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Free Radical Addition Reactions of Alkenes, Anti- Markownikoff’s Orientation

Besides protons and other electrophile species, carbon-carbon double bonds can also be added by other reactive species.

Free radical addition reactions of alkenes

Besides protons and other electrophile species, carbon-carbon double bonds can also be added by other reactive species. Contrary to popular belief, this was revealed first by conflicting reports about the regioselectivity of HBr additions. HBr, which is predominantly present as a product of the Markovnikov Rule, was produced by the acid-induced addition of HBr to 1-butene. Early experiments that used reactants that were contaminated with peroxide produced mostly 1-bromobutane.

According to our research, the anti-Markovnikov product results from an alternative radical chain reaction that occurs as a result of peroxides. The equations below illustrate this.


Peroxide initiators are broken homolytically by thermal or high-energy means, resulting in a weak O-O bond. HBr is then oxidized with a strongly exothermic reaction to yield an alkoxy radical. An atom of bromine is formed once the alkene's π -bond is formed in the first step of the chain reaction. In this regioselective addition, the more stable radical carbon is formed as the intermediate. From HBr, another hydrogen is abstracted by carbon radicals, resulting in new bromine and anti-Markovnikov alkyl bromide. Whenever a chain reaction starts, each step is exothermic, so once begun, the process continues until radicals are lost via termination events.

Ionic chain addition of HBr is slower, but it competes very well with free radical chain addition, particularly in non-polar solvents. However, it is crucial to note the uniqueness of HBr here. It is not favorable for HCl or HI to be radically added since one of the chain reactions becomes endothermic (for HCl, it is the second and for HI, it is the first). The following equation gives an example of a radical addition reaction induced by peroxide to carbon tetrachloride:

RCH=CH2 + CCl4 (peroxide initiator) —> RCHClCH2CCl3

Polymerization of alkenes is probably the most familiar and important application of free radical addition. Due to the relatively favorable energy of radical addition to double bonds, concentrated solutions of alkenes are susceptible to radical-induced polymerization, as the following equation illustrates for propene. Blue R-groups represent radical species initiating or initiating polymer chains, whereas maroon monomers represent propene monomers. An additional radical intermediate is always formed with the addition.

RCH2(CH3)CH· + CH3CH=CH2 —> RCH2(CH3)CH-CH2(CH3)CH· + CH3CH=CH2 —> RCH2(CH3)CHCH2(CH3)CH-CH2(CH3)CH· —> etc.

Anti-Markownikoff's orientation

One molecule of alkene contains at least one double bond, making it a member of the unsaturated hydrocarbon group. They show additional reactions where the double bond is attacked by an electrophile to form additional products due to their pi-electron presence. HBr is generated by adding it to unsymmetrical alkenes in the presence of peroxide, which results in the formation of 1-bromopropane in contradistinction to 2-bromopropane. This reaction can also be called the anti-Markovnikov addition or the Kharash effect after M. S. Kharash, the first to observe it. It is also known as the peroxide effect or Kharash reaction.

In addition to the anti-Markovnikov addition, additional reactions of alkenes are also an exception to Markovnikov's rule. Inorganic chemistry represents one of the few reactions that follow a free radical mechanism rather than the electrophilic addition proposed by Markovnikov. HCl or HI do not show the same reaction as HBr.

Mechanism

Free radicals are found to be responsible for the Anti Markovnikov addition reaction. It is believed that peroxide contributes to the formation of free radicals. The following is a general explanation of how anti-Markovnikov addition reactions work:
  • Free radicals are generated via homolytic cleavage of peroxide compounds.
  • Hemolysis produces halide radicals from free radicals attacked on hydrogen halide
  • By attacking alkene molecules with halide radicals, alkyl radicals are formed.
  • During homolytic cleavage of the hydrogen halide bond, an alkyl radical reacts with hydrogen halide to form an alkyl halide.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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