Step-by-Step Guide to Anti Markovnikov Addition Mechanism
As suggested by Markovnikov, the Anti Markovnikov mechanism is one of the few reactions following free radical mechanisms in organic chemistry in place of electrophilic addition. This reaction is seen only with HBr (Hydrogen Bromide), but not with HCl (Hydrochloric Acid) or HI (Hydrogen Iodide).
Explanation
Alkanes belong to an unsaturated hydrocarbon group, which means one molecule of an alkane contains at least one double bond. Due to the existence of 'pi' electrons, alkenes show the anti-markovnikov's addition reactions in which the electrophile attacks the carbon-carbon double bond to create additional products. When Hydrogen Bromide (HBr) is added to unsymmetrical alkenes in the peroxide presence, 1-bromopropane is formed, contrary to 2-bromopropane.
Better, this reaction is called either the Anti-Markovnikov addition or the Kharash effect after the name of M. S. Kharash, who discovered it first. Also, this reaction is known as either Peroxide or Kharash effect.
When any polar molecule is added to an unsymmetrical alkene in the presence of any organic peroxide, the negative part of the molecule is added to that carbon atom connected to more Hydrogen atoms than the other unsaturated carbon atom. This is known as the peroxide effect.
CH3-CH=CH2+HBr \[ \xrightarrow[]{(ORGANIC \, PEROXIDE)} \] CH3-CH(H)-CH2(Br)
The Mechanism of Anti-Markovnikov's Addition Rule With an Example
The most common type of Anti-Markovnikov addition mechanism is free radical addition. This type of mechanism is applicable only for HBr - not HCl or HI - with either hydrogen peroxide (H2O2) or benzoyl peroxide (C14H10O4). Peroxide is an essential part. It acts as a catalyst that breaks HBr into Br and H radicals (any chemical species with one unpaired electron is referred to as a radical).
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Here The Phases (a) and (b) are explained as,
Br added to 1° carbanion for stability.
2° carbanion reacts with another molecule of HBr to give the major product
The Br (Bromine) radical attacks the alkene first. It attacks the less substituted carbon because the carbon radical so formed will be at more substituted carbon for greater stability. Then, the radical carbon attacks the hydrogen of another HBr (Hydrogen Bromide) molecule, liberating another Br (Bromine) radical, and hence the reaction is carried forward.
Anti Markovnikov Halogenation
Halogenation of alkanes refers to the addition of a halogen to the C = C double bond of an alkane. An Anti-Markovnikov halogenation is a free radical reaction of the hydrogen bromide to an alkene.
In a Markovnikov addition of HBr (Hydrogen Bromide) to propene, the H (Hydrogen) adds to the C atom with more H atoms. The resultant product is 2-bromopropane pictured below.
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In the presence of peroxides, H adds to the C atom with a lower count of H atoms. This refers to anti-Markovnikov addition. The resultant product is 1-bromopropane.
The reason for the Anti-Markovnikov addition is, it is the Br (Bromine) atom that attacks the alkene. It attacks the C (Carbon) atom with the most H (Hydrogen) atoms. So the H adds to the C atom with the less number of H atoms.
Anti-Markovnikov Rule
Anti-Markovnikov rule narrates the regiochemistry where the substitute is bonded to fewer carbon substitutes instead of more carbon substitutes. One such process is quite unusual, as carbocations commonly formed during alkene, or alkyne reactions, tend to favour more substituted carbon. This happens because the substitution of carbocation allows for more hyperconjugation and induction to make the carbohydrate more stable.
In his paper, Morris Selig Karasch explained this process for the first time, 'Addition of Hydrogen Bromide to Allyl Bromide' in 1933.1 Examples of Anti-Markovnikov rule include the Radical Addition of HBr and Hydroboration-Oxidation. A free radical is any chemical substance with an unpaired electron. Here, the resultant carbon forms based on more carbon substituents. Some of the examples of the Anti-Markovnikov rule are Primary carbon (least substituted), Secondary carbon (medium substituted), and Tertiary carbon (most substituted).
Anti-Markovnikov Radical addition of Haloalkane will only happen to HBr, and Hydrogen Peroxide ( H2O2) MUST be there. Hydrogen Peroxide is necessary for this process because it is the chemical that starts off the chain reaction at the initiation step itself. HI (Hydrogen Iodide) and HCl (Hydrochloric Acid) can't be used in radical reactions. In their radical reaction, one of the radical reaction steps is initiation Endothermic, as recalled from Chem 118A, which means the reaction is unfavourable.
To demonstrate the example of the anti-Markovnikov rule of regiochemistry, let us use 2-Methylpropene as an example below.
Initiation Steps
Hydrogen Peroxide is an unstable molecule. If we shine it or heat it with sunlight, two free OH radicals will be formed. These OH radicals will move on and attack HBr, which will take the Hydrogen and produce a Bromine radical. Hydrogen radicals do not form because they appear to be highly unstable with only one electron. Thus bromine radical will be readily formed, which is more stable.
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Propagation Steps
The Bromine Radical will move on and attack the less substituted carbon of alkene. This happens because, after the bromine radicals attack the alkene, a carbon radical will be formed. A carbon radical is highly stable when it relies on a more substituted carbon due to hyperconjugation and induction. Thereby, the radical will be established at the more substituted carbon, whereas the bromine is bonded to the less substituted carbon. Once a carbon radical is formed, it will move on and attack the Hydrogen of an HBr, which a bromine radical will again be formed.
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Termination Steps
There are the Termination Steps also. But we are not concerned about the termination steps as they are just the radicals combining to create the waste products. For example, two bromine radicals combine to produce bromine. This radical addition of bromine to an alkene by a radical addition reaction will move on until all the alkene turns into bromoalkane. And this process will take a bit more time to finish.
FAQs on Anti Markovnikov Addition Reaction Explained
1. What is the Anti-Markovnikov addition reaction as explained in CBSE Chemistry?
The Anti-Markovnikov addition, also known as the Kharasch effect or peroxide effect, is an organic reaction that describes the regiochemistry of adding a reagent like HBr across an unsymmetrical alkene. Contrary to Markovnikov's rule, the hydrogen atom adds to the carbon atom with fewer hydrogen atoms, and the bromine atom adds to the carbon with more hydrogen atoms. This reaction specifically occurs in the presence of a peroxide, which initiates a free-radical mechanism.
2. Can you provide a clear example of an Anti-Markovnikov addition reaction?
A classic example is the reaction of propene with hydrogen bromide (HBr) in the presence of a peroxide (like Benzoyl peroxide).
- Reactants: Propene (CH₃-CH=CH₂) + HBr
- Condition: In the presence of peroxide (R-O-O-R)
- Product: The reaction yields 1-bromopropane (CH₃-CH₂-CH₂-Br) as the major product.
Here, the bromine atom attaches to the terminal carbon (which initially has more hydrogens), and the hydrogen atom attaches to the central carbon, which is the opposite of the Markovnikov product (2-bromopropane).
3. How does the Anti-Markovnikov rule fundamentally differ from the standard Markovnikov's rule?
The fundamental difference lies in the reaction mechanism and the resulting intermediate, which dictates the final product's structure.
- Markovnikov's Rule: Proceeds via an electrophilic addition mechanism, forming the most stable carbocation intermediate. The hydrogen (electrophile) adds to the carbon with more hydrogen atoms to achieve this stability.
- Anti-Markovnikov Rule: Proceeds via a free-radical addition mechanism, initiated by peroxide. The reaction forms the most stable free radical intermediate. The bromine radical adds first to the carbon with more hydrogens, creating a more stable secondary free radical on the adjacent carbon.
In essence, Markovnikov's rule is governed by carbocation stability, while the Anti-Markovnikov rule is governed by free-radical stability.
4. What is the step-by-step mechanism of the Anti-Markovnikov addition of HBr?
The mechanism is a free-radical chain reaction with three main stages:
- 1. Initiation: The peroxide first decomposes to form free radicals. This radical then abstracts a hydrogen from HBr to generate a bromine radical (Br•).
- 2. Propagation: The highly reactive bromine radical attacks the double bond of the unsymmetrical alkene. It adds to the carbon with more hydrogen atoms to form the more stable (secondary) alkyl free radical. This alkyl radical then abstracts a hydrogen atom from another HBr molecule, forming the final product and regenerating a bromine radical, which continues the chain.
- 3. Termination: The reaction stops when the free radicals combine with each other, for example, two bromine radicals combining to form Br₂.
5. Why is the Anti-Markovnikov effect (peroxide effect) observed only with HBr and not with HCl or HI?
This specificity is due to the thermodynamics of the propagation steps. The H-X bond energy is the critical factor.
- For HCl: The H-Cl bond is very strong. The step where the alkyl radical abstracts a hydrogen from HCl is highly endothermic (requires too much energy) and therefore too slow to sustain the chain reaction.
- For HI: The H-I bond is weak, so it breaks easily. However, the resulting iodine radicals (I•) are not very reactive. Instead of adding to the alkene's double bond, they tend to recombine with each other to form iodine molecules (I₂), terminating the chain reaction before it can effectively propagate.
The H-Br bond has the ideal strength to make both propagation steps exothermic and energetically favourable, allowing the free-radical chain reaction to proceed efficiently.
6. What is the specific role of peroxide in an Anti-Markovnikov addition?
The peroxide acts as a radical initiator. Its sole purpose is to start the free-radical chain reaction. The weak O-O bond in the peroxide breaks easily upon exposure to heat or UV light, generating initial free radicals. These radicals are necessary to produce the first bromine radical (Br•) from an HBr molecule. Without the peroxide, there is no initiator to start the free-radical pathway, and the reaction defaults to the standard electrophilic addition mechanism, leading to the Markovnikov product.
7. Is 'Anti-Markovnikov addition' the same concept as 'anti-addition' in organic chemistry?
No, they describe two different aspects of a reaction.
- Anti-Markovnikov addition refers to regioselectivity—it answers the question of 'which atom bonds to which carbon' in an unsymmetrical system. It dictates the constitutional isomer formed.
- Anti-addition refers to stereochemistry—it answers 'how the atoms add' relative to each other, specifically that they add to opposite faces of the double bond.
The free-radical addition of HBr (an Anti-Markovnikov reaction) is not stereospecific; it results in a mixture of both syn- and anti-addition products because the intermediate radical is planar, allowing the final hydrogen to add from either face.
8. Does the Anti-Markovnikov rule apply to molecules other than alkenes, for example, alkynes?
Yes, the principle of anti-Markovnikov addition can apply to other reactions where the mechanism avoids the standard carbocation intermediate. For instance, the hydroboration-oxidation of alkynes is a well-known example. In this two-step reaction, the addition of borane followed by oxidation results in the -H and -OH groups adding across the triple bond in an anti-Markovnikov fashion, leading to the formation of an enol which tautomerizes to an aldehyde (from a terminal alkyne).
