amide

Meaning: An amide has the form RC(=O)NR′₂, with nitrogen bonded directly to a carbonyl carbon. The nitrogen lone pair delocalizes into the C=O through resonance, so the C–N bond has partial double-bond character. The amide group is therefore planar with restricted rotation, and the nitrogen is essentially non-basic. The same resonance stabilizes the group, so amides are the least reactive common carboxylic acid derivatives in nucleophilic acyl substitution. Hydrolysis needs strong acid or base and heat, giving a carboxylic acid (or carboxylate) and an amine (or ammonium). The peptide bond is an amide.

On the MCAT: Expect reactivity rankings (acid chloride > anhydride > ester > amide), products of amide hydrolysis, or an amine attacking an acid chloride to form an amide. Protein passages use the planar, rigid peptide bond to explain limited backbone rotation. In IR spectra, an amide C=O absorbs near 1650 cm⁻¹, a little lower than a ketone, and N–H stretches appear above 3200 cm⁻¹.

Common confusion: Students treat the amide nitrogen like an amine nitrogen and call it basic. Its lone pair is tied up in resonance with the carbonyl, so amides are far weaker bases than amines. Protonation occurs, weakly, on the oxygen rather than the nitrogen.