Part 1: Carboxylic acids and ester formation

Use structure, weak-acid behaviour and esterification stoichiometry to predict products and explain properties.

Keep the two oxygen atoms distinct

A carboxylic acid contains –C(=O)OH; an ester contains –C(=O)OR. The carbonyl carbon is part of the parent chain. CH₃CH₂COOH is propanoic acid; HOOCCH₂COOH is propanedioic acid. For an ester, name the alcohol-derived group first and the acid-derived group second: CH₃COOCH₂CH₂CH₃ is propyl ethanoate, not ethyl propanoate.

The group directly attached to the single-bonded ester oxygen came from the alcohol. Hydrolysing the drawn ester and naming its two fragments is a useful way to check an unfamiliar name. Keep the ester C–O–C connection explicit in displayed or skeletal structures.

Weak acids still react with carbonate

In water, a carboxylic acid only partially dissociates. The conjugate carboxylate ion is stabilised by delocalisation across its two oxygens; the negative charge is not confined to one permanently single-bonded oxygen. Weak does not mean dilute or incapable of neutralisation. Reaction with a base removes H⁺ and draws further acid through the equilibrium.

Small carboxylic acids hydrogen-bond with water and are soluble; the non-polar hydrocarbon group becomes more influential as it grows. The acids also hydrogen-bond to one another, contributing to relatively high boiling points. Solubility and acidity are separate properties. Electron-withdrawing substituents can stabilise a conjugate base and increase acidity, but do not impose a universal simple acidity order from chain length alone.

Predict salts and identify the gas

Carboxylic acids react with reactive metals, alkalis and carbonates. Sodium metal releases H₂; carbonate releases CO₂. The different gases come from different reaction types, so do not report a hydrogen test for carbonate effervescence. Carbonate effervescence is useful evidence for an acid among suitable organic candidates, but is not unique to carboxylic acids if other acids are allowed.

An acid catalyst accelerates a reversible reaction

Heat a carboxylic acid and an alcohol under reflux with an acid catalyst, commonly concentrated sulfuric acid, to form an ester and water. Reflux returns condensed volatile material while heating; it does not collect a distillate. The reaction is reversible, so using excess reactant or removing a product can favour ester formation. A catalyst speeds equilibration without changing the equilibrium constant.

Pure simple esters lack an O–H donor and do not hydrogen-bond to one another, but their oxygen atoms can accept hydrogen bonds from water. Water solubility varies with structure; do not label every ester completely insoluble. Many volatile esters are used in flavourings/perfumes, selected esters are solvents, and larger ester plasticisers help polymer chains move past one another. A pleasant smell does not demonstrate safety.

Worked example: limiting reagent and atom economy

Constructed preparation: 6.00 g ethanoic acid, Mᵣ = 60.0, reacts with 9.00 g propan-1-ol, Mᵣ = 60.0. Their amounts are 0.100 and 0.150 mol; acid is limiting in the 1:1 equation. Propyl ethanoate has Mᵣ = 102.0, giving 10.2 g theoretical product. Isolating 7.14 g gives 70.0% yield.

Ideal reaction atom economy for the ester is 102.0/(60.0 + 60.0) × 100 = 85.0%. Use stoichiometric reactant masses, not the experimentally chosen excess, in this equation-based calculation. Yield and atom economy measure different things. The later Organic Synthesis practical lesson covers washing, drying and distilling the liquid.

Quick checks

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

Sources