Amide functional groups in peptides are often described as having "partial double-bond character" between the carbonyl carbon and the adjacent nitrogen, resulting in a nearly planar local geometry.
Consider acetamide, CH₃CONH₂, as a model compound.
(a) Draw a complete Lewis diagram of acetamide that shows all resonance contributors relevant to delocalization between the carbonyl carbon and the nitrogen lone pair.
(b) Predict the hybridization of (i) the carbonyl carbon, (ii) the carbonyl oxygen, and (iii) the nitrogen atom. Explain how resonance affects your choice for the nitrogen atom.
(c) Explain why rotation about the C–N bond in acetamide is significantly restricted compared with rotation about the C–N bond in CH₃NH₂. Your answer should reference orbital overlap and the energetic consequence of disrupting that overlap.
(d) A researcher records the ¹H NMR spectrum of acetamide at 25 °C and again at 150 °C. Predict qualitatively how the splitting pattern of the two nonequivalent hydrogen environments on the NH₂ group is expected to change with temperature, and relate this observation to rotational freedom about the C–N bond.