8.2 Halogenation of Alkenes: Addition of X2
3 min read•may 7, 2024
of is a key reaction in organic chemistry. It involves adding halogen molecules across carbon-carbon double bonds, forming new carbon-halogen bonds. This process occurs through a three-step mechanism, resulting in products with specific .
Understanding halogenation is crucial for grasping addition reactions and stereochemistry. It has applications in synthesis and occurs naturally in marine organisms. Factors like solvent choice and alkene structure influence the reaction's outcome, making it a versatile tool in organic synthesis.
Halogenation of Alkenes
Mechanism of alkene halogenation
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- Halogenation of alkenes involves the addition of a halogen molecule (, such as , , or ) across the carbon-carbon double bond
- Mechanism proceeds through three main steps:
- of the halogen to the alkene
- Halogen molecule acts as an and attacks the electron-rich double bond
- More electropositive halogen atom forms a bond with one alkene carbon, while the other halogen atom becomes a counterion (X-)
- Formation of a intermediate
- Positively charged halogen atom forms a three-membered ring with the two alkene carbons, creating a cyclic ( for Br2)
- Halonium ion is a bridged structure with the halogen atom bonded to both carbons
- Nucleophilic attack by the halide counterion
- Negatively charged halide counterion (X-) attacks the more substituted carbon of the halonium ion
- Opens the three-membered ring and forms the final vicinal dihalide product with the halogen atoms on adjacent carbons
- of the halogen to the alkene
- Addition of the halogen occurs in an anti fashion, with the halogen atoms adding to opposite faces of the planar alkene
Anti stereochemistry in cycloalkene reactions
- Halogen addition to results in a product with
- Diaxial orientation: Both halogen substituents are oriented in the axial position on the ring
- Anti stereochemistry: Halogen atoms are added to opposite faces of the cycloalkene plane
- Mechanism involves:
- Electrophilic addition of the halogen to form a halonium ion intermediate
- Nucleophilic attack by the halide counterion from the opposite face ()
- Results in the of the halogen atoms across the former double bond
- Anti stereochemistry is a consequence of the backside attack by the halide counterion
- Halonium ion intermediate locks the conformation of the ring, preventing bond rotation
- Halide counterion can only attack from the opposite face, leading to the diaxial, anti product
- Example: Bromination of cyclohexene yields with both bromine atoms in axial positions
Biological halogenation in marine organisms
- Many marine organisms (algae, sponges, corals) produce halogenated organic compounds
- Compounds often have antibacterial, antifungal, or antifouling properties for defense against predators or competitors
- Halogenation reactions in marine organisms are catalyzed by enzymes called
- Haloperoxidases use hydrogen peroxide (H2O2) to oxidize halide ions (X-) to (HOX)
- Hypohalous acids react with organic substrates (alkenes, aromatics) to form halogenated products
- Examples of halogenated compounds produced by marine organisms:
- and phenols in some red algae species
- and fatty acids in certain sponges and corals
- derivatives in some brown algae species
- Enzymatic halogenation reactions in marine organisms are highly selective and occur under mild conditions
- Contrasts with laboratory halogenation reactions that often require harsh conditions and lack selectivity
- Understanding enzymatic halogenation in marine organisms can inspire development of new, environmentally friendly halogenation methods for organic synthesis
Factors Affecting Halogenation Reactions
- Stereochemistry: The addition of halogens to alkenes results in anti addition, influencing the 3D arrangement of atoms in the product
- : The rate of halogenation is influenced by factors such as concentration, temperature, and the nature of the alkene and halogen
- : The choice of solvent can impact the reaction rate and product distribution in halogenation reactions
- : In unsymmetrical alkenes, the halogen addition may preferentially occur at one carbon over the other
- intermediates: In some cases, carbocations can form as intermediates, leading to rearrangements or side products
- Role of nucleophiles and electrophiles: The halogen acts as an electrophile in the initial step, while the halide ion acts as a in the final step of the mechanism
Key Terms to Review (31)
Alkenes: Alkenes are a class of unsaturated organic compounds characterized by the presence of a carbon-carbon double bond. They are an important functional group in organic chemistry, with a wide range of applications and reactivity. Alkenes are closely related to the topics of chirality, isomerism, electrophilic addition reactions, halogenation, hydration, the E2 reaction, infrared spectroscopy, 13C NMR spectroscopy, alcohol preparation, and the Wittig reaction.
Anti Addition: Anti addition refers to the stereochemical outcome of an electrophilic addition reaction, where the incoming electrophilic species adds to the opposite face of the alkene or alkyne relative to the existing substituents. This results in the formation of the anti-addition product, where the new substituents are arranged in an anti-configuration.
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.
Backside Attack: A backside attack is a type of nucleophilic substitution reaction where the attacking nucleophile approaches the carbon atom from the opposite side of the leaving group. This orientation of the attack is a key characteristic that distinguishes the SN2 reaction mechanism from the SN1 reaction mechanism.
Br2: Br2, or bromine, is a diatomic halogen element that plays a crucial role in the addition reactions of alkenes, specifically in the processes of halogenation and halohydrin formation.
Brominated Indoles: Brominated indoles are a class of organic compounds that contain a bromine atom attached to an indole ring structure. These compounds are commonly used in various applications, including pharmaceuticals, agrochemicals, and materials science.
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.
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.
Carbocation: A carbocation is a positively charged carbon atom that is part of an organic molecule. These reactive intermediates play a crucial role in various organic reactions, including electrophilic additions, nucleophilic substitutions, and elimination reactions.
Chlorinated Terpenes: Chlorinated terpenes are a class of organic compounds that consist of terpene structures with one or more chlorine atoms attached. These compounds are often found in natural sources and can exhibit a variety of biological and chemical properties.
Cl2: Cl2, or chlorine gas, is a highly reactive diatomic molecule composed of two chlorine atoms. It is a key reactant in the halogenation of alkenes, as well as the formation of halohydrins from alkenes through the addition of HO-X.
Cycloalkenes: Cycloalkenes are a class of organic compounds that consist of a cyclic ring structure with at least one carbon-carbon double bond. These molecules are closely related to both cycloalkanes and alkenes, combining the cyclic nature of the former with the unsaturated character of the latter.
Diaxial: Diaxial refers to the orientation of substituents in a cyclohexane ring where they are positioned along the axis of the ring, rather than in the equatorial plane. This term is particularly relevant in the context of halogenation of alkenes through the addition of X2.
Electrophile: An electrophile is a species that is attracted to electron-rich regions and seeks to form new bonds by accepting electron density. Electrophiles play a crucial role in many organic reactions, including polar reactions, electrophilic aromatic substitution, and nucleophilic acyl substitution, among others.
Electrophilic Addition: Electrophilic addition is a type of organic reaction where an electrophile, a species that is attracted to electrons, adds to the carbon-carbon double bond of an alkene. This results in the formation of a new carbon-carbon single bond and the incorporation of the electrophile into the molecule.
Electrophilic addition reaction: An electrophilic addition reaction is a chemical process in which an electrophile reacts with a nucleophile, typically an alkene or alkyne, forming a new sigma bond by adding across the double or triple bond. This reaction is key in organic synthesis, resulting in the addition of atoms or groups to the carbon atoms involved in the multiple bond.
Halogenation: Halogenation is the process of introducing a halogen atom (fluorine, chlorine, bromine, or iodine) into an organic compound, typically through a substitution or addition reaction. This term is closely tied to various topics in organic chemistry, including functional groups, alkane properties, reaction mechanisms, and the reactivity of different classes of organic compounds.
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.
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.
Haloperoxidases: Haloperoxidases are a class of enzymes that catalyze the addition of halogens, such as chlorine, bromine, or iodine, to organic compounds. They play a crucial role in the halogenation of alkenes, specifically in the context of the addition of X2 (where X is a halogen) to alkenes.
Hypohalous Acids: Hypohalous acids are a class of weak acids formed when a halogen (such as chlorine, bromine, or iodine) reacts with water. They play a crucial role in the halogenation of alkenes through the addition of X2, as described in topic 8.2 Halogenation of Alkenes.
I2: I2, or diatomic iodine, is a chemical compound consisting of two iodine atoms bonded together. It is a key element in the context of the addition of halogens to alkenes, as well as the formation of halohydrins from alkenes.
Iodinated Tyrosine: Iodinated tyrosine refers to the addition of iodine atoms to the amino acid tyrosine. This process is particularly relevant in the context of the halogenation of alkenes, specifically the addition of dihalogen molecules (X2) to carbon-carbon double bonds.
Nucleophile: A nucleophile is a species that donates a pair of electrons to form a covalent bond with another atom or molecule. Nucleophiles are central to understanding many organic reactions, including polar reactions, electrophilic addition reactions, and nucleophilic substitution reactions.
Reaction Kinetics: Reaction kinetics is the study of the rates and mechanisms of chemical reactions. It examines the factors that influence the speed and efficiency of a reaction, such as temperature, pressure, and the presence of catalysts. This concept is crucial in understanding organic reactions, as the rate and pathway of a reaction can have a significant impact on the products formed and the overall efficiency of the process.
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.
Solvent Effects: Solvent effects refer to the influence that the surrounding solvent environment can have on the behavior and properties of chemical reactions, molecules, and spectroscopic measurements. The nature and polarity of the solvent can significantly impact the energetics, kinetics, and outcomes of various organic chemistry processes.
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.
Trans-1,2-dibromocyclohexane: trans-1,2-dibromocyclohexane is a cyclic organic compound with two bromine atoms attached to adjacent carbon atoms in a trans configuration on a cyclohexane ring. It is a key intermediate in the halogenation of alkenes through the addition of X2 (where X = halogen).
Vicinal Dihalide: A vicinal dihalide is a compound that contains two halogen atoms (such as chlorine, bromine, or iodine) attached to adjacent carbon atoms in an organic molecule. These compounds are important intermediates in various organic reactions, particularly in the preparation and reactions of alkynes.
X2: X2 refers to the diatomic halogen molecules, such as chlorine (Cl2), bromine (Br2), and iodine (I2), which are commonly involved in addition reactions with alkenes and alkynes. These halogen molecules can add across the carbon-carbon double or triple bonds, introducing new functional groups and altering the structure of the organic compounds.