What is Anti Markovnikov Addition Reaction Mechanism Rule and Examples
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).
(Image will be uploaded soon)
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.
(Image will be Uploded soon)
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.
(Image will be uploaded soon)
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.
(Image will be uploaded soon)
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 and Peroxide Effect Explained
1. What is Anti Markovnikov addition reaction?
The Anti Markovnikov addition reaction is an addition reaction in which the hydrogen atom attaches to the more substituted carbon of an alkene, while the other group (such as Br) attaches to the less substituted carbon. This orientation is opposite to Markovnikov’s rule.
- Occurs mainly in the presence of peroxides (ROOR).
- Common with HBr and alkenes.
- Proceeds through a free radical mechanism.
- Example: CH3–CH=CH2 + HBr → CH3–CH2–CH2Br (in presence of peroxide).
2. Why does Anti Markovnikov addition occur in the presence of peroxide?
Anti Markovnikov addition occurs in the presence of peroxide because the reaction follows a free radical mechanism instead of a carbocation mechanism. Peroxides (ROOR) decompose to form free radicals, which change the reaction pathway.
- Step 1: ROOR → 2RO• (radical formation).
- Step 2: Br• adds to the alkene to form a more stable radical intermediate.
- Step 3: The radical reacts with HBr to give the Anti Markovnikov product.
3. What is the difference between Markovnikov and Anti Markovnikov addition?
The key difference is that Markovnikov addition attaches hydrogen to the carbon with more hydrogens, while Anti Markovnikov addition attaches hydrogen to the carbon with fewer hydrogens.
- Markovnikov rule: Follows carbocation mechanism; no peroxide required.
- Anti Markovnikov rule: Follows free radical mechanism; requires peroxide.
- Markovnikov product: Halogen on more substituted carbon.
- Anti Markovnikov product: Halogen on less substituted carbon.
- Without peroxide: CH3–CHBr–CH3
- With peroxide: CH3–CH2–CH2Br
4. Does Anti Markovnikov addition occur with HCl and HI?
Anti Markovnikov addition does not occur with HCl or HI because the free radical mechanism is not energetically favorable for these acids. The peroxide effect is observed only with HBr.
- For HCl: Radical formation is endothermic and unstable.
- For HI: Side reactions dominate.
- Only HBr shows a suitable bond energy balance for radical propagation.
5. What is the mechanism of Anti Markovnikov addition of HBr to alkenes?
The mechanism of Anti Markovnikov addition of HBr involves a free radical chain reaction with initiation, propagation, and termination steps.
- Initiation: ROOR → 2RO•
- Propagation Step 1: RO• + HBr → ROH + Br•
- Propagation Step 2: Br• + alkene → more stable radical
- Propagation Step 3: Radical + HBr → product + Br•
6. Can you give an example of Anti Markovnikov addition reaction?
A classic example of Anti Markovnikov addition is the reaction of propene with HBr in the presence of peroxide. The reaction is:
- CH3–CH=CH2 + HBr → CH3–CH2–CH2Br (ROOR present)
7. What is the peroxide effect or Kharasch effect?
The peroxide effect or Kharasch effect is the Anti Markovnikov addition of HBr to alkenes in the presence of organic peroxides. It was discovered by Morris Kharasch.
- Requires peroxide (ROOR).
- Occurs only with HBr.
- Proceeds via free radical mechanism.
8. Why is Anti Markovnikov addition important in organic chemistry?
Anti Markovnikov addition is important because it allows selective formation of less substituted alkyl halides using controlled reaction conditions. It is widely used in synthesis.
- Helps prepare primary alkyl bromides from alkenes.
- Demonstrates radical reaction mechanisms.
- Important in industrial and laboratory organic synthesis.
9. What type of intermediate is formed in Anti Markovnikov addition?
The intermediate formed in Anti Markovnikov addition is a free radical, not a carbocation. The stability of this radical determines the product formed.
- More substituted radicals are more stable.
- Radical stability order: tertiary > secondary > primary.
- This stability controls the orientation of bromine addition.
10. How do you predict the product of an Anti Markovnikov addition reaction?
To predict the product of an Anti Markovnikov addition, attach hydrogen to the more substituted carbon and the halogen to the less substituted carbon in the presence of peroxide. Follow these steps:
- Step 1: Identify the double bond in the alkene.
- Step 2: Confirm presence of HBr + ROOR.
- Step 3: Add Br to the less substituted carbon.
- Step 4: Add H to the more substituted carbon.
