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Organic Chemistry
Table of Contents
Unit 9 Overview: Alkynes: Intro to Organic Synthesis
9.1 Naming Alkynes
9.2 Preparation of Alkynes: Elimination Reactions of Dihalides
9.3 Reactions of Alkynes: Addition of HX and X2
9.4 Hydration of Alkynes
9.5 Reduction of Alkynes
9.6 Oxidative Cleavage of Alkynes
9.7 Alkyne Acidity: Formation of Acetylide Anions
9.8 Alkylation of Acetylide Anions
9.9 An Introduction to Organic Synthesis

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9.2 Preparation of Alkynes: Elimination Reactions of Dihalides

Citation:

Alkynes, those fascinating molecules with triple bonds, can be created through elimination reactions. This process involves removing hydrogen halides from dihalides or converting alkenes to alkynes. It's all about kicking out those pesky hydrogens and halogens to form that tight triple bond.

Strong bases are the key players in these reactions, pulling off protons to make alkynes. The starting material's structure determines whether you end up with a straight or bent alkyne. It's like molecular origami - the way you fold it affects the final shape.

Preparation of Alkynes through Elimination Reactions

Alkyne preparation via elimination

  • Alkynes prepared by eliminating two equivalents of hydrogen halide (HX) from vicinal dihalides (dihalides on adjacent carbons) known as a double dehydrohalogenation reaction
  • Requires a strong base such as sodium amide ($NaNH_2$) or potassium tert-butoxide ($KOC(CH_3)_3$) to abstract protons
  • Reaction proceeds through an E2 mechanism
    • Base abstracts a proton from one of the carbons bearing a halogen forming a vinylic halide intermediate
    • Second equivalent of base then abstracts the remaining proton forming the alkyne product
  • Stereochemistry of the starting dihalide determines the stereochemistry of the alkyne product
    • Vicinal dihalide with halogens on opposite faces of the molecule (anti) gives a linear alkyne
    • Vicinal dihalide with halogens on the same face (syn) gives a bent alkyne

Alkene to alkyne conversion

  • Alkynes synthesized from alkenes via a two-step process involving halogenation followed by dehydrohalogenation
  • Step 1: Halogenation of the alkene
    • Treatment of an alkene with $X_2$ (where X = Cl, Br) adds halogens across the double bond forming a vicinal dihalide
    • Reaction proceeds through an electrophilic addition mechanism
    • Stereochemistry of addition is anti with halogens adding to opposite faces of the alkene
  • Step 2: Dehydrohalogenation of the vicinal dihalide
    • Vicinal dihalide treated with a strong base to eliminate two equivalents of HX forming the alkyne
    • Proceeds through an E2 mechanism as described in the previous objective
  • Overall stereochemistry of the reaction is anti addition followed by anti elimination resulting in a linear alkyne product

Vinylic halides in alkyne synthesis

  • Vinylic halides are alkenes with a halogen substituent directly attached to one of the carbons of the double bond
  • Serve as intermediates in the synthesis of alkynes from dihalides
    1. In the double dehydrohalogenation of a vicinal dihalide, the first equivalent of base abstracts a proton to form a vinylic halide
    2. The second equivalent of base then abstracts the remaining proton from the vinylic halide to form the alkyne
  • Can also be used to directly synthesize alkynes through a single dehydrohalogenation
    • Treatment of a vinylic halide with a strong base eliminates one equivalent of HX forming the alkyne
    • Reaction proceeds through an E2 mechanism
  • Stereochemistry of the vinylic halide determines the stereochemistry of the alkyne product
    • Vinylic halide with the halogen and hydrogen on opposite faces (trans) gives a linear alkyne
    • Vinylic halide with the halogen and hydrogen on the same face (cis) gives a bent alkyne

Additional Alkyne Preparation Methods

  • Dehydration of alcohols: Terminal alkynes can be prepared by the dehydration of certain alcohols using strong bases
  • Alkyl halides: Internal alkynes can be synthesized from alkyl halides through elimination reactions
  • Strong bases (e.g., sodium amide) are commonly used in these preparations to facilitate the elimination process

Key Terms to Review (23)

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.
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.
Triple bonds: A triple bond is a chemical bond where three pairs of electrons are shared between two atoms. It is the strongest and shortest type of covalent bond found in molecules.
Sp Hybridization: sp Hybridization is a concept in organic chemistry that describes the formation of hybrid atomic orbitals through the combination of one s orbital and one p orbital, resulting in the creation of two equivalent sp hybrid orbitals. This hybridization is particularly important in understanding the structure and bonding patterns of certain organic compounds, such as alkynes.
Triple Bond: A triple bond is a covalent bond in which three pairs of electrons are shared between two atoms, resulting in a very strong and stable chemical connection. This type of bond is particularly important in the context of organic chemistry, as it is a key structural feature in certain classes of compounds known as alkynes.
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.
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.
Dehydrohalogenation: Dehydrohalogenation is an elimination reaction in organic chemistry where a hydrogen atom and a halogen atom (such as chlorine, bromine, or iodine) are removed from an organic compound, resulting in the formation of an alkene or alkyne. This process is a key step in the preparation of unsaturated hydrocarbons from alkyl halides.
Dehydration: Dehydration is a chemical process in which water is removed from a compound, typically resulting in the formation of a new compound with fewer hydrogen and oxygen atoms. This term is particularly relevant in the context of various organic reactions and transformations, where dehydration plays a crucial role in the preparation and interconversion of different functional groups.
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.
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.
Elimination Reaction: An elimination reaction is a type of organic reaction in which two atoms or groups are removed from a molecule, typically resulting in the formation of a carbon-carbon double bond or a carbon-carbon triple bond. This process is an important step in the synthesis of alkenes and alkynes, as well as in various other organic transformations.
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.
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.
Potassium tert-butoxide: Potassium tert-butoxide is a strong base used in organic chemistry, particularly in elimination reactions to prepare alkenes and alkynes. It is a white, crystalline solid that is highly reactive and must be handled with care.
Vinylic Halide: A vinylic halide is an organic compound containing a carbon-carbon double bond (a vinyl group) directly bonded to a halogen atom, such as chlorine, bromine, or iodine. These compounds are important intermediates in the preparation of alkynes through elimination reactions of dihalides, as described in the topic 9.2 Preparation of Alkynes.
Strong Base: A strong base is a type of chemical compound that completely dissociates into its constituent ions in an aqueous solution, resulting in a high concentration of hydroxide ions (OH-) and a high pH. Strong bases are important in various organic chemistry reactions, such as the preparation of alkynes via elimination reactions and the Wittig reaction involving phosphorus ylides.
Sodium Amide: Sodium amide, also known as sodamide, is an inorganic compound with the chemical formula NaNH2. It is a white, crystalline solid that is commonly used as a strong base and nucleophile in organic chemistry reactions, particularly in the preparation of alkynes and the formation of acetylide anions.
Syn Elimination: Syn elimination, also known as syn-elimination or syn-periplanar elimination, is a type of E2 elimination reaction in organic chemistry. It refers to the removal of two substituents from adjacent carbon atoms in a molecule, resulting in the formation of a carbon-carbon double bond, or an alkene, with the new double bond oriented in a syn configuration relative to the leaving groups.
Vicinal: Vicinal refers to the spatial relationship between two substituents or functional groups that are positioned next to each other, typically on adjacent carbon atoms, in a molecule.
E2 Mechanism: The E2 mechanism, or bimolecular elimination reaction, is a type of organic reaction where a base removes a hydrogen atom and a leaving group from adjacent carbon atoms, resulting in the formation of a carbon-carbon double bond. This mechanism is particularly important in the preparation of alkynes through the elimination of dihalides, as well as in the dehydration of aldol products to form enones.
Anti Elimination: Anti elimination is a type of elimination reaction where the leaving groups depart in an anti-periplanar fashion, resulting in the formation of an alkyne. This process is particularly important in the context of preparing alkynes from dihalides through elimination reactions.
Double Dehydrohalogenation: Double dehydrohalogenation is a type of elimination reaction that involves the removal of two halogen atoms (typically bromine or chlorine) and two hydrogen atoms from an organic compound, resulting in the formation of an alkyne. This process is a key method for the preparation of alkynes from dihalide precursors.