Carboxylic Acids, Esters, Ester Hydrolysis & Biodiesel (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about carboxylic acids, esters, ester hydrolysis and biodiesel.

These are covered together because carboxylic acids react with alcohols to form esters, ester hydrolysis is that reaction run in reverse, and biodiesel is a real industrial application of forming esters at scale.

Carboxylic acids and esters are two sides of the same reaction. Carboxylic acids react with alcohols to form esters, in a reaction called esterification. Ester hydrolysis is esterification run in reverse, breaking an ester back down into an alcohol and a carboxylic acid. Biodiesel is what happens when that ester-forming reaction is used industrially, converting vegetable oils into a fuel.

This video starts with carboxylic acids and how they form esters, then covers how esters are hydrolysed back apart, and finishes with biodiesel production.

Carboxylic acids contain the carboxyl group, COOH, and follow the general formula CnH2n+1COOH. In water they're only partially ionised, which makes them weak acids. The equilibrium lies well to the left, so the concentration of H plus ions is much lower than the concentration of the acid itself. That's still enough hydrogen ion concentration to react with sodium carbonate or sodium hydrogen carbonate solutions, producing carbon dioxide, which when bubbled through limewater, is a useful test for a carboxylic acid. Carboxylic acids also react with alcohols, in the presence of a concentrated acid catalyst, to form esters. This condensation reaction is called esterification, and it's the reaction the rest of this video builds on.

Esters contain the ester group, COO, and are named after the alcohol first, then the acid, with the acid's -oic acid ending changed to -oate, for example, ethyl ethanoate. Esters have fruity, sweet smells and are used as food flavourings and fragrances. They're also used as solvents and as plasticisers, where they sit between polymer chains in PVC to make it more flexible. Vegetable oils and animal fats are naturally occurring esters too. Triglycerides formed from glycerol and three fatty acid chains. Esters are the product side of the esterification reaction covered a moment ago.

Ester hydrolysis is the reverse of esterification. Heating an ester under reflux with a dilute acid sets up an equilibrium, so the reaction doesn't go to completion and produces a carboxylic acid and an alcohol. Heating it under reflux with a dilute alkali instead is irreversible. The ester is fully hydrolysed, forming a carboxylate salt and an alcohol. The salt can be acidified afterwards to recover the carboxylic acid. The same alkaline hydrolysis applied to vegetable oils and animal fats is called saponification where triglycerides are broken down into glycerol and the sodium or potassium salts of fatty acids, which are soaps.

Biodiesel is made by transesterification, reacting a vegetable oil's triglycerides with methanol to form methyl esters and glycerol. Both acid and alkaline catalysts can be used, though alkaline catalysts such as sodium or potassium hydroxide are more common industrially. Because the reaction is reversible, an excess of methanol is used to push the equilibrium towards the esters, and under optimal conditions this can achieve yields of around 98 percent. Biodiesel is the same ester-forming chemistry from earlier in this video, just run at an industrial scale to make a fuel.

When drawing esters, especially in vegetable oil structures, make sure the C-O-O linkage is drawn the right way round. And remember that concentrated sulfuric acid is a catalyst for esterification, it isn't used up in the reaction, which is why the reaction is reversible and needs to be heated under reflux.

Carboxylic acids react with alcohols to form esters. Ester hydrolysis just reverses that reaction, splitting the ester back into an alcohol and a carboxylic acid or its salt, if hydrolysed under alkaline conditions, which goes to completion rather than reaching an equilibrium. Biodiesel is that same ester-forming reaction used industrially, converting vegetable oils into methyl esters for fuel.

On Save My Exams, you'll find exam specific revision, made by trusted examiners. Study with revision notes, practice with exam questions, and improve your grade by targeting your weak spots. Visit the links in the description.