SolutionsPhysical and Bond Properties
1. Hydrogen bonds
Explain why DMF (N,N-dimethylformamide, mass 73) boils at 153 °C and N-methylacetamide (mass 73) at 205 °C.
N-Methylacetamide has an N–H and forms hydrogen bonds between molecules; DMF has no N–H. Both are very polar, which is why they boil well above an alkane of similar mass.
2. Amides are planar
Rotation about the C–N bond of an amide is strongly hindered (about 75-85 kJ/mol). Why?
Because of the contribution of the third resonance form, which gives the C–N bond double-bond character: its length (1.37 Å) lies between that of a single C–N (1.47 Å) and a double one. The atoms of the amide group lie in a plane.