A-Level Carboxylic acids, acyl chlorides and esters: worked solution

Question

Both methylamine (CH3NH2) and ethanamide (CH3C(O)NH2) possess a C-N bond. In methylamine, this bond gives rise to an infrared absorption near 1100 cm-1. Propose the position of the C-N infrared absorption in ethanamide and account for your suggestion.

In ethanamide the nitrogen lone pair is delocalised into the adjacent C=O group (amide resonance), giving the C-N bond partial double-bond character. This makes the C-N bond stronger and stiffer (larger force constant) than the pure single bond in methylamine, so it vibrates at a higher frequency. The absorption therefore appears at a higher wavenumber than 1100 cm-1.