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Acetamide is the amide $\mathrm{C}{\mathrm{H}}_3\mathrm{CON}{\mathrm{H}}_2$. Donation of the nitrogen lone pair into the carbonyl system gives the carbon-nitrogen bond partial double-bond character.

This favors a relatively planar local geometry and reduces nitrogen basicity compared with a simple amine. Acetamide provides a small-molecule model for the electronic features of the peptide bond.

An amide bond, in the carboxamide sense used here, is the carbon–nitrogen covalent bond in the unit $\mathrm{R}-\mathrm{C}(=\mathrm{O})-\mathrm{N}{\mathrm{R}}^{\prime }{\mathrm{R}}^{\prime \prime }$. The nitrogen is directly attached to the carbonyl group's carbon. Merely finding a carbon–nitrogen bond elsewhere in a molecule does not identify an amide bond. The broader IUPAC class name amide also covers other acid-derived compounds; this article concerns the carbonyl linkage.^[ref]

For example, acetamide has connectivity $\mathrm{C}{\mathrm{H}}_3-\mathrm{C}(=\mathrm{O})-\mathrm{N}{\mathrm{H}}_2$. Its amide C–N bond is the bond from the carbonyl carbon to nitrogen, rather than the adjacent C–C bond. A peptide bond is a biologically important instance of this connectivity in a chain of amino acids.

Chemical resonance describes electron sharing through the nitrogen and carbonyl group. Two useful contributing structures areThese are descriptions contributing to one electronic structure, rather than two molecules rapidly alternating. The amide bond consequently has partial double bond character, and the ordinary amide group favors approximate planarity. Linus Pauling's lecture connects that structural picture to measured bond lengths and explains why hypothetical resonance forms remain useful.^[ref]

This supplies a useful modeling constraint: a chain containing peptide bonds cannot be treated as if every backbone single bond rotates equally freely. Approximate amide planarity constrains the conformations available without asserting that every amide is perfectly planar.

A carboxamide is an amide of a carboxylic acid, with a carbonyl group directly bonded to nitrogen. Its general connectivity is $\mathrm{R}-\mathrm{C}(=\mathrm{O})-\mathrm{N}{\mathrm{R}}^{\prime }{\mathrm{R}}^{\prime \prime }$, where the substituents shown can include hydrogen. The term identifies a class of compounds or a structural unit; amide bond identifies the carbonyl C–N bond within it.^[ref]

For example, acetamide, $\mathrm{C}{\mathrm{H}}_3\mathrm{C}(=\mathrm{O})\mathrm{N}{\mathrm{H}}_2$, contains one carboxamide group. Replacing an N–H covalent bond by an N–C covalent bond can leave the defining carbonyl C–N connectivity intact. Conversely, replacing the carbonyl oxygen by another group changes the functional class even though a C–N bond remains. Connectivity, rather than merely an inventory of elements, supplies the classification.

A formal atom balance illustrates carboxamide hydrolysis:Both sides contain two carbons, seven hydrogens, one nitrogen and two oxygens. The equation balances neutral molecular formulas; the actual reaction products can be protonated or deprotonated according to pH. It does not assert that mixing neutral reactants gives rapid conversion. Ordinary amide hydrolysis needs suitable conditions or enzyme catalysis, consistent with the persistence of peptide bonds in water.^[ref]

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.

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.

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.