9.3 Reactions of Alkynes: Addition of HX and X2
2 min read•may 7, 2024
Alkynes, with their triple bonds, are eager to react. They undergo , forming carbocations along the way. This process mirrors alkene reactions but with unique twists in stability and product formation.
and additions to alkynes follow specific rules. guides where protons land, while determines the final product's shape. These reactions transform alkynes from sp to sp2 , changing their very nature.
Addition of HX and X2 to Alkynes
Electrophilic addition to alkynes
Top images from around the web for Electrophilic addition to alkynes
Organic chemistry 18: Electrophilic addition to alkenes View original
Is this image relevant?
Organic chemistry 18: Electrophilic addition to alkenes View original
Is this image relevant?
Organic chemistry 18: Electrophilic addition to alkenes View original
Is this image relevant?
Organic chemistry 18: Electrophilic addition to alkenes View original
Is this image relevant?
Organic chemistry 18: Electrophilic addition to alkenes View original
Is this image relevant?
1 of 3
Top images from around the web for Electrophilic addition to alkynes
Organic chemistry 18: Electrophilic addition to alkenes View original
Is this image relevant?
Organic chemistry 18: Electrophilic addition to alkenes View original
Is this image relevant?
Organic chemistry 18: Electrophilic addition to alkenes View original
Is this image relevant?
Organic chemistry 18: Electrophilic addition to alkenes View original
Is this image relevant?
Organic chemistry 18: Electrophilic addition to alkenes View original
Is this image relevant?
1 of 3
- Electrophiles add to alkynes in a two-step process involving a intermediate
- Step 1: Electrophile (proton or halogen) adds to one of the carbons forming a vinylic carbocation
- determined by the stability of the vinylic carbocation intermediate (more stable carbocation favored according to Markovnikov's rule)
- Step 2: Nucleophile ( or solvent) attacks the vinylic carbocation leading to the final product
- Stereochemistry of the product determined by the approach of the nucleophile from either the top or bottom face of the planar vinylic carbocation
- Step 1: Electrophile (proton or halogen) adds to one of the carbons forming a vinylic carbocation
- The reaction involves breaking one of the pi bonds in the alkyne
Mechanism of HX alkyne addition
- Proton () from HX adds to one of the alkyne carbons forming a vinylic carbocation intermediate
- Proton adds to the carbon that will yield the more stable carbocation (Markovnikov's rule)
- Halide ion () attacks the vinylic carbocation leading to the final product
- Similar two-step mechanism to alkene addition involving carbocation intermediate
- Alkynes yield vinylic carbocations while alkenes yield alkyl carbocations
- Vinylic carbocations more stable than primary alkyl carbocations but less stable than secondary and tertiary alkyl carbocations (stability: 3° > 2° > vinylic > 1°)
- Increased stability of vinylic carbocations compared to primary alkyl carbocations due to (positive charge delocalized between the two carbons of the vinylic system)
Products of alkyne HX and X2 reactions
- Addition of HX to alkynes ():
- Markovnikov's rule: Proton adds to the carbon that will yield the more stable carbocation intermediate
- Major product: Halogen attached to the less substituted carbon (more hydrogens)
- Minor product: Halogen attached to the more substituted carbon (fewer hydrogens)
- Stereochemistry: Product is a mixture of E and Z isomers due to approach of the nucleophile from either face of the planar vinylic carbocation
- Markovnikov's rule: Proton adds to the carbon that will yield the more stable carbocation intermediate
- Addition of X2 to alkynes:
- Stereochemistry: with halogen atoms adding to opposite faces of the alkyne in a concerted mechanism
- Product: with (halogens on opposite sides of the double bond)
Hybridization changes during alkyne reactions
- Alkynes start with sp hybridization
- After addition reactions, the product has sp2 hybridization
- This change in hybridization affects the geometry and reactivity of the molecule
Key Terms to Review (22)
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.
Alkyne: An alkyne is a hydrocarbon compound containing a carbon-carbon triple bond. Alkynes are a class of unsaturated organic compounds that play a crucial role in various topics within organic chemistry, including sp hybridization, functional groups, degree of unsaturation, nomenclature, and synthetic transformations.
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.
E Isomer: The E isomer, also known as the trans isomer, is a type of geometric isomerism that can occur in alkenes and other organic compounds. It describes the spatial arrangement of substituents around a carbon-carbon double bond, where the larger substituents are oriented on opposite sides of the double bond.
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.
Halide Ion: A halide ion is a negatively charged ion formed when a halogen atom (fluorine, chlorine, bromine, iodine, or astatine) gains an electron. Halide ions are important in the context of reactions of alkynes, specifically the addition of hydrogen halides (HX) and halogens (X2).
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.
HX: HX is a general notation used to represent the addition of a hydrogen halide (HX) to a substrate, such as an alkyne, in organic chemistry. The 'H' represents the hydrogen atom, and the 'X' represents a halogen atom, typically chlorine (Cl), bromine (Br), or iodine (I).
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.
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.
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.
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.
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 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.
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.
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.
Vinylic: A vinylic position in organic chemistry refers to the position of a hydrogen or substituent that is directly attached to an alkene's sp^2 hybridized carbon atom. This term is often used when discussing reactions involving the addition of elements across the double bond of alkenes.
Vinylic Carbocation: A vinylic carbocation is a positively charged carbon atom that is part of a carbon-carbon double bond. These reactive intermediates are often involved in the reactions of alkynes, such as the addition of hydrohalic acids (HX) and halogens (X2).
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.
Z Isomer: The Z isomer is a type of geometric isomer that occurs when two identical substituents are located on the same side of a carbon-carbon double bond. This configuration results in a zig-zag or 'Z' shape, in contrast to the E isomer where the substituents are on opposite sides.