Past exam of the chemistry B course of the University of Cambridge / 2023 / Paper 1 / B5

The acceptor-strength trend is amide oxygen and amine nitrogen strongest, ketone oxygen weaker, hydrocarbon essentially nonaccepting. trimethylamine and isopropylamine each provide a relatively available nitrogen lone pair; adding N–H bonds changes donor ability much more conspicuously than it changes the acceptor ranking in this comparison. Alkyl substitution and solvation modify the precise values, so the matching amine parameters should be read as the stated empirical strengths rather than a universal equality of all amines.

In acetamide, nitrogen lone-pair donation contributes an ${\mathrm{O}}^{-}-\mathrm{C}={\mathrm{N}}^{+}$ form contributing to resonance in chemistry, increasing electron density at oxygen and making it a strong hydrogen bond acceptor. The amide acceptor is oxygen, not nitrogen: the nitrogen lone pair is delocalized into the carbonyl. acetone also accepts at oxygen, but lacks this nitrogen-to-carbonyl donation and has the smaller hydrogen-bond acceptor parameter. isobutane has neither a heteroatom lone pair nor a comparable acceptor site; its zero value is consistent with the absence of an ordinary directional hydrogen bond acceptor.

The clearest comparison is between the two amines, which have the same molecular formula and molar mass. isopropylamine can donate N–H hydrogen bonds to neighbouring nitrogen lone pairs; trimethylamine has no N–H donor. The resulting intermolecular association raises the primary amine's boiling point, allowing it to remain liquid at room temperature, whereas trimethylamine is gaseous. Independent phase-change data place their normal boiling points on opposite sides of room temperature.^[ref]

acetone has a substantial carbonyl dipole and London dispersion forces sufficient for a room-temperature liquid, even though it cannot build a strong donor–acceptor hydrogen bond network by itself. isobutane has mainly London dispersion forces and a compact shape and remains gaseous. acetamide combines carbonyl acceptors with two N–H donors, allowing an extended, directionally bonded crystal lattice; favourable packing as well as intermolecular association stabilizes its solid state.

These compounds have similar molar masses, so mass alone cannot explain the differences. The hydrogen-bond acceptor parameter also cannot predict the phase on its own: a pure substance needs complementary donors to exploit strong acceptor sites, and freezing additionally depends on crystal packing and entropy. A stronger acceptor does not automatically imply a higher melting point for arbitrary molecules.

Transfer is favourable when new water–solute interactions compensate the cavity and reorganization costs of accommodating the solute. The Gibbs energy also includes the loss of solute–alkane interactions; it is not simply a count of hydrogen bonds. isobutane cannot compensate those costs with strong directional interactions, explaining its positive transfer Gibbs energy. acetone and both amines can accept hydrogen bonds from water and have favourable electrostatic solvation, making their transfer moderately favourable. isopropylamine can additionally donate to water, consistent with a somewhat greater preference for water than trimethylamine.

acetamide combines a strong carbonyl acceptor with N–H donors and has the most favourable hydration in the group, accounting for its much more negative transfer Gibbs energy. Its pure solid lattice is not an extra sublimation penalty to insert here: the stated process starts from a dilute alkane solution and ends in dilute water, rather than dissolving the pure crystal. The nonmonotonic relationship between hydrogen-bond acceptor parameter and transfer Gibbs energy is expected because donor strength, nonpolar surface, solvent cavity formation, and all other solvation contributions also enter.

For matched standard states, $\Delta G_{\mathrm{tr}}^{\circ }=-RT\ln K_{\mathrm{tr}}$, with $K_{\mathrm{tr}}$ the water-to-alkane activity ratio. As an illustrative 298 K calculation, the hydrocarbon value implies $K_{\mathrm{tr}}\simeq 7.0\times 10^{-4}$, whereas the amide value implies approximately $3.6\times 10^4$. These are standard-state partition preferences, not absolute aqueous solubilities. amine protonation could introduce an additional pH-dependent contribution to an experimentally measured distribution coefficient, but no such speciation information is supplied and it is not needed for the molecular comparison.

User-deferred: biology-related part. This part is left unsolved under the instruction to skip biology, including oligonucleotide assembly and protein-receptor recognition. No answer or associated concept research has been added.

User-deferred: biology-related part. This part is left unsolved under the instruction to skip biology, including oligonucleotide assembly and protein-receptor recognition. No answer or associated concept research has been added.

User-deferred: biology-related part. This part is left unsolved under the instruction to skip biology, including oligonucleotide assembly and protein-receptor recognition. No answer or associated concept research has been added.

User-deferred: biology-related part. This part is left unsolved under the instruction to skip biology, including oligonucleotide assembly and protein-receptor recognition. No answer or associated concept research has been added.