What is a distinguishing feature of amides compared to esters?

Amides can be distinguished from esters primarily by the substitution of the hydroxyl group present in esters with an amino group in amides. In simpler terms, while esters are formed from a carboxylic acid and an alcohol, amides derive from carboxylic acids where the hydroxyl group is replaced by an amine or ammonia. This structural difference fundamentally alters their chemical reactivity and properties.

For context, both amides and esters contain a carbonyl group; however, this feature is not unique to amides and is found in both functional groups, which makes it an insufficient distinguishing characteristic. Additionally, while both can have alkyl groups attached, the presence of these alkyl groups is not a defining feature that set them apart. Finally, while amides do form from carboxylic acids, esters also form from the same precursor in a different reaction. The key difference lies in the functional groups themselves, making the substitution of the hydroxyl group with an amino group the most defining feature of amides compared to esters.