The full, readable lecture — the –COOH group and why it is acidic, salts and the CO₂ fizz, esterification, soap and the ester family, with their relative reactivity. As you scroll, the panel on the right shows each idea through an everyday object: vinegar, a lemon, a fizzing antacid, fruity perfume, a soap cauldron and an aspirin tablet.
Open a bottle of vinegar and the sharp, sour tang is acetic (ethanoic) acid, CH₃COOH — a carboxylic acid. Every carboxylic acid carries the same business end: the carboxyl group, –COOH, a carbonyl C=O joined to a hydroxyl –OH on one carbon. The general formula is RCOOH.
| Structure | IUPAC name | Common / source |
|---|---|---|
| HCOOH | methanoic acid | formic — ant & nettle sting |
| CH₃COOH | ethanoic acid | acetic — vinegar |
| CH₃CH₂COOH | propanoic acid | propionic acid |
IUPAC names end in -oic acid; the carboxyl carbon is always carbon number 1.
Bite a lemon and the puckering tartness is citric acid — a single molecule carrying three –COOH groups. In water each group lets its proton go:
A carboxylic acid is far more acidic than an alcohol because the carboxylate ion RCOO⁻ is resonance-stabilised. Once the proton leaves, the negative charge is not stuck on one oxygen — it is shared equally over both oxygens:
Stir an antacid tablet or a spoon of baking soda into something acidic and it fizzes — that froth is carbon dioxide. As acids, –COOH compounds neutralise bases and react with carbonates:
The fruity smell of a banana, a pineapple or a dab of perfume is an ester. A carboxylic acid reacts with an alcohol, over conc. H₂SO₄ as catalyst and dehydrating agent, to make one:
Isotope labelling shows the water comes from the –OH of the acid and the –H of the alcohol. It is an equilibrium: conc. H₂SO₄ removes the water (Le Chatelier), pulling the reaction toward more ester. Use the slider on the right to drive off water and watch the yield — and the smell — grow.
A fat is simply an ester — glycerol joined to three long fatty-acid chains. Boil it with NaOH (lye) in a cauldron and the ester links hydrolyse:
The tablet of aspirin in your medicine cabinet is an ester drug, made in industry by acylating salicylic acid. The fastest acylating agent is the acyl chloride, RCOCl — the –OH of the acid replaced by –Cl:
Two more reactions of the acid itself: LiAlH₄ reduces –COOH all the way to a 1° alcohol; heating the sodium salt with soda-lime decarboxylates it, knocking out –COOH as carbonate.
Picture four doors with locks of falling strength. The better the leaving group and the less the carbonyl is stabilised, the more reactive the derivative:
An acyl chloride has the best leaving group (Cl⁻), so it reacts fastest. An amide's nitrogen lone pair feeds into the carbonyl and stabilises it, so it reacts slowest. A more reactive derivative can always be converted into a less reactive one — never easily the reverse.