’s reactions, in which the electrophile attacks the carbon-carbon double bond to produce extra products due to the presence of ‘pi’ electrons. In the presence of peroxide, when hydrogen bromide (HBr) is added to unsymmetrical alkenes, 1-bromopropane is produced instead of 2-bromopropane. This reaction is commonly referred to as the Anti-Markovnikov addition or the Kharasch effect after the person who discovered it. This response is often referred to as the Peroxide or Kharasch effect.

When a polar molecule is attached to an unsymmetrical alkene in the presence of organic peroxide, the negative half of the molecule is attached to the carbon atom that has more Hydrogen atoms than that of the other unsaturated carbon atom. The peroxide effect is the name for this effect.

Anti-Markovnikov Addition Mechanism

A free radical mechanism is discovered to be involved in the Anti Markovnikov addition reaction. The peroxide component in question contributes to the production of free radicals. The following is a general mechanism for the anti-Markovnikov addition reaction:

• Homolytic cleavage of the peroxide molecule produces a free radical
• Through hemolysis, a produced free radical reacts with hydrogen halide to make a halide radical
• Through hemolysis, a produced halide radical attacks an alkene molecule, forming an alkyl radical
• Through homolytic breakage of the hydrogen halide link, a produced alkyl radical attacks hydrogen halide to yield alkyl halide

Beyond Symmetrical Alkenes

When symmetrical alkenes are added, the result is a symmetrical alkene. Alkene molecules have an asymmetric carbon-carbon double bond (C＝C). They contain the same group on both sides of the carbon-carbon bond. The bonds are formed by reacting alkenes with peroxide compounds or oxygen from the air, which causes them to lose electrons. When this happens, the molecule becomes unstable, and the bonds break apart.

In the absence of organic peroxides or oxygen, the symmetrical alkene produces the same result, but the mechanism is different.

Alkynes with Anti-Markovnikov HBr Addition

The halide binds to the most substituted carbon because it is the most positively polarised in most hydrogen halide reactions with alkynes in a Markovnikov-like way. Highly bonds are linked to 1) carbons or 2) electron-donating compounds such as fluorine and other halides in a more substituted carbon. Nevertheless, there are two types of anti-Markovnikov reactions that occur in alkynes: radical addition of HBr and hydroboration-oxidation events. An anti-Markovnikov addition occurs on a terminating alkyne, or an alkyne at the end of a chain, for alkynes.

In an anti-Markovnikov way, the Br of Hydrogen Bromide (H-Br) connects to the less substituted 1-carbon of the terminal alkyne illustrated below, while the Hydrogen proton binds to the second carbon. Because it has fewer bonds connected to carbons and other substituents, the first carbon is the least substituted carbon. In order for this reaction to continue, the H-Br reagent must also be reacted with heat or another radical activator, such as a peroxide. Because it provides the most durable reaction, the presence of the radical or heat causes the anti-Markovnikov addition.

Example of Peroxide Effect 

The peroxide effect, also known as the Anti Markownikoff’s rule, occurs when HBr is added to asymmetric alkenes against Markownikoff’s rule. According to this effect, the asymmetric attacking reagent’s negative portion attaches to the carbon atom of the double bond of the non-symmetric alkene that has the most hydrogen atoms.” Peroxide is present during this process. Peroxide effect with an example:

CH3-CH=CH2 + HBr (organic peroxide) ⟶ CH3-CH(H)-CH2(Br)

Conclusion

The addition of HBr to an unsymmetrical alkane is called the anti-Markovnikov’s rule because the negative part of the attacking reagent attaches to the carbon atom bearing the higher number of hydrogens. Peroxide is used as a catalyst.