Peroxide Effect
Peroxide Effect
The peroxide effect, also known as the anti-Markovnikov addition or Kharasch effect, is an important concept in organic chemistry, particularly in the context of hydrocarbon reactions. It refers to the observation that the presence of peroxides can alter the typical addition pattern of hydrogen halides to alkenes.
Understanding the Peroxide Effect
Under normal conditions, hydrogen halides (HX, where X is a halogen) add to alkenes following Markovnikov's rule, which states that the hydrogen atom bonds to the carbon with the greater number of hydrogen atoms, and the halogen bonds to the carbon with the fewer hydrogen atoms. However, in the presence of peroxides, the addition follows the anti-Markovnikov rule, where the halogen adds to the less substituted carbon atom.
The peroxide effect is primarily observed with the addition of hydrogen bromide (HBr) to alkenes. It is not seen with other hydrogen halides like hydrogen chloride (HCl) or hydrogen iodide (HI).
Mechanism of the Peroxide Effect
The mechanism for the peroxide effect involves the formation of a free radical intermediate. The process can be divided into three steps: initiation, propagation, and termination.
- Initiation: Peroxides decompose to form free radicals, typically under the influence of heat or light.
$$ R-O-O-R \xrightarrow{heat/light} 2 R\cdot + 2 O=O $$
- Propagation: The alkene reacts with a bromine radical to form a new carbon radical, which then reacts with HBr to form the alkyl bromide and another bromine radical.
$$ R\cdot + HBr \rightarrow R-H + Br\cdot $$ $$ Br\cdot + CH_2=CHR' \rightarrow Br-CH_2-CH\cdot R' $$ $$ Br-CH_2-CH\cdot R' + HBr \rightarrow Br-CH_2-CH_2-R' + Br\cdot $$
- Termination: Two free radicals combine to form a stable molecule, ending the chain reaction.
$$ R\cdot + Br\cdot \rightarrow R-Br $$ $$ 2 Br\cdot \rightarrow Br_2 $$ $$ R\cdot + R'\cdot \rightarrow R-R' $$
Factors Influencing the Peroxide Effect
- Type of Hydrogen Halide: The peroxide effect is most prominent with HBr. HCl and HI do not show this effect because their bond dissociation energies do not favor the formation of the corresponding halogen radicals.
- Presence of Peroxides: Peroxides are necessary to initiate the free radical mechanism.
- Temperature and Light: Heat and light can accelerate the decomposition of peroxides, thus promoting the peroxide effect.
Comparison Table: Markovnikov vs. Anti-Markovnikov Addition
| Factor | Markovnikov Addition | Anti-Markovnikov Addition (Peroxide Effect) |
|---|---|---|
| Rule | Hydrogen adds to the carbon with more hydrogens (more substituted). | Halogen adds to the carbon with more hydrogens (less substituted). |
| Mechanism | Electrophilic addition | Free radical addition |
| Peroxides | Not involved | Required |
| Typical Reagent | HCl, HBr, HI | HBr in the presence of peroxides |
| Example | $$ CH_2=CH_2 + HBr \rightarrow CH_3-CH_2-Br $$ | $$ CH_2=CH_2 + HBr \xrightarrow{peroxides} CH_2Br-CH_3 $$ |
Examples to Explain the Important Points
Example 1: Addition of HBr to Propene without Peroxides
When HBr is added to propene without peroxides, Markovnikov's rule applies:
$$ CH_3-CH=CH_2 + HBr \rightarrow CH_3-CHBr-CH_3 $$
The hydrogen atom adds to the carbon with more hydrogens (the middle carbon), and the bromine adds to the carbon with fewer hydrogens (the end carbon).
Example 2: Addition of HBr to Propene with Peroxides
In the presence of peroxides, the addition of HBr to propene follows the peroxide effect:
$$ CH_3-CH=CH_2 + HBr \xrightarrow{peroxides} CH_3-CH_2-CH_2Br $$
The bromine atom adds to the carbon with more hydrogens (the end carbon), which is the opposite of what Markovnikov's rule predicts.
The peroxide effect is a crucial concept for understanding the reactivity of alkenes with hydrogen halides in the presence of peroxides. It demonstrates how the reaction conditions can significantly alter the outcome of a chemical reaction, leading to different products.