Explain why the urea in particular, or amides in general, are much weaker bases than amines?
Urea and amides in general are much weaker bases compared to amines due to the presence of a carbonyl (C=O) group in their structure. The basicity of a molecule depends on its ability to donate or share an electron pair. In the case of amines, they are able to donate their lone pair of electrons easily, making them fairly strong bases.
However, in the case of urea and amides, the carbonyl group adjacent to the nitrogen atom reduces their basicity. The electronegativity of the oxygen in the carbonyl group causes the nitrogen atom to be partially positive, decreasing its ability to donate electrons. This leads to a decreased basicity of urea and amides compared to amines.
To understand why amides are weaker bases than urea specifically, we can look at the resonance effect. In amides, there is a resonance structure where the lone pair of electrons on the nitrogen can delocalize onto the carbonyl oxygen. This resonance stabilizes the amide and makes it less likely to donate or share its lone pair of electrons, further reducing its basicity.
In summary, the presence of a carbonyl group in urea and amides reduces their basicity compared to amines. The resonance effect in amides further decreases their basicity, making them even weaker bases than urea.