Esters: Structures, Names and Practical (Paper 2)

Explained

Esters: naming them, drawing them, making them

An alcohol reacts with a carboxylic acid to form an ester and water. The reaction is reversible, and it needs a catalyst, usually concentrated sulfuric acid. Get those three facts in place and the rest of the topic is naming and structure.

Where the name comes from

An ester name has two parts, and each part comes from a different reactant. The alcohol supplies the first part, and the acid supplies the second.

Ethanol and ethanoic acid give ethyl ethanoate. Methanol and ethanoic acid give methyl ethanoate. The alcohol's stem becomes the yl, and the acid's stem becomes the oate.

Working backwards is just as examinable. Given propyl methanoate, the alcohol was propanol and the acid was methanoic acid. Split the name at the space and take each half back to its parent.

The ester link

The functional group is the ester link, where a carbon is double bonded to one oxygen and single bonded to another, and that second oxygen joins to the rest of the chain.

When you draw the structure, the oxygen that was part of the alcohol becomes the single bonded one in the link. Being clear about which atom came from where makes the structure much easier to construct correctly and much easier to check.

Making an ester safely

Mix the alcohol and the carboxylic acid with a few drops of concentrated sulfuric acid as the catalyst, and warm the mixture gently in a water bath.

Use a water bath rather than a naked flame. Alcohols are volatile and flammable, which means their vapour ignites easily, and an open flame near a volatile organic liquid is the hazard the question is checking that you have spotted.

Esters have strong, often pleasant smells, which is how the product is identified in the laboratory and why they are used in perfumes and flavourings.

Reversible means incomplete

Because the reaction is reversible, it does not go to completion. Some alcohol and acid remain in the mixture alongside the ester and water.

That is why the product must be separated and purified afterwards, and why a question about yield in this reaction will not expect the answer to be 100 per cent. If asked to explain a low yield, the reversibility is the reason.