Depto. Química Orgánica

PHYSICAL AND BOND PROPERTIES OF AMIDES

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15/07/2017
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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.
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Acetamide, N-methylacetamide and N,N-dimethylacetamide bear boiling points of 220º, 200º and 170ºC, 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 efective 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.
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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.
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Let's take methylamine and acetaldehyde as true single C-N and double C=O bonds.
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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.