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Incoming links: Propyne
The two carbon skeletons are allene, , and propyne, . “Linear” describes the three-carbon skeleton; it does not mean every hydrogen lies on the same line.
For allene, use the contributor . A terminal lone pair attacks a water hydrogen while the O–H pair returns to oxygen, giving and hydroxide. Repeat at the still-dianionic end, then protonate each remaining terminal carbanion. The four successive proton transfers givewith two hydrogens placed on each terminal carbon in this branch.
For propyne, the resonance contributor makes the alternative electron flow clear. Three successive protonations of the singly bonded terminal carbon convert its three lone pairs into C–H bonds; protonation of the acetylide end gives . These are formal protonation pathways for the multiply charged anion, not a claim that the highly charged resonance contributors are separately isolable species. Each transfer produces ; overallAllene has one proton resonance integrating to 4H, ideally a singlet because its four hydrogens are chemically equivalent. Proton-decoupled carbon NMR has two resonances, in ratio two terminal carbons to one central carbon: approximately and ppm, respectively. The unusually downfield central cumulene carbon is not a carbonyl.
Propyne has two proton resonances, integrating 3H near – ppm and 1H near – ppm. If the small four-bond coupling is resolved, they are a methyl doublet and an acetylenic quartet; if unresolved, each can look like a singlet. Proton-decoupled carbon NMR has three resonances: one methyl carbon near ppm and two distinct sp carbons around – ppm. These are approximate predictions, not supplied measured shifts.
The most acidic sites are: acetic-acid O–H, ; phenol O–H, ; ethanal alpha-CH, –; propyne terminal ≡C–H, . These are ordinary approximate organic-chemistry estimates; exact acidities depend on solvent and reference convention.
Acetic acid gives a carboxylate in which negative charge is delocalized over two oxygens through equivalent resonance structures. Phenol gives a phenoxide with delocalization into the aromatic system, but its important contributors are less equivalent and do not stabilize the charge as effectively as a carboxylate.
For ethanal, removal of a methyl hydrogen gives the enolate . The aldehydic hydrogen is not the preferred acidic site: removing it does not give this alpha-carbon/oxygen delocalization. For propyne, loss of the terminal hydrogen gives an acetylide with a lone pair in an orbital. Its greater s character stabilizes negative charge relative to an ordinary or carbanion; the methyl hydrogens are much less acidic.