Part 2: Ester hydrolysis, fats, soaps and biodiesel

Distinguish acidic hydrolysis, alkaline hydrolysis and transesterification, with correct products and mole ratios.

Acid and alkali give different carboxyl products

Heating an ester under reflux with dilute acid and water gives an alcohol and carboxylic acid. The acid catalyses a reversible hydrolysis, the reverse of esterification. Excess water can favour hydrolysis.

Heating with aqueous NaOH gives an alcohol and the sodium carboxylate. Formation of the carboxylate makes the process effectively one-way under these conditions; hydroxide is consumed rather than merely acting as an unchanged catalyst. Acidification after hydrolysis converts the carboxylate into the carboxylic acid.

Split the ester at the acyl–oxygen bond

Identify R–C(=O)–O–R′ before splitting. The acid fragment retains the original carbonyl carbon; the alcohol fragment retains R′ attached to oxygen. For phenyl ethanoate, CH₃COOC₆H₅, the organic hydrolysis fragments are ethanoic acid/ethanoate and phenol, not benzyl alcohol. In excess strong alkali, phenol can also be deprotonated.

Ethyl benzoate, C₆H₅COOCH₂CH₃, gives benzoate and ethanol on alkaline hydrolysis. Acidification can precipitate sparingly soluble benzoic acid. This connects functional-group chemistry to isolating and recrystallising an organic solid; “all organic acids are insoluble” is not the explanation.

Fats and oils contain three ester links

A triglyceride is a triester of glycerol, propane-1,2,3-triol, with three long-chain carboxylic acids. The three chains need not be identical and may contain C=C bonds. Complete hydrolysis requires three ester-link cleavages per triglyceride, producing one glycerol and three acid-derived fragments.

In the illustrative formula below, C₃H₅ represents the glycerol carbon skeleton, and each OCOC₁₅H₃₁ group is an ester-linked acyl chain. A displayed structure must show CH₂–O, CH–O and CH₂–O at the three glycerol positions; the shorthand is not a replacement if every bond is requested.

The soap is the carboxylate salt

Sodium or potassium salts of long-chain carboxylic acids act as soaps. Their charged carboxylate head interacts favourably with water, while the long non-polar tail associates with oily material. Aggregates can surround grease and help disperse it for removal. Calling the un-ionised fatty acid itself the soap misses the ionic head group.

For a constructed sample of 2.50 mmol triglyceride, complete saponification consumes 7.50 mmol OH⁻ and forms 2.50 mmol glycerol plus 7.50 mmol long-chain carboxylate. Any pre-existing free acids also consume hydroxide, so a real oil analysis needs the composition assumptions stated.

Methanol exchanges the alcohol part of the ester

Biodiesel is a mixture of methyl esters of long-chain carboxylic acids. Vegetable oil reacts with methanol in the presence of a suitable catalyst, commonly an alkaline catalyst, to produce those methyl esters and glycerol. This is transesterification, not ordinary hydrolysis: methanol supplies the methyl ester group and no water is a required product of the ideal net reaction.

The carbon in biomass was recently taken from the atmosphere, but cultivation, fertiliser, processing, methanol production, transport and land-use changes affect total emissions. Do not call every biodiesel process automatically carbon-neutral. Compare fuel performance, land/food demand, process energy and waste using stated evidence.

Quick checks

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

Sources