Physical and Bond Properties

Acetamides have much higher boiling points than the corresponding alkanes of similar molecular weight. For instance, in such a small molecule like acetamide, the polarity of the amide group does contribute to stronger intermolecular forces.

Plot of boiling points against molecular weight for alkanes, esters, acyl halides, acids and amides

Acetamide, N-methylacetamide and N,N-dimethylacetamide bear boiling points of 220, 200 and 170 °C, respectively.

Molecular cohesion is higher in acetamide that can accept/give intermolecular hydrogen bonding on its CONH₂ group.

N-methylacetamide lacks a hydrogen in its CONH group as compared to acetamide's CONH₂. The network of intermolecular hydrogen bonding is less effective in N-methylacetamide thus explaining its lower boiling point.

In N,N-dimethylacetamide there are no hydrogens on N and therefore it cannot establish hydrogen bonding hence displaying the lowest boiling point in the series.

Resonance forms of an acid derivative R–CO–L: negative charge on the oxygen and positive charge on the carbon or on the L group

The bond lengths of C=O and C-L bonds can be explained by a larger or smaller contribution of the first and third resonance forms.

Let's take methylamine and acetaldehyde as true single C-N and double C=O bonds.

C=O bond length in acetaldehyde: 1.231 Å

Bond lengths of acetamide: C=O 1.248 Å and C–N 1.374 Å

Any conclusions?

There should be an important contribution of the third resonance form that allows us to explain such a short C-N and long C=O.

The contribution of this third resonance form in amides is the highest in the carboxylic acid derivatives.