Carboxylic acid derivatives are formed by replacing the hydroxyl group of carboxylic acids with a different functional group. The most common carboxylic acid derivatives are:

Acid halides	Acid anhydrides	Esters	Amides

Cyclic esters and cyclic amides are referred to as lactones and lactams, respectively.

All carboxylic acid derivatives contain a carbonyl group. However, nitriles have a cyano group where a carbon atom is triply bonded to a nitrogen atom.

Carboxylic acid derivatives in nature

Amongst the acid derivatives, acid halides and acid anhydrides are not commonly found in nature. Esters are more widespread and found in naturally occurring fats and oils. They are also responsible for the characteristic fragrance of fruits and flowers. Some examples of naturally occurring esters include:

Methyl butanoate	Isopentyl acetate	Butyl Acetate

Amides linkages are most common in biomolecules. For example, proteins are composed of repeating amino acid units linked by an amide (CO–NH) bond.