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Allylic Substitution Reaction

Last updated date: 29th Nov 2023
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Introduction to Allylic Substitution by NBS

The allylic position is a position of a carbon atom present next to the C-C double bonds. N-Bromosuccinimide (NBS) could be a brominating and oxidizer that's used as a supply for bromine in radical reactions (for example, allylic brominations) and numerous electrophilic additions. NBS reacts with HBr to form a low concentration of Bromine to facilitate the allylic bromination reaction.

NBS (N-Bromosuccinimide)

NBS (N-Bromosuccinimide)

What Is Allylic Substitution?

The allylic position is a carbon position present adjacent to the carbon-carbon double bonds. The allylic carbon is attached to a carbon atom that is forming a double bond with a different carbon atom. A substitution could be a reaction during which a group in an organic molecule is replaced with another group.

An allylic substitution could be a substitution reaction during which a group on allylic carbon in a chemical compound is replaced by another group.

Allylic Carbon

Allylic Carbon

NBS as a Reagent for Allylic Substitution Reaction 

NBS (N-Bromo succinimide) could be a gleaming white crystalline solid. The Carbon atom attached to the electron-withdrawing N of succinimide where N has a partial negative charge is nucleophilic. The chemical element, Bromine contains a partial positive charge and is thus electrophilic.

Allylic Bromination

Allylic Bromination

There are unit 2 major reactions NBS is employed for :

  1. Allylic bromination

  2. Replacement of Br2 within the formation of bromohydrin

Allylic bromination is the replacement of hydrogen on a carbon adjacent to a covalent bond (or aromatic ring, during which case it’s referred to as group bromination). NBS is employed as a substitute for Br2 in these cases since Br2 tends to react with double bonds to create dibromides. 

The advantage of NBS is that it provides a low-level concentration of Br2, and the bromination of the covalent bond doesn’t contend with the maximum amount.

Mechanism of Allylic Bromination with NBS

This reaction goes through a radical mechanism, and it's attention-grabbing to note the distinction with the anti-Markovnikov radical bromination: Once Br2 is made, the reaction takes very like alternative free-radical halogenation reactions: homolytic cleavage of the Br2 with lightweight or head (initiation), followed by abstraction of the allylic H (propagation step #1) and the ensuant reaction of this radical with another equivalent of Br2 to provide the specified product. The remaining Br radical then reacts with another equivalent of the organic compound during this chain reaction till the limiting chemical agent is consumed.

Step 1: The first step of allylic bromination is the homolytic cleavage of the N-Br bond (initiation) of the N-bromosuccinimide (NBS)

Homolytic Cleavage of N-Br Bond

Homolytic Cleavage of N-Br Bond.

The imide group  stabilizes the radical by two additional resonance structures which help to initiate the homolysis of the N-Br bond:

Resonance Stabilization of Succinimide Radical

Resonance Stabilization of Succinimide Radical.

Step 2: After this, the Br radical rip off an allylic H forming the corresponding allylic radical: 

Br Radical Abstracting the Allylic Hydrogen

Br Radical Abstracting the Allylic Hydrogen.

Step 3: The HBr produced in this step then reacts with NBS producing Br2 in low concentration. (initiation step)

Production of Br2

Production of Br2.

Step 4: In the next step, the Br2 is then quickly captured by the allylic radical, thus keeping the concentration of HBr and Br2 at a minimum suppressing the competing electrophilic addition to the double bond. (propagation step)

Allylic Substitution Reaction

Allylic Substitution Reaction

The process repeats until the termination step.

NBS as a Reagent For Bromohydrin Formation From Alkenes

NBS may also function as a substitute for Br2 within the formation of bromohydrins. Alkenes react with Br2 to create “bromonium ions “, which are 3-atom rings with a charge on the Br atom. Well, NBS will make bromonium ions with alkenes. Once water (or alcohol) is employed as a solvent, it'll attack the bromonium ion, leading to the formation of the bromohydrin.  Also, the stereochemistry is usually “trans“.

Formation of The Trans Product

Formation of The Trans Product.

The trans product is formed due to the nucleophilic attack on the face opposite to the bromonium ion.

Formation of Bromohydrin

Formation of Bromohydrin.


The allylic position is in addition to a vinylic position. The allylic carbon is bonded to an atom that's doubly bonded to a distinct atom. An allylic substitution may be a substitution reaction in which a substance on an allylic carbon in a compound is replaced by another substance. The supply of Br2 is that of the NBS, which, besides manufacturing the Br radical, generates an occasional concentration of Br2

Competitive Exams after 12th Science

FAQs on Allylic Substitution Reaction

1. What are nucleophilicity and the basic strength of nucleophiles?

Nucleophilicity is the ability of the nucleophiles to give their lone pairs to a positive centre. It's a kinetic term that relates to the speed at which the nucleophile attacks the substrates (R - LG). The subsequent factors are accustomed to comparing the nucleophilicity of various nucleophiles. The essential strength of nucleophiles is the ability to give lone pairs of electrons by a species. Each of the terms basic strength and nucleophilicity square measure reasonably similar and are connected directly. There's one distinction between these 2 terms, nucleophilicity could be a kinetic term, whereas basic strength could be a physical science term.

2. How will the NBS chemical agent work?

The major reaction of  NBS: is allylic bromination which is the most typical reaction, and also the alternative one is the replacement of Br2 for the formation of bromohydrins. Chemical group bromination could be a replacement of the H on a carbon atom present next to the covalent bond or to the aromatic ring that is termed group bromination since Br2 tends to react with alkenes to make dibromides. The utilization of NBS is that it provides a low-level concentration of Br2, and bromination of the olefin bond doesn't contend the maximum amount.

3. What is an electrophilic aromatic substitution reaction?

Electrophilic aromatic substitution reactions are the organic reactions during which an electrophile replaces an atom connected to an aromatic ring. The replacement of an atom from a benzene ring with an electrophile is concerned in these reactions. Examples like aromatic nitrations, aromatic sulphonation, and Friedel-Crafts reactions. In an electrophilic aromatic substitution reaction, the aromaticity of the aromatic system is preserved. The aromaticity of the aromatic ring isn't lost once bromobenzene is made from the reaction of aromatic hydrocarbons and  Bromine, as an example.