Anti-Markovnikov Addition: Explanation, How It Works, Rule, FAQs

As suggested by Markovnikov, the anti-Markovnikov mechanism is one of the few reactions in organic chemistry that follows a free radical mechanism rather than electrophilic addition. This reaction is only seen with HBr (hydrogen bromide) and not with HCl (hydrochloric acid) or HI (hydrogen iodide).

Explanation

Alkanes belong to unsaturated hydrocarbon groups. This means that an alkane molecule contains at least one double bond. Due to the presence of 'pi' electrons, alkenes exhibit anti-Markovnikov reactions in which electrophiles attack the carbon-carbon double bond to form additional products. When hydrogen bromide (HBr) is added to an unsymmetrical alkene in the presence of peroxide, 1-bromopropane is formed as opposed to 2-bromopropane.

Fortunately, this reaction is named after M. S. Kharash, who first discovered it, the anti-Markovnikov addition. This reaction is also known as the peroxide effect or Clash effect.

When a polar molecule is added to an unsymmetrical alkene in the presence of an organic peroxide, the negative part of the molecule is added to a carbon atom that is attached to more hydrogen atoms than to other unsaturated carbon atoms. It is called the peroxide effect.

How the anti-Markovnikov addition law works and examples

• Free radical addition is the most common type of anti-Markovnikov addition mechanism. This type of mechanism only applies to HBr (not HCl or HI) containing hydrogen peroxide (H2O2) or benzoyl peroxide (C14H10O4). Peroxide is an integral part. It acts as a catalyst for the decomposition of HBr into Br and H radicals (species with an unpaired electron are called radicals).

• Br was added to the 1° carbanion for stability.

• The 2° carbanion reacts with another HBr molecule to form the main product

• The Br (bromine) radical attacks the alkene first. The carbon radicals thus formed attack less substituted carbons as more substituted carbons become more stable. The carbon radical then attacks the hydrogen of the other HBr (hydrogen bromide) molecule, releasing the other Br (bromine) radical, thus allowing the reaction to proceed.

Anti Markovnikov Halogenation

Halogenation of alkanes refers to the addition of halogen to the C=C double bond of an alkane. Anti-Markovnikov halogenation is the free radical reaction of hydrogen bromide to alkenes.

In the Markovnikov addition of HBr (hydrogen bromide) to propene, H (hydrogen) is added to the C atom and more H atoms are added. The obtained product is 2-bromopropane.

In the presence of peroxides, H is added to C atoms with a small number of H atoms. This refers to the addition of the anti-Markovnikov. The obtained product is 1-bromopropane.

The reason for the addition of the anti-Markovnikov is that it is the Br (bromine) atom that attacks the alkene. It attacks the C (carbon) atom that has the most H (hydrogen) atom. Therefore, H is added to C atoms with fewer H atoms.

Anti-Markovnikov Rule

The anti-Markovnikov law speaks of geochemistry in which substituents attach to less carbon substituents rather than more carbon substituents. One of these processes is very unusual because the carbocations usually formed during alkene or alkyne reactions have a greater affinity for substituted carbons. This is because many combinations and derivatives are possible, making the carbohydrate more stable.

Morris Selig Karasch first described this process in his 1933 paper, "Addition of Hydrogen Bromide to Allyl Bromide." Free radicals are chemicals with unpaired electrons. Here, the resulting carbon is formed based on more carbon substitutes. Examples of anti-Markovnikov rules are primary (least substituted) carbons, secondary (moderately substituted) carbons, and tertiary (most substituted) carbons.

Anti-Markovnikov radical addition of haloalkanes occurs only on HBr and requires the presence of hydrogen peroxide (H2O2). Hydrogen peroxide is essential to this process because it is the chemical that initiates the chain reaction in the initiation step. HI (hydrogen iodide) and HCl (hydrochloric acid) cannot be used for radical reactions. In those radical reactions, one of the steps in the radical reaction is exothermic, meaning that the reaction is unfavorable, as recalled from Chem 118A.

To illustrate the anti-Markovnikov law in geochemistry, let us use 2-methylpropene as the following example-

• Start the procedure:

Hydrogen peroxide is an unstable molecule. Upon exposure to sunlight or heat, two OH free radicals are formed. These OH radicals migrate and attack HBr, which takes up hydrogen and produces bromine radicals. Hydrogen radicals do not form because they only have one electron and appear to be very unstable. Therefore, bromine radicals are easily formed and become more stable.

• Propagation methods:

The bromine radical moves forward and attacks the less substituted carbon of the alkene. This is because the carbon radical is formed after the bromine radical attacks the alkene. Carbon radicals are very stable when they rely on more substituted carbons via hyperconjugation and induction. It forms a radical bond at the highly substituted carbon and bromine with the less substituted carbon. Once the carbon radical is formed, it moves and attacks the HBr hydrogen to form the bromine radical again.

• Termination method:

There is also a cancellation method. However, we are not concerned with the termination steps, since they simply combine radicals to produce a waste product. Like, when two bromine radicals combine to form bromine. Addition of bromine radical to alkenes by this radical addition reaction continues until all alkenes are converted to bromoalkanes. And it takes a bit more time to complete this process.