Reactions of Surface-Confined Terminal Alkynes Mediated by Diverse Regulation Strategies
1Interdisciplinary Materials Research Center, School of Materials Science and Engineering, Tongji University, Shanghai 201804, China.
Abstract:
Terminal alkynes, characterized by sp-hybridized carbon atoms at the molecular termini, possess high electron density and exceptional chemical reactivity. These properties make them ideal candidates for the synthesis of one-dimensional molecular wires and two-dimensional networks. Advances in nanoscale characterization techniques, such as scanning tunneling microscopy and atomic force microscopy, have enabled the real-space visualization of molecular assembly and chemical reactions of terminal alkynes and in situ atomic-level manipulations under surface-confined conditions. In addition, through the combination of spectroscopic measurements, physicochemical properties of and information about resulting nanostructures have been achieved. Moreover, density functional theory calculations provide deeper insights into the underlying reaction pathways and mechanisms. From this perspective, this review summarizes recent progress in the assembly and chemical transformations of terminal alkynes on noble metal surfaces. It discusses strategies for structural modulation and reaction selectivity control, including direct incorporation of heteroatoms or functional groups into precursors, the selection of metal surfaces, the introduction of extrinsic components into molecular systems, and atomic-scale manipulations using scanning probes.
Related Concept Videos
02:27Preparation of Alkynes: Alkylation Reaction
Alkylation of terminal alkynes with primary alkyl halides in the presence of a strong base like sodium amide is one of the common methods for the synthesis of longer carbon-chain alkynes. For example, treatment of 1-propyne with sodium amide followed by reaction with ethyl bromide yields 2-pentyne.
02:47Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation
One of the convenient methods for the preparation of aldehydes and ketones is via hydration of alkynes. Hydroboration-oxidation of alkynes is an indirect hydration reaction in which an alkyne is treated with borane followed by oxidation with alkaline peroxide to form an enol that rapidly converts into an aldehyde or a ketone. Terminal alkynes form aldehydes, whereas internal alkynes give ketones as the final product.
02:42Acidity of 1-Alkynes
The acidic strength of hydrocarbons follows the order: Alkynes > Alkenes > Alkanes. The strength of an acid is commonly expressed in units of pKa — the lower the pKa, the stronger the acid. Among the hydrocarbons, terminal alkynes have lower pKa values and are, therefore, more acidic. For example, the pKa values for ethane, ethene, and acetylene are 51, 44, and 25, respectively, as shown here.
02:40Alkynes to Aldehydes and Ketones: Acid-Catalyzed Hydration
Analogous to alkenes, alkynes also undergo acid-catalyzed hydration. While the addition of water to an alkene gives an alcohol, hydration of alkynes produces different products such as aldehydes and ketones.
02:24Reduction of Alkynes to cis-Alkenes: Catalytic Hydrogenation
Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves two sequential syn additions of hydrogen via a cis-alkene intermediate.
02:38Electrophilic Addition to Alkynes: Halogenation
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a π bond. In alkynes, the presence of two π bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.