Organic Chemistry Unit 9 ReviewAlkynes: Intro to Organic Synthesis

Alkynes are unsaturated hydrocarbons with a carbon-carbon triple bond. They're key players in organic synthesis, serving as versatile building blocks for creating complex molecules. Their unique structure and reactivity make them valuable in pharmaceutical and polymer industries. Alkynes undergo addition reactions, breaking π bonds to form new σ bonds. They can be terminal or internal, with sp hybridization giving them a linear shape. Less reactive than alkenes, alkynes still offer a wide range of synthetic possibilities for organic chemists.

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

unit 9 review

Key Concepts

Alkynes contain a carbon-carbon triple bond consisting of one σ (sigma) bond and two π (pi) bonds
Alkynes are unsaturated hydrocarbons with the general formula C $_n$ $_{n}$ H $_{2n-2}$ $_{2 n - 2}$
- Simplest alkyne is acetylene (C $_2$ H $_2$ ) which has two carbon atoms and two hydrogen atoms
Alkynes undergo addition reactions due to the presence of the reactive triple bond
- Addition reactions involve the breaking of the π bonds and the formation of new σ bonds
Alkynes can be terminal (with the triple bond at the end of the carbon chain) or internal (with the triple bond in the middle of the carbon chain)
Alkynes are linear molecules with sp hybridization of the carbon atoms involved in the triple bond
Alkynes are less reactive than alkenes due to the higher bond energy of the triple bond compared to the double bond
Alkynes can be used as building blocks in organic synthesis to create more complex molecules (pharmaceuticals, polymers)

Structure and Bonding

In alkynes, each carbon atom involved in the triple bond is sp hybridized
- sp hybridization involves the mixing of one s orbital and one p orbital to form two sp hybrid orbitals
The two sp hybrid orbitals are oriented at an angle of 180° to each other, resulting in a linear geometry around the triple bond
The remaining two unhybridized p orbitals on each carbon atom overlap to form two π bonds perpendicular to the σ bond
- The two π bonds are formed by the sideways overlap of the p orbitals above and below the plane of the molecule
The carbon-carbon triple bond is shorter (1.20 Å) and stronger (839 kJ/mol) than a carbon-carbon double bond (1.34 Å, 614 kJ/mol) or a carbon-carbon single bond (1.54 Å, 347 kJ/mol)
The high electron density of the triple bond makes alkynes slightly acidic and capable of forming acetylide anions (R-C≡C $^-$ ) in the presence of strong bases (sodium amide, NaNH $_2$ )
The linear geometry of alkynes results in a lack of cis-trans isomerism, unlike alkenes

Nomenclature

Alkynes are named by replacing the "-ane" suffix of the corresponding alkane with "-yne"
- The position of the triple bond is indicated by the number of the first carbon atom involved in the triple bond (propyne, 1-butyne)
For alkynes with multiple triple bonds, the suffix is modified to "-diyne", "-triyne", etc., and the positions of all triple bonds are indicated (1,3-butadiyne)
The longest continuous chain containing the triple bond is used as the base name, and substituents are named and numbered accordingly (4-methyl-2-pentyne)
Common names for simple alkynes include acetylene (ethyne), propyne (methylacetylene), and butyne (ethylacetylene)
For complex alkynes with multiple functional groups, the order of priority for naming is: carboxylic acids > esters > aldehydes > ketones > alcohols > amines > alkyne > alkene > alkane
When the triple bond is not part of the longest carbon chain, it is named as an ethynyl substituent (ethynylbenzene)

Physical Properties

Alkynes are nonpolar molecules due to the symmetric distribution of electrons in the triple bond
Lower alkynes (C $_2$ $_{2}$ to C $_4$ $_{4}$ ) are gases at room temperature, while higher alkynes are liquids or solids
- Acetylene (C $_2$ H $_2$ ) is a colorless, flammable gas with a garlic-like odor
Alkynes have higher boiling points than alkanes and alkenes of similar molecular weight due to the increased surface area for van der Waals interactions
- Boiling point of ethyne (acetylene) is -84°C, compared to ethene (ethylene) at -104°C and ethane at -89°C
Alkynes are slightly soluble in water due to their ability to form hydrogen bonds with water molecules through the acidic terminal hydrogen atom
Alkynes have a higher refractive index than alkenes and alkanes, making them useful in optical materials and coatings
The melting points of alkynes are generally lower than those of alkenes with the same number of carbon atoms due to the linear geometry and reduced intermolecular interactions

Synthesis Methods

Dehydrohalogenation of vicinal dihalides (dihalides on adjacent carbon atoms) using a strong base (sodium amide, NaNH $_2$ $_{2}$ , or potassium hydroxide, KOH)
- Involves the elimination of two molecules of hydrogen halide (HX) from the vicinal dihalide
Dehalogenation of tetrahalides using zinc metal in the presence of a strong base (sodium hydroxide, NaOH)
- Involves the removal of two molecules of a halogen (X $_2$ ) from the tetrahalide
Alkylation of terminal alkynes using a strong base (sodium amide, NaNH $_2$ $_{2}$ ) followed by an alkyl halide (R-X)
- The strong base deprotonates the terminal alkyne to form an acetylide anion, which then acts as a nucleophile and attacks the alkyl halide
Reaction of calcium carbide (CaC $_2$ $_{2}$ ) with water to produce acetylene (C $_2$ $_{2}$ H $_2$ $_{2}$ )
- CaC $_2$ + 2H $_2$ O → C $_2$ H $_2$ + Ca(OH) $_2$
Pyrolysis (thermal decomposition) of alkanes at high temperatures (1000-1200°C) in the absence of oxygen
- Involves the breaking of carbon-carbon and carbon-hydrogen bonds to form alkynes and other unsaturated hydrocarbons
Kolbe electrolysis of dicarboxylic acids using a high current density
- Involves the decarboxylation and dimerization of the carboxylate anions to form alkynes

Reactions and Mechanisms

Electrophilic addition reactions with halogens (X $_2$ $_{2}$ ), hydrogen halides (HX), and water (H $_2$ $_{2}$ O) in the presence of a mercury(II) salt catalyst
- Involves the breaking of the π bonds and the formation of new σ bonds through the addition of electrophiles across the triple bond
Nucleophilic addition reactions with Grignard reagents (R-MgX), organolithium compounds (R-Li), and hydrogen (H $_2$ $_{2}$ ) in the presence of a metal catalyst (Lindlar catalyst)
- Involves the addition of nucleophiles to the triple bond, forming new carbon-carbon or carbon-hydrogen bonds
Hydration of alkynes using a mercury(II) salt catalyst (mercury(II) sulfate, HgSO $_4$ $_{4}$ ) in the presence of water and a strong acid (sulfuric acid, H $_2$ $_{2}$ SO $_4$ $_{4}$ )
- Involves the addition of water across the triple bond to form an enol, which then tautomerizes to a ketone
Hydrogenation of alkynes using a metal catalyst (Lindlar catalyst, palladium on calcium carbonate poisoned with lead acetate) and hydrogen gas
- Involves the addition of hydrogen atoms across the triple bond to form alkenes (partial hydrogenation) or alkanes (complete hydrogenation)
Oxidative cleavage of alkynes using a strong oxidizing agent (potassium permanganate, KMnO $_4$ $_{4}$ , or ozone, O $_3$ $_{3}$ )
- Involves the breaking of the carbon-carbon triple bond to form carboxylic acids or ketones, depending on the structure of the alkyne
Cycloaddition reactions with alkenes (Diels-Alder reaction) or other alkynes ([2+2] cycloaddition) to form cyclic compounds
- Involves the formation of new carbon-carbon bonds through the concerted addition of the alkyne to the alkene or another alkyne

Applications in Organic Synthesis

Alkynes serve as versatile building blocks in organic synthesis due to their ability to undergo a wide range of reactions
Alkynes can be used to synthesize alkenes through partial hydrogenation using a Lindlar catalyst
- Useful in the synthesis of cis-alkenes, as the hydrogenation proceeds with syn addition of hydrogen atoms
Alkynes can be converted to aldehydes or ketones through hydration in the presence of a mercury(II) salt catalyst followed by tautomerization
- Provides a route to synthesize carbonyl compounds with specific substituents
Terminal alkynes can be used to form new carbon-carbon bonds through alkylation reactions with alkyl halides
- Allows for the extension of carbon chains and the introduction of functional groups
Alkynes can participate in cycloaddition reactions (Diels-Alder reaction) with dienes to form substituted cyclohexenes
- Enables the synthesis of complex cyclic compounds with control over regio- and stereochemistry
Alkynes can be used to synthesize aromatic compounds through trimerization reactions in the presence of a metal catalyst
- Example: Cyclotrimerization of acetylene to form benzene
Alkynes can be converted to carboxylic acids through oxidative cleavage using strong oxidizing agents (KMnO $_4$ $_{4}$ or O $_3$ $_{3}$ )
- Provides a method to synthesize carboxylic acids with specific substituents
Alkynes can be used as monomers in polymerization reactions to form conjugated polymers with interesting electronic and optical properties
- Example: Polyacetylene, a conductive polymer used in organic electronics

Practice Problems and Examples

Draw the structure of the following alkynes and name them according to the IUPAC system: a) HC≡C-CH $_2$ -CH $_3$ b) CH $_3$ -C≡C-CH $_2$ -CH $_2$ -CH $_3$ c) HC≡C-CH(CH $_3$ )-CH $_2$ -CH $_3$
Propose a synthesis for the following alkynes starting from the given compounds: a) 1-Butyne from 1,2-dibromoethane b) 2-Pentyne from 1-butene c) 4-Methyl-2-pentyne from acetylene and 2-bromopropane
Predict the products of the following reactions and provide the reaction conditions: a) Addition of HBr to 2-butyne b) Hydration of 3-hexyne using mercury(II) sulfate catalyst c) Hydrogenation of 1-hexyne using Lindlar catalyst
Complete the following reactions and provide the missing reagents or products: a) HC≡CH + 2Na → ? b) CH $_3$ -C≡C-CH $_3$ + ? → CH $_3$ -CH=CH-CH $_3$ c) HC≡C-CH $_2$ -CH $_2$ -CH $_3$ + KMnO $_4$ → ?
Design a multi-step synthesis for the following target molecules using alkynes as starting materials or intermediates: a) 3-Hexanone from acetylene b) cis-3-Hexene from 1-butyne c) p-Toluic acid (4-methylbenzoic acid) from propyne

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