Chapter 9: Alkyne Reactions

Overview of Alkyne Reactivity

Alkynes are hydrocarbons containing a carbon-carbon triple bond, which consists of one sigma (σ) and two pi (π) bonds. Their unique bonding leads to distinct chemical reactivity compared to alkenes and alkanes.

• Alkynes have two π bonds, whereas alkenes have one.

• Many alkene reactions also apply to alkynes, but with key differences:

  • The product is typically an alkenyl compound rather than an alkane.

  • The reaction can occur twice, once for each π bond.

• Terminal alkynes (with a triple bond at the end of the carbon chain) can give different products and undergo reactions that internal alkynes cannot.

Preparation of Alkynes

Alkynes are less common in nature but can be synthesized in the laboratory. The most common method involves the elimination of alkyl dihalides.

• Alkyl dihalides (vicinal or geminal) are prepared from alkenes.

• Treatment with a strong base (e.g., NaNH2) induces double elimination to form the alkyne.

Example Reaction:

Addition of HX or X2 to Alkynes

Alkynes undergo electrophilic addition reactions similar to alkenes, but the process can occur twice due to the presence of two π bonds.

• The first addition yields an alkene, which can further react to form a dihalide or tetrahalide.

• The mechanism involves the formation of a vinyl carbocation, which is less stable than alkyl carbocations.

Example:

• 1-Hexyne + HBr → 2-Bromo-1-hexene → 2,2-Dibromohexane

• 1-Butyne + Br2 → (E)-1,2-Dibromo-1-butene → 1,1,2,2-Tetrabromobutane

Mechanism Comparison Table

Hydration of Alkynes

Hydration adds water across the triple bond, forming an enol (vinyl alcohol), which rapidly tautomerizes to a carbonyl compound.

• Internal alkynes: Markovnikov's rule does not apply due to symmetry.

• Terminal alkynes: Markovnikov's rule applies; the more substituted carbon receives the hydroxyl group.

• The reaction occurs only once due to tautomerization.

Key Reactions:

• Acid-catalyzed hydration:

• Mercury(II)-catalyzed hydration:

• Hydroboration-oxidation:

Tautomerization of Vinyl Alcohols

Vinyl alcohols (enols) formed during hydration spontaneously rearrange to carbonyl compounds (aldehydes or ketones) via tautomerization.

• This is an equilibrium reaction favoring the carbonyl form.

• Can be catalyzed by acids or bases, with different mechanisms.

General Reaction:

Acid vs. Base Catalyzed Tautomerization

Hydrogenation of Alkynes

Alkynes can be reduced to alkanes by catalytic hydrogenation. The process requires a catalyst due to the strength of the H-H bond.

• Palladium on carbon (Pd/C) catalyzes syn addition of H2 across the triple bond.

• The reaction is not chemoselective; the intermediate alkene is further reduced to an alkane.

General Reaction:

Selective Reduction of Alkynes

Alkynes can be selectively reduced to either cis or trans alkenes using specific reagents.

• Cis-alkene: Lindlar catalyst (poisoned Pd) allows partial hydrogenation to the cis-alkene.

• Trans-alkene: Dissolving metal reduction (Na or Li in liquid NH3) yields the trans-alkene via a radical mechanism.

Summary Table:

Alkylation of Terminal Alkynes (Acetylide Anions)

Terminal alkynes have acidic hydrogen atoms (pKa ≈ 24) and can be deprotonated to form acetylide anions, which are strong nucleophiles.

• Deprotonation with a strong base (e.g., NaNH2).

• Alkylation with a primary alkyl halide via nucleophilic substitution (SN2 mechanism).

General Reaction:

Organic Synthesis Using Alkynes

Alkynes are valuable intermediates in organic synthesis, allowing for the construction of complex molecules through carbon-carbon bond formation.

• Retrosynthesis: Planning synthetic routes by working backward from the target molecule.

• Alkynes can be used to introduce functional groups or extend carbon chains.

Example: Synthesis of alcohols, ketones, or aldehydes from alkynes via hydration and tautomerization.

Click Chemistry with Alkynes

Click chemistry refers to a class of biocompatible reactions that are fast, selective, and yield high-purity products. The copper-catalyzed azide-alkyne cycloaddition is a prominent example.

• Alkynes react with azides (R-N3) in the presence of a Cu catalyst to form 1,2,3-triazoles.

• Widely used in chemical labeling and biological studies due to the inertness of alkynes and azides under physiological conditions.

General Reaction:

Additional info: Strain-promoted click reagents have been developed for cell studies, allowing for catalyst-free reactions in biological environments.