Carboxylic acids and esters (A-level only) — AQA A-Level Chemistry

Esterification is a reversible reaction: an alcohol and a carboxylic acid heated under reflux with a few drops of concentrated sulfuric acid reach an equilibrium mixture, so the yield is always well short of the theoretical maximum. Ethanol and ethanoic acid give ethyl ethanoate and water, and the sulfuric acid is a catalyst that is regenerated, not a reactant in the equation. To push the equilibrium over you use an excess of the cheaper reagent or distil the ester out as it forms. The crude distillate still holds unreacted acid, so it is shaken in a separating funnel with sodium carbonate solution; that neutralises the ethanoic and sulfuric acids and releases carbon dioxide, which is why pressure builds and the funnel has to be vented.

Esters are volatile and have strong, pleasant smells, which is why they are used in perfumes and food flavourings; short-chain esters are what a pear drop or a banana sweet is imitating. Volatility comes from the bonding: the C=O makes the molecule polar, so there are permanent dipole-dipole forces, but there is no O-H left, so ester molecules cannot hydrogen bond to one another and the forces holding them together are relatively weak. That same polarity with low boiling point makes esters good solvents for glues, inks and nail varnish remover. As plasticisers they are mixed into a polymer such as PVC, where the ester molecules sit between the chains and hold them apart, weakening the forces between chains so the chains slide and the plastic becomes flexible.

Glycerol is propane-1,2,3-triol, a three-carbon chain carrying an -OH on every carbon. Each -OH can be esterified by a long-chain carboxylic acid, so one glycerol plus three acid molecules gives a triglyceride with three ester linkages and three molecules of water. The three chains need not be the same. In a fat the chains are saturated, so they are straight, pack closely and have large contact areas for van der Waals forces, which makes fats solids at room temperature. In an oil the chains carry Z (cis) C=C bonds that kink them, so they pack badly, the van der Waals forces are weaker and the oil is a liquid. Bromine adds across each C=C, so the mole ratio of bromine to oil counts the double bonds per molecule.

Both routes cut the ester at the same place, between the carbonyl carbon and the single-bonded oxygen, but they end in different places. Heating under reflux with dilute sulfuric acid is reversible: water attacks the δ⁺ carbonyl carbon, and you reach an equilibrium containing the alcohol, the carboxylic acid and unreacted ester, so a large excess of water is used to push it over. Heating under reflux with aqueous sodium hydroxide goes to completion, because the product is the carboxylate ion, which carries a negative charge and is not attacked by the alcohol, so the reverse reaction cannot happen. The alcohol is the same either way; the acid is obtained from the salt afterwards by adding a strong acid such as dilute hydrochloric acid.

Saponification is alkaline hydrolysis carried out on a triglyceride. Three ester links means three moles of sodium hydroxide per mole of fat, and the products are three molecules of a sodium salt of a long-chain carboxylic acid, RCOO⁻Na⁺, plus one molecule of glycerol. That salt is the soap. It works because it has two incompatible ends: a long non-polar hydrocarbon tail, which mixes with grease, and an ionic COO⁻ head, which is attracted to water. Soap molecules surround a grease droplet with their tails inwards and heads outwards, forming a micelle whose charged surface keeps droplets apart so the grease is lifted and suspended. In hard water the calcium and magnesium salts are insoluble, so scum forms instead.

Biodiesel molecules have the form RCOOCH₃, where R is a long hydrocarbon chain of typically 15 to 17 carbons and the methyl group sits on the single-bonded oxygen. It is a mixture rather than a single compound because the vegetable oil it came from contains several different fatty acid chains, of different lengths and with different numbers of C=C bonds, and each one becomes its own methyl ester. That mixture burns in a diesel engine much like the hydrocarbon fuel it replaces. Biodiesel is often called carbon neutral because the carbon dioxide released on burning was taken from the air by the crop as it grew, but the claim is only partly true: fuel is burned to farm, harvest, transport and process the oil, and land is taken out of food production.

Biodiesel is made by transesterification: the triglyceride is warmed with a large excess of methanol and a catalyst, usually sodium hydroxide or potassium hydroxide, though a strong acid also works. Methanol replaces the glycerol at each of the three ester links, so one mole of triglyceride needs three moles of methanol and gives three moles of methyl ester plus one mole of glycerol. The reaction is reversible, which is why the methanol is in excess: it shifts the position of equilibrium towards the esters. The glycerol is denser and separates as a lower layer, so it is run off and sold, and removing it also pulls the equilibrium forwards. The catalyst is recovered rather than consumed.