6.4 An Example of a Polar Reaction: Addition of HBr to Ethylene
2 min read•may 7, 2024
addition to alkenes is a key reaction in organic chemistry. It's all about how a hydrogen and bromine atom attach to a carbon-carbon , following a specific pattern called ###'s_Rule_0###.
This reaction showcases the reactivity difference between alkenes and alkanes. Alkenes, with their electron-rich double bonds, are much more reactive than alkanes, making them prime targets for reactions like this one.
Addition of HBr to Alkenes
Mechanism of HBr addition to ethylene
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- adds H and Br across the
- Step 1: of the alkene
- Electrophilic H from HBr attacks the electron-rich double bond forming a intermediate
- Planar structure with a positive charge on a carbon, two substituents and an empty p orbital
- Electrophilic H from HBr attacks the electron-rich double bond forming a intermediate
- Step 2: by Br
- Lone pair of electrons on Br attacks the carbocation and bonds to it, forming the final product
- follows Markovnikov's rule
- H adds to the less substituted carbon of the double bond
- Br adds to the more substituted carbon, stabilizing the carbocation intermediate
- : The addition of HBr to results in an anti-addition product
Reactivity of alkenes vs alkanes
- Alkenes have a high electron density in the double bond with two shared electron pairs between the carbons, making the electrons more exposed and accessible
- Alkanes have only single bonds where electrons are less accessible and more tightly held
- Electrophiles (H) are attracted to the electron-rich double bond, initiating the electrophilic
- Alkanes lack the electron density and accessibility for electrophilic attack, requiring more extreme conditions for reactions
- : Alkenes have sp2 hybridized carbons, while alkanes have sp3 hybridized carbons
Products of HBr addition to alkenes
- + HBr produces
- H adds to the less substituted carbon
- Br adds to the more substituted carbon
- + HCl produces
- H adds to the terminal carbon (less substituted)
- Cl adds to the more substituted carbon, forming a tertiary carbocation intermediate
- Markovnikov's rule predicts the major product based on the stability of the carbocation intermediate
- More stable carbocations have more alkyl substituents (3° > 2° > 1°)
- stabilization can affect carbocation stability and product distribution
Factors Affecting the Reaction
- : The rate-determining step is typically the formation of the carbocation intermediate
- : Polar protic solvents can stabilize charged intermediates and affect reaction rates
- Temperature: Higher temperatures can increase reaction rates and potentially affect product distribution
Key Terms to Review (36)
2-chloro-2-methylpropane: 2-chloro-2-methylpropane, also known as tert-butyl chloride, is an organic compound with the chemical formula (CH3)3CCl. It is a polar molecule that is used in various organic reactions, particularly in the context of addition reactions and Markovnikov's rule.
2-methylpropene: 2-methylpropene, also known as isobutylene, is a branched-chain alkene with the molecular formula C₄H₈. It is an important organic compound that is relevant in the context of several topics in organic chemistry, including the addition of HBr to ethylene, the stability of alkenes, Markovnikov's rule, and the Hammond postulate.
Addition Reaction: An addition reaction is a type of chemical reaction where two or more reactants combine to form a single product. In the context of organic chemistry, addition reactions typically involve the addition of atoms or molecules to an alkene or alkyne, resulting in the formation of a new compound with a different structure and properties.
Alkyl halide: An alkyl halide is an organic compound in which one or more hydrogen atoms in an alkane (saturated hydrocarbon) have been replaced by a halogen atom (fluorine, chlorine, bromine, or iodine). This substitution results in a molecule with distinct chemical and physical properties compared to its alkane precursor.
Alkyl Halide: An alkyl halide is a type of organic compound that consists of an alkyl group (a hydrocarbon chain) bonded to a halogen atom (fluorine, chlorine, bromine, or iodine). These compounds are important intermediates in many organic reactions, including polar reactions, elimination reactions, and substitution reactions.
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.
Bridged Ion: A bridged ion is a type of carbocation intermediate that forms during certain polar addition reactions, such as the addition of hydrogen bromide (HBr) to ethylene. It features a three-membered ring structure where a single carbon atom is shared between two other carbon atoms, creating a 'bridge' between them.
Bromocyclohexane: Bromocyclohexane is a cyclic organic compound with the chemical formula C6H11Br. It is formed by the addition of hydrogen bromide (HBr) to cyclohexene, a common cycloalkene. The bromine atom is covalently bonded to one of the carbon atoms in the cyclohexane ring, resulting in a polar molecule.
Bromoethane: Bromoethane, also known as ethyl bromide, is a colorless, volatile organic compound with the chemical formula C2H5Br. It is an important intermediate in organic synthesis and is commonly used in the context of the addition of hydrogen bromide (HBr) to ethylene, which is an example of a polar reaction.
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.
Cyclohexene: Cyclohexene is a cyclic alkene compound with the molecular formula C₆H₁₀. It is a versatile organic compound that plays an important role in various chemical reactions, including the addition of hydrogen bromide (HBr) and cycloaddition reactions.
Dipole-Dipole: Dipole-dipole interactions are a type of intermolecular force that occurs between polar molecules, where the partially positive end of one molecule is attracted to the partially negative end of another molecule. These attractive forces play a crucial role in the context of the addition of HBr to ethylene, a polar reaction.
Double bond: A double bond in organic chemistry is a chemical bond between two atoms involving four bonding electrons instead of the usual two. It results in stronger attraction and shorter distance between the bonded atoms compared to a single bond.
Double Bond: A double bond is a covalent chemical bond that forms between two atoms, with the sharing of four valence electrons. This type of bond is commonly found in organic compounds, particularly in alkenes, and is a key structural feature that influences the properties and reactivity of these molecules.
Electronegativity: Electronegativity is a measure of an atom's ability to attract shared electrons in a chemical bond. It is a fundamental concept in understanding the nature and strength of chemical bonds, as well as predicting the polarity and reactivity of molecules.
Electronegativity (EN): Electronegativity is a measure of an atom's ability to attract and hold onto electrons when it is part of a compound. The higher the electronegativity value, the more strongly an atom can pull electrons towards itself.
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.
Ethylene: Ethylene is a colorless, flammable gas with the chemical formula C₂H₄. It is the simplest alkene and is widely used in the chemical industry for the production of various organic compounds and polymers. Ethylene is a key term that connects to several important topics in organic chemistry, including the structure of alkenes, chemical bonding, and industrial applications.
HBr: HBr, or hydrobromic acid, is a strong acid composed of hydrogen (H) and bromine (Br). It is an important reagent in organic chemistry, commonly used in various reactions and processes, including the addition of HBr to alkenes, the preparation of alkyl halides from alcohols, and electrophilic additions to conjugated dienes.
Hybridization: Hybridization is a fundamental concept in chemistry that describes the process of mixing atomic orbitals to form new hybrid orbitals, which are used to explain the geometry and bonding patterns of molecules. This term is closely related to the development of chemical bonding theory, valence bond theory, and molecular orbital theory, as well as the structure and properties of various organic compounds.
Hydrogen Bonding: Hydrogen bonding is a special type of dipole-dipole interaction that occurs when a hydrogen atom covalently bonded to a highly electronegative element, such as nitrogen, oxygen, or fluorine, experiences an attractive force with another nearby highly electronegative element. This intermolecular force is stronger than a typical dipole-dipole interaction and has a significant impact on the physical and chemical properties of many organic compounds.
Markovnikov: Markovnikov is a principle that describes the preferred regiochemistry of electrophilic addition reactions to unsymmetrical alkenes. It states that in the addition of an unsymmetrical reagent, such as a hydrogen halide, to an alkene, the electrophilic portion of the reagent will add to the carbon of the alkene that can best stabilize the resulting carbocation intermediate.
Markovnikov’s rule: Markovnikov's rule predicts the outcome of the electrophilic addition of hydrogen halides to alkenes, stating that the hydrogen atom will attach to the carbon with more hydrogen atoms, and the halide will attach to the more substituted carbon. This rule helps in determining the major product of addition reactions in organic chemistry.
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.
Nucleophilic Attack: Nucleophilic attack is a fundamental chemical reaction in which a nucleophile, an electron-rich species, attacks an electrophilic (electron-deficient) center, forming a new covalent bond. This process is central to understanding many organic reactions, including polar reactions, addition reactions, and substitution reactions.
Pi Bond: A pi (π) bond is a type of covalent chemical bond formed by the side-to-side overlap of atomic orbitals, resulting in electron density concentrated above and below the internuclear axis between two atoms. Pi bonds are crucial in the structure and reactivity of many organic compounds.
Polar Reaction: A polar reaction is a type of chemical reaction where the breaking and forming of bonds involves the unequal sharing of electrons, leading to the formation of partial positive and negative charges on the atoms involved. This type of reaction is common in organic chemistry and is exemplified by the addition of hydrogen bromide (HBr) to ethylene.
Protonation: Protonation is the process of adding a proton (H+) to a molecule or atom, resulting in the formation of a positively charged species. This fundamental chemical reaction is central to various organic chemistry topics, as it can significantly influence the reactivity and stability of molecules.
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.
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.