Catalytic Hydrogenation Reactions
Catalytic hydrogenation uses transition metal catalysts (like Pt) to add hydrogen across carbon–carbon multiple bonds. When applied to alkynes, this reaction can lead to full saturation to give alkanes if excess hydrogen and catalyst are used.
Nucleophilic Substitution via Acetylide Anion Formation
This concept refers to the generation of a nucleophilic acetylide anion from an alkyne through deprotonation (using strong bases like sodium amide). The resulting acetylide can then undergo nucleophilic substitution reactions with electrophilic alkyl halides to form new carbon–carbon bonds, extending the carbon chain.
Dissolving Metal Reductions
This reaction type involves the reduction of alkynes using metals (such as sodium) in liquid ammonia, leading to the formation of alkenes. The reduction often yields trans alkenes due to the radical anion intermediates and subsequent protonation steps.
Stereoselective Partial Hydrogenation (Lindlar Catalyst)
This concept involves the use of a poisoned catalyst (Lindlar catalyst) to stop the hydrogenation reaction at the alkene stage rather than fully reducing the alkyne to an alkane. The process typically yields a cis or syn alkene due to the surface-controlled addition of hydrogen atoms.
Radical Addition Mechanisms
Under conditions involving peroxides, radical pathways can dominate, leading to anti-Markovnikov outcomes in hydrohalogenation reactions. The formation of radicals changes the addition pathway, which can result in different regio- and stereochemical products compared to the ionic mechanism.
Oxymercuration-Hydration of Alkynes
This reaction involves the addition of a water molecule across the triple bond in the presence of mercury(II) sulfate and aqueous acid, avoiding carbocation rearrangements. The initial product is an enol, which then tautomerizes to yield a carbonyl compound, typically a ketone.
Halogenation of Alkynes
Halogenation involves the addition of halogens, like Br2, to alkynes resulting in vicinal or geminal dihalides. The reaction mechanism typically follows anti addition, and the stereochemistry of the final product is influenced by the reaction medium and the number of halogen equivalents used.
Markovnikov vs Anti-Markovnikov Addition
These concepts explain the regiochemistry observed in electrophilic additions to unsaturated systems. In the Markovnikov addition, the electrophile (such as HBr) adds so that the hydrogen attaches to the carbon with more hydrogens, while the halide attaches to the more substituted carbon. Under different conditions, such as in the presence of peroxides, the addition can reverse (anti-Markovnikov), giving rise to different isomeric products.
Electrophilic Addition to Alkynes
This concept covers the general reactivity of alkynes towards electrophiles. When an alkyne is treated with electrophiles like HBr or Br2, addition reactions occur across the triple bond. The mechanism and final product depend on the number and type of electrophiles used and can lead to single or multiple addition products.
Hydroboration-Oxidation of Alkynes
In this two-step reaction, an alkyne reacts with a borane reagent to form a trialkylborane intermediate, which upon oxidation with hydrogen peroxide in basic media yields an enol that rapidly tautomerizes to give an aldehyde or ketone. This process is stereospecific and typically proceeds via syn addition, leading to anti-Markovnikov products.