Chapter 9: Alkynes: An Introduction to Organic Synthesis
A triple bond is one sigma bond and two perpendicular pi bonds
Each carbon of a carbon-carbon triple bond is sp hybridized. One 2s orbital mixes with one 2p orbital to produce two sp hybrid orbitals with 50 percent s character, and two 2p orbitals are left unhybridized. The two sp hybrids on a given carbon point 180 degrees apart, so an sp carbon and the two atoms bonded to it are collinear. This linear geometry is the most visible structural consequence of the triple bond: propyne is a straight rod, and a triple bond cannot be accommodated inside a small ring. The sigma bond of the triple bond comes from head-on overlap of one sp orbital from each…
Number the chain to give the triple bond the lower locant
Alkyne naming follows the same substitutive rules as alkanes, with the suffix -yne replacing -ane. First find the longest continuous carbon chain that includes both carbons of the triple bond; a longer chain that misses the triple bond is not the parent. Number the chain from the end nearer the triple bond, so the first of the two triply bonded carbons gets the lower locant, and cite that locant just before the suffix: CH3-C(triple bond)C-CH3 is but-2-yne, and CH(triple bond)C-CH2CH3 is but-1-yne rather than but-3-yne. Substituents are cited as prefixes with their own locants, assigned by the…
HX adds with Markovnikov regiochemistry through a vinyl cation
Hydrogen halides add to alkynes by the electrophilic addition pattern already familiar from alkenes. The pi electrons of the triple bond attack the proton of H-X, and the halide ion then adds to the resulting cation. The regiochemistry is Markovnikov: the proton bonds to the carbon that already carries more hydrogens, which places the positive charge, and ultimately the halide, on the more substituted carbon. Propyne and one equivalent of HBr therefore give 2-bromopropene rather than 1-bromopropene. The intermediate deserves attention because it is where alkyne and alkene reactivity diverge.…
Two hydration routes give a ketone or an aldehyde
Adding water across a triple bond is one of the most useful ways to reach a carbonyl compound, and the chapter presents two complementary routes that differ only in regiochemistry. The first uses aqueous sulfuric acid with a mercuric ion catalyst. Water is delivered in the Markovnikov sense, so the oxygen bonds to the more substituted carbon of the triple bond. Applied to a terminal alkyne this places the oxygen on the internal carbon, and the isolated product is a methyl ketone: propyne gives acetone. The second route is hydroboration-oxidation, and it must be run with a bulky borane —…
The reagent, not the substrate, sets the geometry
Reduction of an internal alkyne is the clearest example in this chapter of a reagent-controlled outcome: one substrate, three products, selected entirely by the conditions. Hydrogen gas over an active heterogeneous catalyst such as palladium on carbon reduces the triple bond all the way. The intermediate alkene is itself a good substrate for the same catalyst, so the reaction does not stop, and the isolated product is the alkane. To stop after one addition the catalyst must be deliberately deactivated; the Lindlar catalyst — palladium deposited on calcium carbonate and poisoned with lead…
s character makes the terminal C-H acidic enough to remove
Hydrogens bonded to carbon are normally not acidic in any useful sense, which makes the terminal alkyne a striking exception. The C-H at the end of a terminal alkyne has a pKa of about 25, compared with roughly 44 for a vinylic C-H and roughly 60 for an alkane C-H. The explanation is entirely about the conjugate base. When the proton is removed, the electron pair that formed the C-H bond stays behind as a lone pair in the same orbital. For a terminal alkyne that orbital is an sp hybrid with 50 percent s character; for an alkene it is sp2 with 33 percent; for an alkane it is sp3 with 25…
Acetylide alkylation builds the carbon skeleton
Most reactions in an introductory course rearrange functional groups on a fixed skeleton. Acetylide alkylation is different: it makes a carbon-carbon bond, and that is why this chapter is subtitled an introduction to organic synthesis. The sequence has two operations. First, sodium amide removes the terminal C-H to give an acetylide anion, whose lone pair sits in an sp orbital on carbon. Second, that anion attacks the electrophilic carbon of an alkyl halide. The attack is a textbook SN2: the nucleophile approaches from the side opposite the halide, the new carbon-carbon bond forms as the…
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