Every carboxylic acid has the –COOH group sitting at the end of a carbon chain. Think of it as a carbon double-bonded to one oxygen (like a ketone) and single-bonded to an –OH group, both attached to the same carbon. That carbon is always numbered carbon 1 — which is why you never need locant numbers in the name (unlike "propan-2-ol" or similar).

Naming pattern

Carboxylic acids are named: alkan + oic acid. So the 2-carbon chain (ethane) becomes ethanoic acid, the 3-carbon chain (propane) becomes propanoic acid, and so on. Some countries call the whole family "alkanoic acids" instead of "carboxylic acids" — same thing.

Why the boiling points are so high

Carboxylic acids contain two polarised groups: the C=O and the O–H. Because of the O–H bond, molecules can form hydrogen bonds with each other — and in fact, two carboxylic acid molecules can pair up into a "dimer," hydrogen-bonding to each other at both ends like a molecular handshake. That's a lot of intermolecular attraction to break, which is why carboxylic acids have unusually high melting and boiling points for their size — often higher than alcohols of similar molar mass.

Solubility — and why it fades with chain length

The same O–H group that gives high boiling points also lets short-chain carboxylic acids hydrogen-bond with water molecules — so methanoic, ethanoic and propanoic acid mix with water in any proportion. But as the hydrocarbon "tail" gets longer, it starts wedging itself between water molecules, physically breaking up the water's own hydrogen-bond network. The tail is non-polar and water can't hydrogen bond to it, so solubility drops off. Acids with more than 8 carbons are solids at room temperature and barely dissolve in cold water (though hot water helps a bit).

Route 1: Oxidation of primary alcohols & aldehydes

Primary alcohols oxidise in two stages: first to an aldehyde, then all the way to a carboxylic acid. Aldehydes can also be oxidised straight to carboxylic acids in a single step. The oxidising agent is either acidified potassium dichromate(VI), K₂Cr₂O₇, or acidified potassium manganate(VII), KMnO₄ — always under reflux, so that the volatile intermediate doesn't escape before it's fully oxidised.

You can spot that oxidation is happening because the oxidising agent itself gets reduced, and that shows up as a colour change:

• K₂Cr₂O₇: orange dichromate ions (Cr₂O₇²⁻) → green Cr³⁺ ions
• KMnO₄: purple manganate ions (MnO₄⁻) → colourless Mn²⁺ ions

Route 2: Hydrolysis of nitriles

A nitrile has a –C≡N group. Hydrolysing it (breaking it apart with water, using either dilute acid or dilute alkali) converts the –CN into a –COOH group, adding one carbon's worth of carboxylic acid onto the chain.

• Acid hydrolysis (dilute HCl + water, heat): gives the carboxylic acid directly, plus an ammonium salt as a by-product.
• Alkaline hydrolysis (dilute NaOH + water, heat): gives a sodium carboxylate salt plus ammonia gas — you then need a separate acidification step to convert the carboxylate salt into the actual carboxylic acid.

Carboxylic acids are weak acids

In water, carboxylic acids only partially ionise:

The equilibrium sits well over to the left, meaning most molecules stay as CH₃COOH — only a small fraction ionises to give H⁺ and the carboxylate ion (CH₃COO⁻). That's exactly what "weak acid" means: not fully dissociated (compare with strong acids like HCl, which ionise essentially completely).

The carbonate test — a classic identification reaction

Even though the H⁺ concentration is low, it's still enough to react with carbonates and hydrogen carbonates, producing fizzing CO₂ gas. This is a genuinely useful lab test: if bubbling/fizzing occurs with sodium carbonate or sodium bicarbonate solution, you likely have a carboxylic acid (or another acid) present.

Reduction with LiAlH₄

Lithium tetrahydridoaluminate (LiAlH₄) is a powerful reducing agent — used suspended in dry ether at room temperature (not water, because LiAlH₄ reacts violently with water). It reduces a carboxylic acid all the way down to a primary alcohol.

Notice it takes 4 [H] — more than the 2[H] you'd use to reduce an aldehyde to an alcohol — because you're removing an oxygen as well as adding hydrogens. Water is added at the very end of the reaction simply to destroy/quench any leftover LiAlH₄ safely.

Reaction with bases: metal oxides, alkalis, carbonates

Carboxylic acids behave like any other acid when reacting with these three types of base — the products just follow the standard acid + base rules you already know:

Reaction with phosphorus(V) chloride, PCl₅

Solid PCl₅ converts a carboxylic acid into an acyl chloride — this is actually one of the standard ways to make acyl chlorides in the lab. You get "steamy fumes" of HCl gas as a visible sign the reaction is happening (a classic exam observation to quote!).

The three liquid/gas products (acyl chloride, phosphorus trichloride oxide, HCl) can be separated by fractional distillation because they have different boiling points.

Reaction with alcohols → esters (esterification)

Heating a carboxylic acid with an alcohol, using concentrated H₂SO₄ as a catalyst, gives an ester in a condensation reaction (water is eliminated). Naming is a two-part job: the first part comes from the alcohol, the second part from the acid.

Acyl chlorides (–COCl) and acid anhydrides are both "derivatives" of carboxylic acids — made by swapping out the –OH group for something else (a Cl atom for acyl chlorides, or an alkanoate group for anhydrides). Naming follows the same pattern as the parent acid, but swaps "-oic acid" for "-oyl chloride" or "-oic anhydride."

The mechanism pattern: nucleophilic addition–elimination

Acyl chlorides are much more reactive than carboxylic acids because Cl is a great leaving group. Almost every reaction of an acyl chloride follows the same two-step pattern:

1. Addition — a nucleophile (water, an alcohol, or ammonia/an amine) attacks and adds across the C=O bond.
2. Elimination — a small molecule (HCl, or a Cl⁻ ion that quickly becomes HCl) is kicked out.

Hydrolysis (with water)

Formation of esters (with alcohols)

This route (acyl chloride + alcohol) is actually a better way to make esters in the lab than the carboxylic acid + alcohol route, because it's faster, doesn't need a catalyst, and doesn't set up a reversible equilibrium — it goes essentially to completion.

Formation of amides (with ammonia or amines)

This one has an extra twist. A lone pair on the nitrogen attacks the carbonyl carbon (same addition–elimination idea), and a Cl⁻ is eliminated, forming HCl. But that HCl doesn't just sit there — it's immediately neutralised by a second molecule of ammonia or amine (which must be present in excess), forming an ammonium salt.

Hydrolysis is literally the reverse of esterification — breaking the ester bond apart using water, with either acid or alkali as a catalyst/reagent. Which one you use completely changes how the reaction behaves.

Acid hydrolysis — reversible, equilibrium

Heating an ester under reflux with dilute acid (e.g. dilute H₂SO₄) sets up an equilibrium — the reaction does not go to completion, meaning at any moment you have a mixture of ester + water and carboxylic acid + alcohol.

Alkaline hydrolysis — irreversible, goes to completion

Heating an ester under reflux with dilute alkali (e.g. NaOH) is irreversible — the ester is fully hydrolysed. But there's a catch: the direct product isn't the carboxylic acid, it's the sodium carboxylate salt (because the acidic –COOH that would form immediately reacts with the excess NaOH present). If you actually want the carboxylic acid itself, you need a separate acidification step afterwards.

Addition polymers (like polyethene) are built from monomers with C=C double bonds joining together with nothing lost. Condensation polymers work differently: monomers link together with a small molecule (usually water) eliminated at every join. You can spot a condensation polymer because the repeating monomers are connected by ester bonds (→ polyester) or amide bonds (→ polyamide, like nylon — covered elsewhere).

Route 1: diol + dicarboxylic acid

A polyester needs two things reacting repeatedly:

• A diol — a molecule with 2 –OH groups, one at each end (e.g. ethane-1,2-diol).
• A dicarboxylic acid — a molecule with 2 –COOH groups, one at each end (e.g. benzene-1,4-dicarboxylic acid).

Every time an –OH end reacts with a –COOH end, an ester bond forms and a water molecule is eliminated. Because both monomers have functional groups at both ends, the chain can just keep growing and growing — that's what makes it a polymer rather than a single small molecule.

The example in the chapter is exactly this reaction — it produces poly(ethylene terephthalate), PET, which you'll know by its trade names Terylene or Dacron (used in polyester fabric and plastic bottles).

Route 2: a single hydroxycarboxylic acid

Instead of using two separate monomers, you can use one monomer that already has both functional groups — an –OH at one end and a –COOH at the other. This is called a hydroxycarboxylic acid (e.g. 2-hydroxybutanoic acid). Since each individual molecule has one of each group, they can link up head-to-tail indefinitely, again releasing water at every ester bond formed.