Past exam of the chemistry course of the year 1 of the natural sciences course of the University of Cambridge / 2024 / Paper 1 / 3 / a / Solution

The most acidic sites are: acetic-acid O–H, $\mathrm{p}{K}_a\approx 4.8$; phenol O–H, $\approx 10$; ethanal alpha-CH${}_3$, $\approx 17$–$20$; propyne terminal ≡C–H, $\approx 25$. 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 ${}^{-}\mathrm{C}{\mathrm{H}}_2-\mathrm{CH}=\mathrm{O}\leftrightarrow \mathrm{C}{\mathrm{H}}_2=\mathrm{CH}-{\mathrm{O}}^{-}$. 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 $sp$ orbital. Its greater s character stabilizes negative charge relative to an ordinary $s{p}^2$ or $s{p}^3$ carbanion; the methyl hydrogens are much less acidic.