16.1 Electrophilic Aromatic Substitution Reactions: Bromination
2 min read•may 7, 2024
of is a classic example of . This reaction showcases how aromatic compounds react with electrophiles, replacing a hydrogen atom with a atom while maintaining the ring's .
The mechanism involves a catalyst, forming a that attacks the benzene ring. This creates a -stabilized before regenerating the aromatic system. Understanding this process is key to grasping aromatic reactivity and stability.
Electrophilic Aromatic Substitution: Bromination
Mechanism of benzene bromination
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- (EAS) reaction where an replaces a hydrogen on an aromatic ring
- Bromination of benzene specific example of EAS ()
- Bromine () acts as the electrophile
- catalyst () required for reaction to proceed
- Mechanism steps:
- reacts with forming an electrophilic () and
- electrophilically attacks benzene forming a resonance-stabilized intermediate ()
- removes a proton from the arenium ion regenerating the aromatic system and producing and
- The catalyst () regenerated allowing the reaction to continue
- The electrophilic attack on the benzene ring is the rate-determining step of the reaction
Alkenes vs aromatic electrophilic reactions
- Electrophilic addition to alkenes:
- Alkenes more reactive than aromatic compounds due to higher electron density and lack of resonance stabilization
- Reaction typically with lower
- Proceeds through a carbocation intermediate not stabilized by resonance
- Electrophilic aromatic substitution:
- Aromatic compounds less reactive than alkenes due to resonance stabilization
- Reaction typically with higher activation energy
- Proceeds through a resonance-stabilized arenium ion intermediate
- Requires a strong electrophile and often a catalyst to overcome the aromatic stabilization energy
Substitution products in aromatic systems
- Aromatic compounds highly stable due to cyclic, planar structure and continuous overlapping p-orbitals
- Allows for delocalization of electrons resulting in resonance stabilization
- In EAS, aromatic system temporarily disrupted during formation of arenium ion intermediate
- Resonance stabilization of arenium ion lowers energy of this intermediate
- Final step of EAS involves restoration of aromaticity through loss of a proton
- Energetically favored step as it regenerates the stable aromatic system
- Addition products would result in permanent loss of aromaticity and resonance stabilization
- Energetically unfavorable compared to which maintains aromatic character of the compound
Substituent effects on EAS reactions
- Substituents on the aromatic ring can affect the reaction in various ways:
- increase the reactivity of the ring towards electrophilic attack
- decrease the reactivity of the ring towards electrophilic attack
- Substituents can also influence , directing the incoming electrophile to specific positions on the ring
- These effects are important in predicting the outcome and of EAS reactions
Key Terms to Review (28)
Activating Groups: Activating groups are substituents on an aromatic ring that increase the reactivity of the ring towards electrophilic aromatic substitution reactions. These groups facilitate the addition of electrophiles to the aromatic system by stabilizing the intermediate carbocation formed during the reaction.
Activation Energy: Activation energy is the minimum amount of energy required to initiate a chemical reaction. It represents the energy barrier that reactants must overcome in order to form products. This concept is central to understanding the mechanisms and kinetics of organic reactions.
Activation energy, ΔG‡: Activation energy (ΔG‡) is the minimum amount of energy required to initiate a chemical reaction, specifically the energy needed to reach the transition state from the reactants. It's a crucial factor in determining the rate at which a reaction will occur in organic chemistry.
Addition Product: An addition product is a compound formed through the addition of two or more reactants, typically across a carbon-carbon double bond, resulting in the saturation of the double bond and the formation of a new single bond.
Arenium Ion: The arenium ion, also known as the arene cation, is a positively charged intermediate species that arises during electrophilic aromatic substitution reactions. It is a key concept in understanding the mechanisms of various aromatic substitution reactions, including bromination, other aromatic substitutions, and the Friedel-Crafts reactions.
Aromaticity: Aromaticity is a fundamental concept in organic chemistry that describes the unique stability and reactivity of certain cyclic compounds with delocalized pi electron systems. This term is central to understanding the structure, stability, and reactivity of a wide range of organic compounds, including benzene and other aromatic heterocycles.
Benzene: Benzene is a planar, aromatic hydrocarbon compound with the chemical formula C6H6. It is a key building block in organic chemistry and has a unique resonance structure that contributes to its stability and reactivity.
Bromination: Bromination is a chemical reaction in which bromine atoms are introduced into organic compounds, often in the context of electrophilic aromatic substitution reactions or reactions of phenols.
Bromine: Bromine is a dense, reddish-brown liquid halogen element that is highly reactive and widely used in organic chemistry. It is particularly relevant in the context of electrophilic addition reactions of alkenes, the naming and structures of alkyl halides, and electrophilic aromatic substitution reactions involving bromination.
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.
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.
Deactivating groups: In the context of electrophilic aromatic substitution reactions, deactivating groups are substituents attached to a benzene ring that decrease the reactivity of the ring towards an electrophile. They accomplish this by either withdrawing electron density from the ring or by being meta-directing in orientation.
Deactivating Groups: Deactivating groups are substituents on an aromatic ring that decrease the reactivity of the ring towards electrophilic aromatic substitution reactions. These groups make the aromatic ring less susceptible to further substitution by electrophiles.
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 aromatic substitution: Electrophilic aromatic substitution is a chemical reaction in which an atom, typically hydrogen, attached to an aromatic system, such as benzene, is replaced by an electrophile. This process preserves the aromaticity of the compound while introducing a functional group.
Electrophilic Aromatic Substitution: Electrophilic aromatic substitution is a fundamental organic reaction in which an electrophile (a species that is attracted to electrons) replaces a hydrogen atom on an aromatic ring, resulting in the formation of a new carbon-electrophile bond. This reaction is crucial in understanding the behavior and reactivity of aromatic compounds, which are prevalent in many organic molecules and have widespread applications.
Endothermic: An endothermic process is a chemical reaction or physical change that absorbs energy from the surroundings in the form of heat. This term is particularly relevant in the context of describing reaction equilibria, rates, and energy changes, as well as understanding electrophilic aromatic substitution reactions involving bromination.
Exothermic: Exothermic refers to a chemical reaction or process that releases energy in the form of heat to the surrounding environment. These reactions release more energy than they absorb, resulting in an overall decrease in the system's internal energy.
FeBr3: FeBr3, or ferric bromide, is a chemical compound consisting of one iron (Fe) atom bonded to three bromine (Br) atoms. It is an important reagent in the context of electrophilic aromatic substitution reactions, particularly in the bromination of aromatic compounds.
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.
Lewis acid: A Lewis acid is a chemical species that accepts an electron pair from another molecule during the formation of a covalent bond. Unlike the traditional concept of acids and bases, which focuses on proton donors and acceptors, the Lewis definition emphasizes the role of electron pair donors and acceptors.
Lewis Acid: A Lewis acid is an electron pair acceptor that can form a coordinate covalent bond with a Lewis base, which is an electron pair donor. Lewis acids play a crucial role in various organic chemistry reactions, including electrophilic aromatic substitution, the Friedel-Crafts reaction, and cyanohydrin formation.
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.
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.
Substitution Product: A substitution product is the result of a chemical reaction where an atom or functional group in a molecule is replaced by a different atom or functional group. This is a key concept in the context of electrophilic aromatic substitution reactions, specifically the bromination of aromatic compounds.