Amide 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.