Alkynes: Structure, Reactivity, and Synthetic Transformations

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Alkynes

Nomenclature

Alkynes are hydrocarbons containing a carbon-carbon triple bond (C≡C). Their nomenclature follows IUPAC rules, with the suffix -yne indicating the presence of a triple bond. The parent chain must include the triple bond, and numbering is done to give the triple bond the lowest possible locant. The locant is assigned to the first carbon of the triple bond.

Terminal alkyne: The triple bond is at the end of the chain.
Internal alkyne: The triple bond is within the chain, not at the terminus.

Structural Features

Alkynes can be classified as terminal or internal based on the position of the triple bond. Terminal alkynes have a hydrogen atom attached to the triple-bonded carbon, while internal alkynes do not.

Terminal alkyne: R–C≡C–H
Internal alkyne: R–C≡C–R'

Alkyne Acidity

Acidic Properties of Terminal Alkynes

Terminal alkynes possess an acidic proton (pKa ≈ 25), which can be deprotonated by a strong base. The conjugate base formed is an alkynide ion. Only bases with conjugate acid pKa values greater than 25 (e.g., NH2⁻, pKa ≈ 38; RC⁻, pKa ≈ 50) are suitable for deprotonation.

Acidic proton: Present in terminal alkynes.
Strong base required: Water is not suitable as a solvent.

Acidity comparison: deprotonation of terminal alkyne by sodium amide

Preparation of Alkynes

Elimination Reactions

Alkynes can be synthesized by elimination reactions from dihalides. Geminal dihalides (two halides on the same carbon) and vicinal dihalides (halides on adjacent carbons) undergo E2 elimination with strong base to yield alkynes.

Internal alkynes: Elimination of geminal or vicinal dihalides.
Terminal alkynes: Requires a hydrogen on the geminal carbon; strong base deprotonates the terminal alkyne, and a weak proton source is used to protonate the alkynide ion.

Preparation of alkynes from dihalides using NaNH2/NH3 and water

Alkylation of Terminal Alkynes

Alkylation Mechanism

Terminal alkynes can be alkylated by first deprotonating with a strong base to form an alkynide ion, which then undergoes SN2 reaction with a methyl or primary alkyl halide. Secondary or tertiary alkyl halides typically lead to elimination rather than substitution.

Step 1: Deprotonation with strong base (e.g., NaNH2).
Step 2: SN2 reaction with alkyl halide.

Alkylation of terminal alkyne via SN2 mechanism

Reduction of Alkynes

Hydrogenation

Alkynes can be reduced to alkenes or alkanes by hydrogenation. Complete hydrogenation with platinum (Pt) yields alkanes, while partial hydrogenation with a poisoned catalyst (e.g., Lindlar's catalyst) produces cis-alkenes.

Complete hydrogenation: Two equivalents of H2 convert alkyne to alkane.
Partial hydrogenation: Produces cis-alkene intermediate.

Hydrogenation of alkenes and alkynes to alkanes Stepwise hydrogenation of alkyne to alkene and then alkane

Lindlar's Catalyst

Lindlar's catalyst is a poisoned palladium catalyst used for selective hydrogenation of alkynes to cis-alkenes. It consists of Pd/BaSO4 and quinoline.

Structure and composition of Lindlar's catalyst

Dissolving Metal Reduction

Dissolving metal reduction (e.g., Na/NH3) converts alkynes to trans-alkenes via anti-addition. This method is useful for synthesizing trans-alkenes from alkynes.

Reduction of alkyne to trans-alkene using sodium in liquid ammonia

Hydrohalogenation of Alkynes

Markovnikov and Anti-Markovnikov Addition

Alkynes react with hydrogen halides (HX) to form vinyl halides (1 equiv) or geminal dihalides (2 equiv). Markovnikov addition occurs in the absence of peroxides, while anti-Markovnikov addition is promoted by peroxides (ROOR).

Markovnikov addition: Halide adds to the more substituted carbon.
Anti-Markovnikov addition: Halide adds to the less substituted carbon.

Hydrohalogenation of alkyne to vinyl halide Anti-Markovnikov addition of HBr to alkyne with peroxides

Hydration of Alkynes

Acid-Catalyzed Hydration

Alkynes undergo hydration to form enol intermediates, which tautomerize to ketones. Acid-catalyzed hydration (using HgSO4 or Hg(OAc)2) follows Markovnikov regiochemistry.

Enol intermediate: Not isolated; rapidly tautomerizes to ketone.
Regiochemistry: Markovnikov addition.

Acid-catalyzed hydration of alkyne to ketone via enol intermediate

Hydroboration-Oxidation

Hydroboration-oxidation of alkynes gives anti-Markovnikov hydration products. The enol intermediate tautomerizes to a ketone (internal alkyne) or an aldehyde (terminal alkyne). Bulky borane reagents prevent multiple additions.

Reagents: BH3, THF; H2O2, NaOH; disiamylborane; 9-BBN.
Product: Ketone or aldehyde, depending on alkyne type.

Hydroboration-oxidation of alkyne to aldehyde

Oxidative Cleavage of Alkynes

Ozonolysis and Permanganate Oxidation

Oxidative cleavage of alkynes breaks the triple bond to yield carboxylic acids. Internal alkynes produce two carboxylic acids, while terminal alkynes yield one carboxylic acid and carbon dioxide. Ozonolysis (O3, H2O) and permanganate oxidation (KMnO4, NaOH, heat, H3O+) give similar products.

Internal alkyne: Two carboxylic acids.
Terminal alkyne: One carboxylic acid and CO2.

Ozonolysis of internal alkyne to carboxylic acids Ozonolysis of terminal alkyne to carboxylic acid and carbon dioxide

Synthesis and Retrosynthesis

Multi-Step Synthesis

Alkyne chemistry is central to organic synthesis, allowing for the construction of complex molecules via multi-step reactions. Key considerations include changes in the carbon skeleton and functional group transformations.

Carbon skeleton: Gain or loss of carbon atoms.
Functional group: Conversion and positional changes.

Example: Synthesis of 3-heptyne from acetylene involves alkylation and elimination steps.

Additional info: Alkynes are versatile intermediates in organic synthesis, enabling the formation of a wide range of functional groups and carbon frameworks.