The triple bond of alkynes is made up of a sigma bond (from sp–sp overlap) and two pi bonds (from p–p orbital overlap). The sigma bond is stronger (83 kcal/mole) and the two pi bonds in total contribute 117 kcal/mole). Removing one on the pi bonds requires 54 kcal/mole. Terminal alkynes are relatively acidic, with pKa values of about 25.
Alkynes have a weak absorption at 2100 but only unsymetrical alkynes absorb. The proton of terminal alkynes is shielded by the triple bond and occurs at approximately 2 ppm. Hydrogens on carbons bound to the sp carbons of an alkyne are deshield and occur at approximately 2.5.
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Alkynes are most valuable as a source of both cis- and trans- alkenes by reduction.
Addition of Br2 to an alkene followed by treatment with a strong base such as NaNH2 results in the formation of an alkyne where the triple bond is between the two carbon atoms that were sp2 in the alkene.
The anion resulting from deprotonation of a terminal alkyne acts as a nucleophile reacting with alkyl halides (and sulfonate esters) in an SN2 reaction. Yields are good with methyl and primary alkyl halides.
Reduction with H2/Pd of an alkyne to an alkene is faster than that of an alkene to an alkane. However, both reactions are fast and for practical control of the reaction, Lindlar catalyst is used. Lindlar catalyst is Pd on CaCO3 that has been poisoned with Pb(OAc)2 and quinoline, an organic base.
Halogen acids can add once to alkynes and then again to the resulting vinyl halides. The regiochemistry of the second addition is dictated by the fact that a cation with a directly attached halogen is more stable than one with a halogen on a neighboring carbon. Acid catalyzed addition of water results in ketones but only symmetrical and terminal alkynes give a single product. The reaction is greatly accelerated by the presence of Hg++ as a Lewis acid which bridges the two carbons and thus no intermediate carbocation is formed initially.
Terminal alkynes undergo addition of HBr in the presence of radicals in an anti-Markovnikov sense. Hydroboration-oxidation converts terminal alkynes to aldehydes.
Alkenyl halides, prepared by the additionof HX to an alkyne with either Markovnikov or Anti-Markovnikov regiochemistry, are useful intermediates. Reaction with Mg affords vinyl Grignard reagents that have the characteristic reactivity as nucleophiles. Thus, the add to ketones and alkdehydes to form allylic alcohols. Skip the Heck reaction.