Important Topic of Chemistry: Peroxide Effect

In the case of alkenes, the peroxide effect is applied, and it is also known as the Anti-Markovnikov’s rule, according to which the negative half of the chemical reagent is added to the carbon of the double bond with the most hydrogen atoms. The peroxide effect refers to the addition of hydrogen bromide (HBr) to unsymmetrical alkenes in violation of the Markownikoff rule. The negative component of the chemical reagent HBr, i.e., the bromide ion, is connected to the carbon atom of the double bond with the least number of hydrogen atoms, according to the Markownikoff rule.

Peroxide Effect

The negative component of the chemical reagent HBr, i.e., the bromide ion, is connected to the carbon atom of the double bond that has the most hydrogen atoms in the peroxide effect. The peroxide effect is named after the fact that this reaction occurs only in the presence of peroxide. Unsymmetrical alkenes are those in which the number of atoms tied on both sides of the double bond is not the same, such as propane, but-1-ene, and so on, whereas symmetrical alkenes are those in which the number of atoms hooked up on both sides of the double bond is the same, such as ethene, but-2-ene, and so on.

Anti-Markovnikov behavior can extend to other chemical reactions than additions to alkenes, but relatively they are rare. For example, the hydration of phenylacetylene results in acetophenone in a Markovnikov addition. But hydration that happens in the presence of a complex ruthenium catalyst gives phenylacetaldehyde in an Anti-Markovnikov addition. Also, a few other anti-Markovnikov reactions involving transition metal catalysts are known.

Kharasch Peroxide Effect

1. Kharasch’s proposal was based on the presence of peroxide in the reaction mixture, which he had no direct evidence for. He used an altered version of the thiocyanate test, an analytical test that is commonly used to assess shelf-stored chemicals for peroxide content because he had no way of isolating the proposed allyl bromide peroxide.
2. In addition to the thiocyanate test, Kharasch demonstrated that adding antioxidants to the reaction mixture permitted the reaction to occur in aerobic conditions, much like it would in vacuo, yielding the slowly developing 1,2-dibromopropane.
3. An antioxidant’s job is to act as a radical scavenger, taking or giving an electron to a radical species. The radical is efficiently neutralized, while the antioxidant itself becomes a radical. Antioxidants, on the other hand, are significantly less reactive radicals since they are usually big and resonance stabilized aromatic molecules that inhibit unwanted oxidations.
4. In this experiment, adding antioxidants to the reaction mixture effectively quenched the peroxide radicals, causing the reaction to proceed to create the 1,2 –dibromopropane product, as seen.
5. Kharasch’s research encouraged more investigation into free radical reactions. Industrial polymerization processes of unsaturated hydrocarbons were achieved as a result of this ongoing study, allowing for the large manufacture of synthetic rubber and plastics.
6. Standard alkanes are halogenated and made significantly more reactive through similar radical reactions. As a result, they make excellent intermediates in organic synthesis.
7. While typical conditions favor one orientation of addition, in some cases having the halide on the less strongly substituted carbon, in the anti-Markovnikov position, may be favourable. In this situation, Morris Kharasch’s work has made it possible to do a free radical addition step, which may be the key to achieving the desired final product.
8. Kharasch’s proposal was based on the presence of peroxide in the reaction mixture, which he had no direct evidence for. He used an altered version of the thiocyanate test, an analytical technique that is commonly used to assess shelf-stored chemicals for peroxide content because he had no way of isolating the proposed allyl bromide peroxide.
9. In adding to the thiocyanate test, Kharasch demonstrated that adding antioxidants to the reaction mixture permitted the reaction to occur in aerobic conditions, much like it would in vacuo, yielding the slowly developing 1,2-dibromopropane.

Because other researchers had found anti-Markovnikov products and attributed them to other reasons, Kharasch investigated a number of variables to determine if they influenced the orientation of HBr added to allyl bromide. Although an increase in temperature appears to direct the orientation of the addition to the anti-Markovnikov product at first glance, Kharasch explained that this temperature effect is secondary to the peroxide effect, as evidenced by the fact that adding antioxidants at high temperatures can produce a high yield of 1,2- dibromopropane.

Also Check: Important Topic of Chemistry Actinides

Significance of Peroxide Effect in NEET exam

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