Alkyne Hydrogenation
Alkyne Hydrogenation - Related Videos
02:24
Reduction of Alkynes to cis-Alkenes: Catalytic Hydrogenation
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2023
Introduction 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. Thermodynamic Stability Catalytic hydrogenation reactions help evaluate the relative thermodynamic stability of hydrocarbons. For example, the heat of hydrogenation of acetylene is −176 kJ/mol, and that of ethylene is −137 kJ/mol. The higher exothermicity associated with...
06:06
Hydrogenation
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2023
Source: Vy M. Dong and Zhiwei Chen, Department of Chemistry, University of California, Irvine, CA This experiment will demonstrate the hydrogenation of chalcone as an example of an alkene hydrogenation reaction (Figure 1). In this experiment, palladium on carbon (Pd/C) will be used as a heterogeneous catalyst for the process. A balloon will be used to supply the hydrogen atmosphere. Figure 1: Diagram showing the hydrogenation of chalcone to 3-phenylpropiophenone.
02:39
Nomenclature of Alkynes
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2025
Alkynes are unsaturated hydrocarbons characterized by the presence of carbon-carbon triple bonds and have a general formula CnH2n-2. The nomenclature of alkynes follows a set of rules similar to alkanes and alkenes; however, alkynes bear the suffix "-yne" instead of "-ane" or "-ene." There are two approaches to naming alkynes: • IUPAC names • Common names IUPAC nomenclature The IUPAC naming system follows a systematic approach to deduce names of alkynes based on their chemical structure.
08:56
Catalytic Reactor: Hydrogenation of Ethylene
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2023
Source: Kerry M. Dooley and Michael G. Benton, Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA The hydrogenation of ethylene (C2H4) to ethane (C2H6) has often been studied as a model reduction reaction in characterizing new metal catalysts.1-2 While supported nickel is not the most active metal catalyst for this reaction, it is active enough that reaction can take place at < 200°C. The reaction typically involves adsorbed, dissociated hydrogen (H2) reacting...
02:42
Acidity of 1-Alkynes
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2023
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. Ethane (pKa = 51) Ethene (pKa = 44)
FAQs
The metal surface brings the alkyne and hydrogen gas into a reactive arrangement through adsorption. Adsorption activates both reactants, allowing hydrogen atoms to add to the carbon–carbon triple bond stepwise rather than in a single event. This surface-mediated pathway explains why the catalyst is central to the reaction and why its activity influences whether reduction stops at an alkene or continues to an alkane.
Reduction can stop after the first stage or proceed through a further reduction, depending on the catalyst and reaction conditions. A catalyst that favors partial reduction preserves the alkene, whereas a more active catalyst promotes additional hydrogen addition and forms the alkane. This distinction gives chemists control over the extent of structural change in the starting alkyne.
Hydrogen atoms usually add syn, meaning they are delivered to the same side of the reacting triple bond during surface-mediated reduction. When the reaction is controlled to stop at the alkene stage, this mode of addition contributes to formation of a cis alkene. The stereochemical outcome matters when a synthesis requires a defined molecular arrangement rather than only a reduced functional group.
Lindlar’s catalyst favors partial reduction of an alkyne to a cis alkene instead of promoting complete conversion to an alkane. Its value lies in selectivity: the triple bond is transformed while the intermediate alkene is retained. By contrast, more active catalysts generally support further hydrogenation, so catalyst choice determines both the reaction endpoint and the structure obtained.
A synthesis begins by identifying whether the desired product is an alkene or an alkane, then selecting a catalyst and reaction conditions that favor that endpoint. Hydrogen gas and the alkyne must interact on the catalyst surface, where the reduction occurs stepwise. Planning therefore centers on matching catalyst activity and conditions to the required degree of reduction and alkene stereochemistry.
The transformation is useful when a synthesis must reduce a triple bond while controlling the resulting molecular structure. It can help construct defined structures, selectively modify functional groups, and prepare compounds used in pharmaceuticals, polymers, and other valuable materials. In chemistry research, its importance comes from combining a straightforward reduction with control over whether an alkene or alkane is produced.