8.3 Halohydrins from Alkenes: Addition of HO-X
2 min read•may 7, 2024
Halohydrins are key players in organic chemistry, formed when halogens and hydroxyl groups add to alkenes. This reaction creates compounds with both a and an alcohol group, opening doors to further transformations.
The process involves a and follows for regiochemistry. Understanding is crucial for predicting products and tackling more complex organic reactions.
Halohydrin Formation and Reactions
Formation of halohydrins from alkenes
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- Halohydrins formed by of halogen (X) and (OH) to alkene
- Halogen and hydroxyl add in anti-stereochemical manner across alkene bond
- Reaction occurs in aqueous conditions with X2 (, , or )
- X2 serves as and H2O serves as
- Reaction proceeds through cyclic intermediate
- Halonium ion forms as electrophilic X2 adds to alkene π bond
- Backside attack of H2O opens halonium ring, leading to of X and OH
- Regiochemistry of addition follows Markovnikov's rule
- More stable intermediate leads to major product
- Halogen preferentially bonds to more substituted carbon (tertiary > secondary > primary)
- Hydroxyl group bonds to less substituted carbon
- Example: Addition of Br2 to 2-methylbut-2-ene forms 3-bromo-2-methylbutan-2-ol as major product (tertiary carbocation intermediate)
- More stable intermediate leads to major product
- of the product is determined by the anti addition of X and OH
NBS as alternative to bromine
- () used as safer bromine source compared to Br2
- NBS is solid, easier to handle than liquid Br2
- NBS less reactive and more selective than Br2
- NBS reacts with H2O to generate low, steady-state concentration of
- HOBr serves as active brominating agent in reaction
- Mechanism with NBS analogous to formation with X2
- Electrophilic addition of HOBr to alkene forms intermediate
- H2O attacks bromonium ring, resulting in anti addition of OH and Br
- Example: NBS addition to cyclohexene forms
Products of halohydrin reactions
- Identify alkene substrate and halogen (X2 or NBS) used in reaction
- Determine major product based on Markovnikov's rule
- Halogen preferentially bonds to more substituted carbon
- Hydroxyl group bonds to less substituted carbon
- Assign of halogen and hydroxyl groups
- Halogen and hydroxyl on opposite sides of former alkene bond
- Example: Addition of Cl2 to trans-2-butene forms (2R,3S)-2,3-dichlorobutan-2-ol and (2S,3R)-2,3-dichlorobutan-2-ol
- Consider potential rearrangements or competing reactions
- Halohydrin formation may compete with simple (HX addition)
- Example: Addition of HBr to 2-methylpropene forms 2-bromo-2-methylpropane as major product
- Carbocation rearrangements may occur, altering product structure
- Example: Addition of Br2 to 3,3-dimethylbut-1-ene forms 2,2-dimethyl-3-bromobutan-3-ol via
- Halohydrin formation may compete with simple (HX addition)
- of the reaction is influenced by the stability of the carbocation intermediate
Reaction Mechanism and Intermediates
- The involves the formation of a halonium ion intermediate
- The nucleophilic water molecule attacks the electrophilic halonium ion
- The reaction proceeds through a carbocation intermediate before forming the final product
Key Terms to Review (33)
1,2-Hydride Shift: A 1,2-hydride shift is a type of rearrangement reaction in organic chemistry where a hydrogen atom migrates from one carbon atom to an adjacent carbon atom. This process is particularly important in the context of the addition of halohydrins to alkenes, as it can lead to the formation of different products.
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.
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.
Bromohydrin: A bromohydrin is an organic compound containing a bromine atom and a hydroxyl group (-OH) attached to adjacent carbon atoms in an alkene-derived molecule. This compound forms through the addition reaction of a bromine and water (HO-X) across the double bond of alkenes.
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.
Chlorohydrin: A chlorohydrin is an organic compound that contains both a chlorine atom and a hydroxyl group (-OH) attached to adjacent carbon atoms, typically formed by the reaction of an alkene with hypochlorous acid (HOCl). This reaction is part of halohydrin formation where the alkene double bond is opened, leading to the addition of -OH and -Cl on neighboring carbons.
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.
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.
Halogen: Halogens are a group of five highly reactive nonmetal elements in the periodic table, including fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). They are known for their strong tendency to gain electrons and form compounds with other elements.
Halohydrin: A halohydrin is a compound containing both a halogen atom (such as chlorine, bromine, or iodine) and a hydroxyl group (-OH) on adjacent carbon atoms. These compounds are formed through the addition of a hydrogen halide (HX, where X is a halogen) to an alkene, resulting in the incorporation of both the halogen and the hydroxyl group.
Halohydrin Formation: Halohydrin formation is the addition of a halogen (X) and a hydroxyl (OH) group to an alkene, resulting in the formation of a halohydrin compound. This process is an important reaction in organic chemistry, particularly in the context of 8.3 Halohydrins from Alkenes: Addition of HO-X.
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.
HOBr: HOBr, or hypobromous acid, is a chemical compound formed by the addition of hydrogen (H) and bromine (Br) to an oxygen (O) atom. It is an important intermediate in the context of the addition of HO-X to alkenes, specifically in the formation of halohydrins.
Hydrohalogenation: Hydrohalogenation is a type of organic reaction where an alkene or alkyne reacts with a hydrogen halide (HX, where X is a halogen such as F, Cl, Br, or I) to form an alkyl halide. This process adds the hydrogen and halogen atoms across the carbon-carbon double or triple bond.
Hydroxyl Group: The hydroxyl group (OH-) is a functional group consisting of an oxygen atom covalently bonded to a hydrogen atom. It is a key structural feature in many organic compounds, particularly alcohols and phenols, and plays a crucial role in their chemical properties and reactivity.
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.
Intermediate: An intermediate is a chemical species that is formed during the course of a chemical reaction, existing temporarily between the reactants and the final products. Intermediates play a crucial role in understanding the mechanisms and energetics of chemical transformations, particularly in the context of organic chemistry topics such as reaction energy diagrams, electrophilic addition reactions, and halohydrin formation.
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.
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.
NBS: NBS, or N-Bromosuccinimide, is a versatile reagent used in organic chemistry, particularly in the context of halohydrin formation from alkenes through the addition of HO-X. It serves as a source of bromine and is commonly employed in electrophilic addition reactions.
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 intermediate: A reaction intermediate is a transient species that is formed during the conversion of reactants into products in a chemical reaction and does not appear in the final reaction equation. It plays a crucial role in determining the pathway and mechanism by which the reaction proceeds.
Reaction mechanism: A reaction mechanism is a step-by-step sequence of elementary reactions by which overall chemical change occurs. It outlines the specific way in which reactants convert to products, including the formation and breaking of bonds.
Reaction Mechanism: A reaction mechanism is the step-by-step sequence of elementary reactions by which overall chemical change occurs. It describes the detailed pathway that a reaction follows, including the formation and rearrangement of chemical bonds, the generation of intermediates, and the movement of electrons. Understanding reaction mechanisms is crucial for predicting the products of a reaction, explaining reactivity trends, and designing new synthetic pathways.
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.
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-2-bromocyclohexanol: trans-2-bromocyclohexanol is a halohydrin compound formed by the addition of hydrobromic acid (HBr) to the alkene cyclohexene. This addition reaction results in the formation of a bromohydrin, where a bromine atom and a hydroxyl group are added across the double bond in a trans configuration.