Use hydrogen bonding and acid–base chemistry to explain properties, then select transformations that retain or extend the carbon skeleton.
COOH is one functional group
A carboxylic acid contains –C(=O)OH, conventionally written –COOH. Both oxygen atoms are attached to the same carbon. CH₃COOH is ethanoic acid; HOCH₂CHO instead contains an alcohol and an aldehyde and is not a carboxylic acid. When naming an acyclic acid, the carboxyl carbon is carbon 1.
A carboxylic acid is a weak Brønsted–Lowry acid in water: only part of it dissociates to carboxylate and H₃O⁺. Weak describes the equilibrium extent of ionisation, not the concentration. The conjugate base has its negative charge delocalised over the two oxygen atoms.
Hydrogen bonding explains boiling and water solubility
A carboxylic acid supplies both an O–H donor and carbonyl-O acceptor, so its molecules form strong intermolecular hydrogen bonds. Paired dimers, especially in the vapour and non-polar solvents, can contain two hydrogen bonds. More energy is needed to separate the molecules than for comparable aldehydes, so boiling temperatures are generally higher.
Small carboxylic acids hydrogen-bond with water and are highly soluble. Increasing the hydrocarbon chain makes the non-polar contribution more important and lowers water solubility. Converting an acid to its carboxylate salt often increases water solubility because ions are strongly hydrated.
Do not say the covalent C=O or O–H bonds are broken during ordinary boiling. Intermolecular attractions are overcome. Hydrogen-bonded association also does not change the molecular formula of each acid molecule.
Choose oxidation or nitrile hydrolysis
For a primary alcohol, heat under reflux with excess acidified potassium dichromate(VI) to obtain the carboxylic acid. Reflux returns volatile material to the flask and gives time for oxidation through the aldehyde to the acid. Removing an aldehyde by distillation while it forms instead limits further oxidation.
An aldehyde needs one [O] to become the acid; a primary alcohol needs two [O], with water also formed. A secondary alcohol normally gives a ketone rather than an acid with an intact carbon chain. Keep the desired skeleton and functional-group position in view.
A nitrile heated under reflux with dilute hydrochloric or sulfuric acid gives a carboxylic acid and ammonium ions. The nitrile carbon becomes the carboxyl carbon. Alkaline hydrolysis instead produces a carboxylate plus ammonia; acidification then releases the free acid. Hydrolysis does not itself add a carbon: the chain was extended when CN was introduced.
Two ways to change the carboxyl group
LiAlH₄ in dry ether, followed by controlled aqueous work-up, reduces a carboxylic acid to a primary alcohol. Four [H] are needed in the overall equation: the carbonyl oxygen is removed as water while the carbonyl carbon becomes CH₂. Sodium borohydride is not the required substitute for this acid reduction.
Phosphorus(V) chloride, PCl₅, converts a carboxylic acid into an acyl chloride by replacing the OH of COOH with Cl. Phosphoryl chloride, POCl₃, and HCl are also formed. Moist air can give steamy acidic fumes. This observation is not unique to acids because other compounds with OH also react with PCl₅.
Worked example: CH₃CH₂COOH → CH₃CH₂COCl gives propanoyl chloride, still with three carbons. CH₃CH₂CH₂Cl would be a different functional group and the wrong transformation. Acyl chlorides are moisture-sensitive and corrosive; appropriate containment and dry apparatus are selected from the procedure’s hazard information.
Use stoichiometry to distinguish hydroxide and carbonate
Hydroxide removes the acidic H, giving a carboxylate salt and water. Carbonate consumes two acid protons and releases CO₂; hydrogencarbonate consumes one. Effervescence that produces a gas turning limewater milky supports an acid within an organic candidate list. Do not label bubbles as “carbon dioxide” until the gas evidence justifies it.
A 20.0 cm³ sample of 0.150 mol dm⁻³ propanoic acid contains 0.150 × 20.0/1000 = 0.00300 mol acid. It needs 0.00300 mol NaOH, but only 0.00150 mol Na₂CO₃. The distinction comes from the balanced proton-transfer equations, not from a memorised one-to-one titration rule.
In acid–base extraction, adding aqueous alkali turns a neutral acid into a hydrated carboxylate that favours the aqueous phase. Acidifying that layer regenerates the acid; whether it precipitates depends on its water solubility.
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official Edexcel mark allocations.
Q1. Why does ethanoic acid normally boil above ethanal?Show answer
Ethanoic acid forms intermolecular hydrogen bonds using its O–H donor and oxygen acceptors. Ethanal has only London and permanent dipole interactions between its own molecules. More energy is required to separate the hydrogen-bonded acid molecules.
Q2. Give the conditions to convert propan-1-ol into propanoic acid and explain reflux.Show answer
Excess acidified potassium dichromate(VI), heated under reflux. Condensed vapour returns to the flask, retaining volatile reactants/intermediate so oxidation can proceed fully to the acid.
Q3. Write the acid-hydrolysis equation for butanenitrile and check nitrogen’s product.Show answer
CH₃CH₂CH₂CN + 2H₂O + H⁺ → CH₃CH₂CH₂COOH + NH₄⁺. The nitrile carbon remains in the four-carbon acid; nitrogen is present as ammonium in acid, not free NH₃.
Q4. How many moles of [H] are required in the shorthand reduction of 0.0250 mol ethanoic acid to ethanol?Show answer
RCOOH + 4[H] → RCH₂OH + H₂O, so 4 × 0.0250 = 0.100 mol [H] equivalents. This is not a claim that 0.100 mol LiAlH₄ is required; [H] is a bookkeeping symbol.
Q5. A student makes an acyl chloride using PCl₅ and draws RCH₂Cl. Correct the structure and name both inorganic coproducts.Show answer
The product is RCOCl: the C=O remains and OH is replaced by Cl. The coproducts are POCl₃ and HCl. Replacing the whole COOH group with CH₂Cl is a connectivity error.
Sources
Sources and examiner guidance (reviewed 9 October 2026)
- Pearson Edexcel 9CH0 specification, Issue 3 — Topic 17, printed pp.38–39; Year 13 content building on 8CH0/9CH0 Topic 6.
- Chemrevise: UK Edexcel Organic Chemistry II — Guide pp.11–13; secondary coverage reference. Explanations, worked data and questions are original Finesse material.
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.