- You recall from CH 334 the bonding components of a C-C triple bond: one sigma and two (perpendicular) pi bonds.
Visualization: the Molecular Orbitals of Acetylene (Ethyne)
- You recall that strong bases can deprotonate terminal alkynes, turning them into nucleophiles that are isoelectronic with cyanide.
- You know two strategies for forming alkynes: dehydrohalogenation of dihalides (geminal or vicinal), and SN2 substitution of an alkyl halide with an acetylide anion.
- You know that the alkyne pi bonds are more reactive than alkene pi bonds. You can recognize an important consequence in the ability to use the Lindlar catalyst (Pd/CaCO3/Pb(OAc)2) to perform a selective syn-hydrogenation of an alkyne to an alkene, even in the presence of other alkenes.
- You know that alkynes can be reduced by Na in liquid NH3 to form trans-alkenes.
Visualization: YouTube video of Na dissolving in NH3
- You recognize that electrophilic addition reactions behave as they do for alkenes, leading to Markiownikov addition. Usually it is difficult to stop at a single addition, and the intermediate alkene adds a second equivalent of the reagent.
- You know that the addition product in acid catalyzed hydration (including oxymercuration) is called an enol and that this unstable compound spontaneously rearranges ("tautomerizes") to form a ketone.
- You know how to apply the major anti-Markownikov additions to alkynes: HBr/peroxides, and hydroboration/oxidation. Unlike electrophilic additions, these reactions can be stopped after addition of one equivalent because of the different reactivity of the first alkyne pi bond.
- You know the Heck reaction couples an alkenyl halide (made from an alkyne) with a second alkene using a soluble Pd catalyst to form a conjugated diene.
Recommended end-of-chapter problems: 13-29, 13-30, 13-35, 13-38, 13-39, 13-42 (consider possible side reactions for each), 13-44, 13-46, 13-47, 13-50, 13-51.
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