Part 1: Carboxylic acids: properties and reactions

Explain physical properties through intermolecular forces and use proton transfer to balance carboxylic-acid reactions.

Treat COOH as one functional group

A carboxylic acid contains –C(=O)OH. The carbonyl and hydroxyl belong to a single carboxyl group; do not predict that it behaves as an independent ketone plus alcohol. The carboxyl carbon is included in the parent chain and numbered carbon 1.

CH₃CH₂COOH is propanoic acid, with three carbons. A dicarboxylic acid has two COOH groups and can donate two protons in complete neutralisation. Carbon counting and the number of acidic groups determine the stoichiometry before any calculation.

Carboxylic acids are weak acids in aqueous solution: only a fraction of their molecules donate H⁺ to water. Weak does not mean dilute and does not prevent essentially complete reaction with a sufficient amount of a strong base.

Hydrogen bonding explains two different trends

Carboxylic-acid molecules can form hydrogen bonds using O–H as a donor and oxygen lone pairs as acceptors. These strong intermolecular attractions contribute to relatively high boiling points compared with similarly sized molecules unable to donate hydrogen bonds. Hydrogen-bonded pairs are especially relevant in suitable non-aqueous environments.

Short-chain acids dissolve readily in water because their polar COOH groups form hydrogen bonds with water. As the hydrocarbon chain becomes longer, a larger non-polar region must be accommodated, and solubility generally decreases. Do not claim that a longer acid stops having hydrogen bonds: the balance between its polar group and non-polar chain changes.

When discussing boiling, say intermolecular forces are overcome; ordinary boiling does not break the covalent C–C, C=O or O–H bonds within the acid molecule. When discussing solubility, compare interactions with water rather than quoting boiling-point arguments alone.

One acidic proton gives a predictable set of reactions

With a reactive metal, the acid forms a carboxylate salt and hydrogen. For magnesium, two acid molecules provide the two H⁺ equivalents needed per Mg atom: 2RCOOH + Mg → (RCOO)₂Mg + H₂. The salt’s formula follows the metal charge.

With a metal oxide or hydroxide, neutralisation forms the carboxylate salt and water. With carbonate, carbon dioxide is also formed. Effervescence with carbonate is useful evidence distinguishing a carboxylic acid from phenol or an ordinary alcohol under the standard test conditions.

Write RCOO⁻ after deprotonation, not RCO⁻: both oxygens remain. The OH hydrogen is removed, while the carbon skeleton and carbonyl connectivity are retained.

Worked example: count acid groups before using volume

An original sample contains 0.750 g of HOOC–CH₂–COOH, Mr 104.0. The amount of acid is 0.750/104.0 = 0.00721 mol. Because each molecule has two COOH groups, complete neutralisation needs 0.0144 mol OH⁻.

With 0.400 mol dm⁻³ NaOH, volume = n/c = 0.0361 dm³ = 36.1 cm³. The factor of two comes from two acidic groups, not from the word “weak”. Ignoring one COOH group would halve the calculated volume.

For a product drawing after complete neutralisation, replace both COOH groups with COO⁻ and include two Na⁺ counterions if a salt formula is requested. In an acidified work-up the groups return to COOH. The solution conditions determine which form is appropriate.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources