A carboxylic acid's defining functional group, a carbon double-bonded to one oxygen and single-bonded to a hydroxyl group, makes the molecule genuinely acidic, capable of donating a hydrogen ion, and it also serves as the reactive starting point for a whole family of closely related derivatives. Esters, amides and several other derivative types are all formed by swapping that hydroxyl group for a different chemical partner while keeping the same underlying carbon-oxygen backbone intact, which is exactly why this one functional group turns up, in one form or another, in substances as different as vinegar, aspirin and soap.
The carboxylic acid group's structure is what makes the molecule genuinely acidic
The carboxylic acid group's specific arrangement, that double-bonded oxygen pulling electron density away from the neighbouring hydroxyl group, weakens the bond holding that hydroxyl's hydrogen in place, making it considerably easier for the molecule to release that hydrogen as a hydrogen ion than an ordinary alcohol's hydroxyl group would. This structural feature is exactly what earns carboxylic acids their genuine acidity, and it's why a molecule like acetic acid, the active component in vinegar, actually behaves as a real, if comparatively weak, acid.
Swapping the hydroxyl group for a different partner produces an entire derivative family
Reacting a carboxylic acid so that its hydroxyl group is replaced by a different chemical group produces a whole family of related derivatives, replacing it with an alcohol-derived group produces an ester, replacing it with a nitrogen-based group produces an amide, and each derivative keeps the same underlying carbon-oxygen backbone while gaining genuinely different chemical properties from whatever new partner replaced the original hydroxyl. This shared structural lineage is exactly why esters, amides and carboxylic acids themselves are studied together as one connected family rather than as unrelated compounds, and it's why understanding the parent carboxylic acid's reactivity explains so much about its many practical derivatives.
What we're still unsure about
That the carboxylic acid functional group's structure explains both its genuine acidity and its ability to form a family of structurally related derivatives is well established, thoroughly confirmed organic chemistry. What's more genuinely a matter of ongoing predictive nuance is exactly how strongly acidic a specific carboxylic acid actually turns out to be, since neighbouring groups elsewhere on the same molecule can meaningfully strengthen or weaken that acidity beyond what the basic carboxylic acid group alone would suggest, and chemists continue to refine predictive models accounting for these neighbouring-group effects across the genuinely enormous range of real carboxylic acid structures found in nature and industry.
This sits inside Carboxylic Acids & Their Derivatives, one of eight topics in Organic Chemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.