An ester forms when an alcohol reacts with a carboxylic acid, and water is the other product. The skill is to name the ester from its parents, write the formula and work backwards from an ester to its alcohol and acid. It builds on alcohol and acid changes inside organic reactions.
How do an alcohol and an acid form an ester?
The reaction is called esterification. The alcohol gives up a hydrogen from its OH group, and the acid gives up its OH, and together they leave as one water molecule. The two halves join through a new link, the ester group, written -COO-.
An acid catalyst, such as concentrated sulfuric acid, is used. The reaction is reversible, so the equation uses a reversible arrow.
Naming rule: the alcohol names the first word, with the ending -yl. The acid names the second word, with -anoic acid changed to -anoate. So methanol gives methyl, and propanoic acid gives propanoate.
Worked example
Ethanol reacts with ethanoic acid. Name the ester and write the equation.
Step 1, parents: ethanol, C₂H₅OH, and ethanoic acid, CH₃COOH.
Step 2, name: the alcohol part is ethyl. The acid part is ethanoate. The ester is ethyl ethanoate.
Step 3, equation: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
Step 4, check atoms: left C₂H₄O₂ + C₂H₆O gives 4 C, 10 H, 3 O. Right C₄H₈O₂ + H₂O gives 4 C, 10 H, 3 O. Balanced.
Step 5, mass of ester: Mr of ethyl ethanoate is 4(12) + 8(1) + 2(16) = 88. If 4.6 g of ethanol (Mr 46, so 0.10 mol) reacts with excess acid, the most that could form is 0.10 mol, which is 8.8 g. The real yield is lower because the reaction is reversible.
The mistake to watch for
A common slip is to name the ester in the wrong order.
Mistaken name: “ethanoyl ethanol” for the ester from ethanol and ethanoic acid.
The alcohol and acid names have been swapped and the endings are wrong.
The correction: alcohol first with -yl, acid second with -oate. Ethyl ethanoate is correct.
Another slip is to forget the water. Always include it, as in the equation above.
Check yourself
1. Name the ester made from propan-1-ol and ethanoic acid.
Show answer
Propyl ethanoate. The alcohol gives propyl and the acid gives ethanoate.
2. Methanol reacts with butanoic acid. Name the ester and say what else forms.
Show answer
Methyl butanoate, and water forms as the second product.
3. An ester is named ethyl propanoate. What alcohol and acid would you get on splitting it?
Show answer
Ethanol (from ethyl) and propanoic acid (from propanoate).
Where this leads next
Long molecules built from repeating units use a related idea, covered in recognising a polymer repeat unit. For the earlier steps in the chain, revisit alcohol and acid transformations. Then test everything with the organic reactions practice set.
If you can follow the naming rule but still slip under time pressure, our teachers can coach that in online one-to-one Chemistry tuition.