Carboxylic acidsEdexcel A-Level Chemistry: Revision notes
Section 1
The carboxylic acid functional group
Carboxylic acids contain the carboxyl group, –COOH, which combines a C=O and an O–H on the same carbon. They are named with the suffix -oic acid, with the carboxyl carbon as carbon 1 (ethanoic acid, CH₃COOH; propanoic acid, CH₃CH₂COOH). They are weak acids: they only partly ionise in water, RCOOH ⇌ RCOO⁻ + H⁺.
Section 2
Hydrogen bonding, boiling temperatures and solubility
The O–H group can form hydrogen bonds with the C=O oxygen of another molecule, so carboxylic acids have higher boiling temperatures than alcohols, aldehydes and alkanes of similar relative molecular mass. Ethanoic acid (118 °C) boils far above propanal (49 °C).
The –COOH group also hydrogen bonds with water, so the shorter acids (methanoic, ethanoic, propanoic) mix with water in all proportions. As the hydrocarbon chain gets longer the non-polar chain outweighs the hydrogen bonding and solubility falls sharply (hexanoic acid is only slightly soluble).
When explaining boiling temperatures, say that intermolecular forces are overcome. Never say that covalent bonds break.
Section 3
Preparing carboxylic acids
Oxidation of a primary alcohol: heat under reflux with an excess of acidified potassium dichromate(VI). The solution turns orange to green. Reflux returns the intermediate aldehyde to the flask until it is oxidised fully to the acid. CH₃CH₂OH + 2[O] → CH₃COOH + H₂O.
Oxidation of an aldehyde: the same oxidising agent converts RCHO to RCOOH: CH₃CHO + [O] → CH₃COOH.
Hydrolysis of a nitrile: heat the nitrile under reflux with dilute acid: RCN + 2H₂O + H⁺ → RCOOH + NH₄⁺. A nitrile is made from a halogenoalkane and KCN, so this route adds one carbon to the chain, which oxidation of an alcohol cannot do.
Section 4
Reactions with bases and with LiAlH₄
Salts: carboxylic acids react with metals, bases and carbonates to form carboxylate salts. With sodium carbonate there is effervescence (CO₂): 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂. With sodium hydroxide: CH₃COOH + NaOH → CH₃COONa + H₂O.
Reduction: LiAlH₄ in dry ether reduces a carboxylic acid to a primary alcohol: CH₃COOH + 4[H] → CH₃CH₂OH + H₂O. The ether must be dry.
Section 5
Reactions with PCl₅ and with alcohols
Phosphorus(V) chloride replaces the –OH group with –Cl, giving an acyl chloride: CH₃COOH + PCl₅ → CH₃COCl + POCl₃ + HCl. Steamy fumes of HCl are seen.
Esterification: a carboxylic acid heated with an alcohol and a few drops of concentrated sulfuric acid (catalyst) forms an ester and water: CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O. The reaction is reversible, so the yield is limited unless an excess of one reactant is used or the ester is removed by distillation. Esters have fruity smells.
In an ester name the alcohol part comes first: ethanoic acid + propan-1-ol gives propyl ethanoate.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Carboxylic acids
- Ethanoic acid (relative molecular mass 60.0, boiling temperature 118 °C) has a much higher boiling temperature than propanal (relative molecular mass 58.0, boiling temperature 49 °C). Ethanoic acid mixes with water in all proportions, but hexanoic acid, CH₃(CH₂)₄COOH, is only slightly soluble.Explain why ethanoic acid has a much higher boiling temperature than propanal.2 marks
- A student plans to make propanoic acid in two different ways. One route starts from propan-1-ol. The other route starts from propanenitrile.State the colour change seen when propan-1-ol is oxidised to propanoic acid by acidified potassium dichromate(VI), and explain why the mixture must be heated under reflux.2 marks
- Ethanoic acid is used as a starting material in several reactions. In one test, aqueous sodium carbonate is added to ethanoic acid. In other reactions, ethanoic acid is converted into an acyl chloride by phosphorus(V) chloride and into an ester by reaction with ethanol.Describe what is seen when aqueous sodium carbonate is added to ethanoic acid, and write the equation for the reaction.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).