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Organic Chemistry
Table of Contents
Unit 10 Overview: Organohalides
10.1 Names and Structures of Alkyl Halides
10.2 Preparing Alkyl Halides from Alkanes: Radical Halogenation
10.3 Preparing Alkyl Halides from Alkenes: Allylic Bromination
10.4 Stability of the Allyl Radical: Resonance Revisited
10.5 Preparing Alkyl Halides from Alcohols
10.6 Reactions of Alkyl Halides: Grignard Reagents
10.7 Organometallic Coupling Reactions
10.8 Oxidation and Reduction in Organic Chemistry

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10.3 Preparing Alkyl Halides from Alkenes: Allylic Bromination

Citation:

Allylic bromination is a unique reaction that introduces bromine next to a double bond. It uses N-bromosuccinimide and a radical initiator, selectively targeting the allylic position due to the stability of the allylic radical intermediate.

This process stands out from other halogenation methods by its specificity. Unlike electrophilic addition or direct bromination, which add across the double bond, allylic bromination maintains the alkene while functionalizing the adjacent carbon.

Allylic Bromination

Process of allylic bromination

  • Allylic bromination selectively introduces a bromine atom at the allylic position of an alkene (position adjacent to the double bond)
  • Uses N-bromosuccinimide (NBS) as the brominating agent and a radical initiator (benzoyl peroxide or light)
  • Typical reaction conditions:
    • Alkene substrate and NBS (1 equiv.) in CCl4 solvent
    • Catalytic amount of benzoyl peroxide or irradiation with light
    • Heating to reflux temperature (77°C for CCl4)
  • Mechanism:
    1. Initiation: Homolytic cleavage of NBS by heat or light generates bromine radical (Br•)
    2. Hydrogen abstraction: Br• selectively abstracts an allylic hydrogen from the alkene, forming a resonance-stabilized allylic radical
    3. Bromine transfer: The allylic radical reacts with another equivalent of NBS, leading to the allylic bromide product and regenerating Br• to propagate the radical chain reaction
    4. Termination: Two radicals combine or disproportionate to terminate the chain reaction

Selectivity in allylic bromination

  • Bromination occurs exclusively at the allylic position due to the enhanced stability of the allylic radical intermediate compared to other possible radical intermediates
  • Bond dissociation energies (BDEs) of C-H bonds:
    • Allylic C-H: ~87 kcal/mol
    • Alkyl C-H: ~98 kcal/mol
    • Vinyl C-H: ~108 kcal/mol
  • Lower BDE of the allylic C-H bond makes it easier for Br• to abstract an allylic hydrogen compared to alkyl or vinyl hydrogens
  • Allylic radical stability:
    • The unpaired electron in the allylic radical is delocalized through resonance, leading to increased stability
    • The allylic radical can be represented by two resonance structures, distributing the unpaired electron across the $\pi$ system
  • Radicals formed from abstraction of alkyl or vinyl hydrogens do not benefit from resonance stabilization and are less stable

Allylic bromination vs other alkyl halide preparations

  • Electrophilic addition of HX (X = Cl, Br, I):
    • Adds the elements of HX across the double bond, leading to a vicinal dihalide product (1,2-dihalide)
    • Proceeds through a halonium ion intermediate and follows Markovnikov's rule (major product has halogen on more substituted carbon)
    • Does not exhibit the selectivity for the allylic position observed in allylic bromination
  • Bromination with Br2:
    • Adds Br2 across the double bond, leading to a vicinal dibromide product (1,2-dibromide)
    • Proceeds through a bromonium ion intermediate and follows the anti stereochemistry (bromines added on opposite faces of the double bond)
    • Does not exhibit the selectivity for the allylic position observed in allylic bromination
  • Allylic bromination offers a unique approach to selectively introduce a bromine atom at the allylic position, which is not possible with other electrophilic addition reactions

Mechanistic considerations and product formation

  • Allylic bromination is a free radical reaction, which distinguishes it from ionic reactions like electrophilic addition
  • The regioselectivity of allylic bromination is influenced by the relative carbon-hydrogen bond strengths at different positions
  • Resonance stabilization of the allylic radical intermediate plays a crucial role in determining the reaction outcome
  • The stereochemistry of the product can be affected by the planar nature of the allylic radical, potentially leading to a mixture of stereoisomers

Key Terms to Review (30)

Halonium ion: A halonium ion is an intermediate species formed during the addition of a halogen (X2) to an alkene, characterized by a three-membered ring structure consisting of two carbon atoms and one halogen atom. This positively charged ion is highly reactive and plays a crucial role in facilitating the addition of halogens across the double bond of alkenes.
Allylic: In organic chemistry, an allylic position refers to the next carbon atom adjacent to a double bond in an alkene. This position is significant due to its unique reactivity, particularly in the context of allylic bromination where it can be selectively halogenated.
Anti stereochemistry: Anti stereochemistry describes the spatial arrangement in a chemical reaction where two substituents are positioned on opposite sides of a double bond or ring structure after the reaction. It is particularly relevant in the halogenation of alkenes, resulting in products where the added atoms are located across from each other.
Bromonium ion: A bromonium ion is a reactive intermediate formed during the halogenation of alkenes when a bromine molecule reacts with an alkene to form a cyclic structure where the bromine atom is covalently bonded to two carbon atoms. This ion is positively charged and highly electrophilic, making it susceptible to nucleophilic attack.
Bond dissociation energy, D: Bond dissociation energy is the amount of energy required to break a bond between two atoms in a molecule into two separate, radical species. It is measured in kilojoules per mole (kJ/mol) and varies depending on the type of bond and the molecules involved.
Stereochemistry: Stereochemistry is the study of the three-dimensional arrangement of atoms in molecules and how this arrangement affects the chemical and physical properties of the substance. It examines the spatial orientation of atoms and their relationship to one another, which is crucial in understanding many organic chemistry concepts.
Bond Dissociation Energy: Bond dissociation energy is the amount of energy required to break a specific chemical bond between two atoms, separating them into individual, free atoms. This term is crucial in understanding the stability and reactivity of molecules, as well as the energetics of chemical reactions.
Resonance: Resonance is a fundamental concept in organic chemistry that describes the ability of certain molecules to exist in multiple equivalent structures or resonance forms. This phenomenon arises from the delocalization of electrons within the molecule, leading to the stabilization of the overall structure and the distribution of electron density across multiple atoms.
Resonance Stabilization: Resonance stabilization is a phenomenon where the delocalization of electrons in a molecule or ion leads to a more stable configuration compared to a single Lewis structure. This concept is crucial in understanding the behavior and properties of various organic compounds, including their acidity, basicity, reactivity, and stability.
Bromonium Ion: The bromonium ion is a cyclic, three-membered ring intermediate formed during the addition of hydrobromic acid (HBr) or bromine (Br2) to alkenes. It serves as a key intermediate in various organic reactions involving the electrophilic addition of bromine to alkenes.
Regioselectivity: Regioselectivity refers to the preference of a chemical reaction to occur at a specific site or region of a molecule, leading to the formation of one regioisomeric product over another. This concept is particularly important in the context of electrophilic addition reactions of alkenes, electrophilic aromatic substitution, and other organic transformations.
Markovnikov's Rule: Markovnikov's rule is a principle in organic chemistry that describes the orientation of addition reactions involving unsaturated compounds, such as alkenes. It states that in the addition of a hydrogen halide (HX) to an alkene, the hydrogen atom of the HX bond attaches to the carbon atom of the alkene that can best stabilize the resulting carbocation intermediate.
Hydrogen Abstraction: Hydrogen abstraction is a fundamental reaction in organic chemistry where a hydrogen atom is removed from a molecule, often by a reactive species such as a radical or a base. This process is central to various topics in organic chemistry, including radical reactions, biological additions of radicals to alkenes, and the preparation of alkyl halides from alkanes and alkenes.
Homolytic Cleavage: Homolytic cleavage refers to the breaking of a covalent bond in a molecule in a way that results in the formation of two neutral radical species, each retaining one of the shared electrons from the original bond. This process is a key feature in radical reactions and is central to understanding the industrial preparation and use of alkenes, as well as the preparation of alkyl halides from both alkanes and alkenes.
Alkyl Halides: Alkyl halides are organic compounds that consist of an alkyl group (a hydrocarbon chain) bonded to a halogen atom (fluorine, chlorine, bromine, or iodine). They are widely used in organic synthesis and have various applications in chemistry and biology.
Halonium Ion: A halonium ion is a reactive intermediate formed during the electrophilic addition of halogens (X2, where X = F, Cl, Br, I) to alkenes. It is a cyclic, three-membered ring structure that contains a positive charge on one of the carbon atoms and a halogen atom attached to the other two carbon atoms.
Anti-Markovnikov Addition: Anti-Markovnikov addition is a type of electrophilic addition reaction that occurs when a hydrogen-containing molecule, such as water or hydrogen halide, adds to an alkene or alkyne in a way that places the hydrogen on the less substituted carbon. This is in contrast to the Markovnikov addition, which places the hydrogen on the more substituted carbon.
Carbocation Intermediate: A carbocation intermediate is a positively charged carbon atom that acts as a reactive species in various organic chemistry reactions. These intermediates are formed during the course of a reaction and play a crucial role in determining the outcome and mechanism of the transformation.
N-bromosuccinimide: N-bromosuccinimide (NBS) is a versatile organic reagent used in various chemical reactions, particularly in the context of alkene functionalization, aromatic substitution, and oxidation of aromatic compounds. It serves as a source of electrophilic bromine and is commonly employed in reactions such as halohydrin formation, allylic bromination, and aromatic substitution.
Radical Mechanism: A radical mechanism is a type of reaction pathway in organic chemistry where the reactive intermediates involved are free radicals. Free radicals are species with unpaired electrons, making them highly reactive and capable of initiating a chain reaction. Radical mechanisms are particularly relevant in the context of allylic bromination and alpha bromination of carboxylic acids.
Wohl-Ziegler Reaction: The Wohl-Ziegler reaction is a method for the allylic bromination of alkenes, where a bromine atom is introduced at the carbon adjacent to the carbon-carbon double bond. This reaction is an important tool in organic synthesis for the preparation of alkyl halides from alkenes.
Benzoyl Peroxide: Benzoyl peroxide is an organic compound that is widely used as an oxidizing agent, a bleaching agent, and a treatment for acne. It is a white, crystalline solid that decomposes when exposed to heat or light, producing oxygen and other byproducts.
Allylic Bromination: Allylic bromination is a chemical reaction where a bromine atom is added to the carbon atom adjacent to a carbon-carbon double bond in an alkene molecule. This process is used to prepare alkyl halides from alkenes.
Radical Initiator: A radical initiator is a chemical species that triggers the initiation of a radical chain reaction by generating free radicals. These reactive species are essential in various organic reactions, including the preparation of alkyl halides from alkenes through allylic bromination.
Allylic Radical: An allylic radical is a reactive intermediate in organic chemistry that forms when a hydrogen atom is abstracted from the carbon atom adjacent to a carbon-carbon double bond. This reactive species is important in the context of allylic bromination reactions, where it is a key intermediate step.
Free Radical Reaction: A free radical reaction is a type of chemical reaction in which highly reactive, unpaired electron-containing species known as free radicals are involved. These free radicals initiate a chain reaction that propagates through the reactants, leading to the formation of new products.
Carbon-Hydrogen Bond Strength: The carbon-hydrogen bond strength refers to the stability and energy required to break the covalent bond between a carbon atom and a hydrogen atom. This bond strength is a crucial factor in understanding various organic chemistry reactions, including the preparation of alkyl halides from alkenes through allylic bromination.
Radical Chain Reaction: A radical chain reaction is a type of chemical reaction mechanism characterized by the propagation of highly reactive intermediates called free radicals. These reactions often occur in the context of preparing alkyl halides from alkenes, specifically through the process of allylic bromination.
Bromine Radical: A bromine radical is a highly reactive chemical species containing a single, unpaired electron. Bromine radicals play a crucial role in the context of preparing alkyl halides from alkenes through allylic bromination, as well as in the oxidation of aromatic compounds.
Allylic: Allylic refers to a specific position in an organic molecule, particularly in relation to alkenes, where a substituent is bonded to a carbon atom adjacent to the carbon-carbon double bond. This positioning is important because allylic compounds often undergo unique chemical reactions, such as allylic bromination, where bromine can be added selectively at the allylic position. The allylic site is also significant in terms of stability and reactivity, influencing the mechanisms of various reactions.