🥼Organic Chemistry Unit 9 – Alkynes: Intro to Organic Synthesis

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$_n$H$_{2n-2}$
  • 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$_2$ to C$_4$) 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$_2$, 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$_2$) 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$_2$) with water to produce acetylene (C$_2$H$_2$)
  • 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$_2$), hydrogen halides (HX), and water (H$_2$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$_2$) 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$_4$) in the presence of water and a strong acid (sulfuric acid, H$_2$SO$_4$)
  • 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$_4$, or ozone, O$_3$)
  • 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$_4$ or O$_3$)
  • 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

1. 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$_3$

2. 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

3. 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

4. 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$_4$ → ?

5. 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