Allylic/Benzylic Bromination With N-Bromo Succinimide (NBS)

Professor Dave Explains
20 Nov 202004:07
EducationalLearning
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TLDRThe script discusses a selective bromination technique at allylic or benzylic positions, using N-bromo succinimide (NBS) as the reagent. It explains that NBS is preferred over molecular bromine to avoid unwanted dibromination with pi bonds. The mechanism involves the generation of bromine radicals in the presence of hydrobromic acid, which then react with the allylic substrate to form a stable allylic radical, leading to the formation of a carbon-bromine bond. The key is to maintain a minimal concentration of species that react with pi bonds, allowing for successful bromination at the desired positions.

Takeaways
  • 🔍 Allylic and benzylic positions are key targets for selective bromination, referring to carbons adjacent to double bonds or benzene rings, respectively.
  • 🌐 The process of selective bromination at these positions is distinct from radical halogenation of alkanes.
  • 💧 N-bromo succinimide (NBS) is the reagent of choice for allylic or benzylic bromination, as opposed to molecular bromine.
  • 🔬 NBS works in conjunction with a hydrobromic acid catalyst to generate bromine radicals, avoiding unwanted dibromination of pi bonds.
  • ⚛️ The mechanism involves the formation of a bromine radical that abstracts a hydrogen atom from the allylic position, creating an allylic radical.
  • 🌀 Resonance stabilization is crucial for the allylic radical, similar to how an allylic cation is stabilized.
  • 🔗 The allylic radical reacts with a bromine molecule to form a new carbon-bromine bond, propagating the reaction.
  • 🚫 Controlling the concentration of reactive species like hydrobromic acid and molecular bromine is essential to prevent side reactions.
  • 🔄 Homolysis of bromine must occur before it can react with pi bonds, allowing selective bromination to take place.
  • 🌀 The reaction continues until all NBS has been consumed, ensuring complete bromination at the desired positions.
  • 📚 Understanding the mechanism and reagent selection is vital for successful selective bromination at allylic or benzylic positions.
Q & A

  • What is radical halogenation and how is it used in the context of alkanes?

    -Radical halogenation is a chemical reaction where a halogen atom is added to an alkane, typically using a radical mechanism. It is useful for introducing halogens into alkanes, which can then be used in further reactions or serve as functional groups.

  • Why is it easier to predict the bromination site on an alkane compared to other molecules?

    -Bromination on alkanes is easier to predict because the reaction tends to occur at the less substituted carbon, following Markovnikov's rule, which states that in the addition of H-X to an alkene, the hydrogen atom is added to the carbon with more hydrogen atoms.

  • What are the allylic and benzylic positions in the context of bromination?

    -The allylic position refers to the carbon adjacent to a double bond in an alkene, while the benzylic position refers to the carbon adjacent to a benzene ring. These positions are reactive sites for bromination in molecules containing these structural elements.

  • What is the role of N-bromo succinimide (NBS) in the bromination of allylic or benzylic positions?

    -NBS is a reagent used for selective bromination at allylic or benzylic positions. It helps to avoid the formation of unwanted dibrominated products that can occur with the use of molecular bromine on molecules with pi bonds.

  • How does the structure of N-bromo succinimide contribute to its function in bromination reactions?

    -The structure of NBS, with a bromo group attached to the nitrogen atom of succinimide, allows it to generate bromine radicals in the presence of a catalyst like hydrobromic acid. These radicals are responsible for the bromination at the allylic or benzylic positions.

  • Why is it important to minimize the concentration of hydrobromic acid and molecular bromine in the bromination reaction using NBS?

    -Minimizing their concentration ensures that there is enough time for the bromine to undergo homolysis and form radicals before interacting with pi bonds, which would lead to undesired dibromination reactions.

  • What is the significance of resonance stabilization in the formation of an allylic radical?

    -Resonance stabilization is significant because it allows the electron deficiency created by the loss of a hydrogen atom at the allylic position to be delocalized over the adjacent pi bond, making the formation of the allylic radical energetically favorable.

  • How does the bromination at the allylic position proceed after the formation of an allylic radical?

    -After the formation of an allylic radical, it reacts with a bromine molecule, leading to the formation of a new carbon-bromine bond and the regeneration of a bromine radical, thus propagating the bromination reaction.

  • What is the role of homolysis in the bromination reaction using NBS?

    -Homolysis is the process where a bromine molecule is split into two bromine radicals. This step is crucial because it provides the reactive species necessary for the bromination of the allylic or benzylic positions.

  • How does the bromination reaction using NBS ensure selectivity for allylic or benzylic positions over other sites on the molecule?

    -The selectivity is achieved by using NBS, which generates bromine radicals in a controlled manner, allowing them to react preferentially with the allylic or benzylic positions due to their higher reactivity and stabilization through resonance.

  • What are the key factors that contribute to the success of the bromination reaction at the allylic position using NBS?

    -The key factors include the controlled generation of bromine radicals, the resonance stabilization of the allylic radical, and the minimization of competing reactions with pi bonds by keeping the concentration of hydrobromic acid and molecular bromine low.

Outlines
00:00
🧪 Allylic and Benzylic Bromination with NBS

This paragraph introduces the concept of allylic and benzylic bromination, a specific type of chemical reaction where a bromine atom is added to the allylic (adjacent to a double bond) or benzylic (adjacent to a benzene ring) position of a molecule. The reagent used for this purpose is N-bromo succinimide (NBS), which is preferred over molecular bromine to avoid unwanted addition reactions with pi bonds. The mechanism involves the production of bromine radicals in the presence of a hydrobromic acid catalyst, which then react with the hydrogen at the allylic position to form a radical. This radical is stabilized by resonance with the pi bond. The interaction between the allylic radical and a bromine molecule results in the formation of a carbon-bromine bond, propagating another bromine radical and successfully achieving selective bromination at the desired position.

Mindmap
Keywords
💡Radical Halogenation
Radical halogenation is a chemical reaction where a halogen atom is added to an alkane through a radical mechanism. It is central to the video's theme as it sets the stage for discussing specific types of halogenation. In the script, radical halogenation is mentioned as a known process useful for alkanes, highlighting its relevance to the broader topic of halogenation reactions.
💡Bromination
Bromination refers to the chemical process where a bromine atom is added to a molecule. It is a key concept in the video, as the script discusses bromination at specific positions such as allylic or benzylic. The script specifically mentions that bromination can be predictable when it comes to alkanes, emphasizing its significance in the context of the video.
💡Allylic Position
The allylic position denotes the carbon atom adjacent to a double bond in an alkene. This term is crucial to the video's narrative as it defines where bromination can occur in the presence of a double bond. The script uses the term to illustrate the selectivity of the bromination process, showing that the allylic carbon is a preferred site for the reaction.
💡Benzylic Position
The benzylic position is the carbon atom adjacent to a benzene ring. This concept is integral to the video's content, as it identifies another specific site for bromination. The script explains that bromination at the benzylic position is possible and is part of the discussion on selective bromination reactions.
💡N-Bromo Succinimide (NBS)
NBS is a reagent used for selective bromination at allylic or benzylic positions. It is a key component of the video's discussion on bromination, as the script describes its structure and function. NBS is highlighted as a preferred reagent over molecular bromine to avoid undesired addition reactions with pi bonds.
💡Homolysis
Homolysis is the process where a covalent bond is broken, resulting in the formation of free radicals. In the context of the video, homolysis is essential for the bromination mechanism involving NBS, where the script explains that molecular bromine undergoes homolysis to form bromine radicals before interacting with the substrate.
💡Allylic Radical
An allylic radical is a carbon-centered radical formed at the allylic position. The video script describes the formation of this radical during the bromination process, emphasizing its stability due to resonance with the pi bond. This concept is vital for understanding the selectivity and mechanism of the bromination reaction discussed.
💡Resonance Stabilization
Resonance stabilization is a phenomenon where the electron deficiency in a molecule is delocalized, leading to increased stability. The script mentions this concept in relation to the allylic radical, explaining how the radical is stabilized by resonance with the adjacent pi bond, which is crucial for the successful bromination at the allylic position.
💡Molecular Bromine
Molecular bromine refers to bromine in its diatomic molecular form (Br2). The script contrasts NBS with molecular bromine, noting that while bromine is reactive with pi bonds, leading to dibromination, NBS allows for selective bromination without addition reactions, which is central to the video's theme of controlled bromination.
💡Hydrobromic Acid Catalyst
Hydrobromic acid is used as a catalyst in the bromination process to generate small amounts of molecular bromine. The script explains its role in the mechanism, where it facilitates the production of bromine radicals necessary for the reaction. This concept is important for understanding the controlled generation of radicals in the presence of NBS.
💡Radical Chemistry
Radical chemistry involves reactions that proceed through the formation and reactions of radicals. The video script discusses how the bromination at allylic or benzylic positions is a radical chemistry process, highlighting the importance of radicals in the selective bromination mechanism. This concept is central to the video's explanation of the bromination process.
Highlights

Radical halogenation is useful for halogenating alkanes.

Bromination can be predicted on alkanes.

Bromination at allylic or benzylic positions is possible on molecules other than alkanes.

Allylic position refers to the carbon adjacent to a double bond.

Benzylic position refers to the carbon adjacent to a benzene ring.

N-bromo succinimide (NBS) is used for selective bromination at allylic or benzylic positions.

NBS has a bromo group on the nitrogen atom of succinimide.

Molecular bromine tends to cause dibromination with pi bonds, which is avoided with NBS.

In the presence of hydrobromic acid catalyst, small amounts of molecular bromine are produced.

Homolysis of bromine occurs before it can interact with pi bonds.

Bromine radicals are generated in solution.

Bromine radical interacts with the hydrogen at the allylic position, generating HBr and an allylic radical.

Allylic radical is resonance stabilized by the pi bond.

Allylic radical reacts with bromine molecule to form a new carbon-bromine bond.

The reaction continues until all NBS is consumed.

Key to the reaction is minimizing concentration of species that react with pi bonds.

Homolysis and radical chemistry are essential for selective bromination.

Transcripts

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Related Tags
Allylic BrominationBenzylic BrominationNBS ChemistrySelective HalogenationChemical ReactionsOrganic SynthesisRadical ChemistryHalogenation TechniquesChemical MechanismMolecular Brominated