Esters

Esters are Extended-only, short, and astonishingly reliable: name the ester, name the catalyst, draw the linkage. Most series the question is worth 2-4 marks, and most of the errors are name-order errors. Students who can make ethyl ethanoate perfectly in the lab still write “ethanoate ethyl” under pressure.

Making an ester

Heat a carboxylic acid with an alcohol in the presence of a few drops of concentrated sulfuric acid (the catalyst):

CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O

ethanoic acid + ethanol → ethyl ethanoate + water

Three marking points hide in that line: the catalyst (concentrated sulfuric acid), the second product (water), and the reversibility (the ⇌ matters in equilibrium cross-over questions). Because a small molecule (water) is released when the two reactants join, esterification is the model reaction for condensation polymerisation, picked up at scale on the polymers page where polyesters such as PET form the same linkage thousands of times over.

In the practical, the ester is recognised by its sweet, fruity smell, which is also the basis of the uses: flavourings and perfumes.

Naming: alcohol first, acid second

The ester’s name comes in two halves:

• the alcohol contributes the first word, ending -yl (methanol → methyl, ethanol → ethyl, propanol → propyl)
• the carboxylic acid contributes the second word, ending -oate (methanoic → methanoate, ethanoic → ethanoate)

Alcohol	Acid	Ester
ethanol	ethanoic acid	ethyl ethanoate
methanol	butanoic acid	methyl butanoate
propanol	methanoic acid	propyl methanoate
butanol	ethanoic acid	butyl ethanoate

Given an ester name, run it backwards: methyl propanoate came from methanol and propanoic acid. That reverse deduction is the harder and more common exam direction.

Structure and the ester linkage

The ester linkage is −COO−: a carbon double-bonded to one oxygen and single-bonded to a second oxygen that carries the rest of the alcohol chain.

For ethyl ethanoate, CH3COOC2H5: the CH3C(=O)− half comes from ethanoic acid; the −OC2H5 half comes from ethanol. In a displayed formula, draw the C=O explicitly and every C−H bond. Two checks before moving on: exactly one C=O, and no O−H left anywhere (the acid’s O−H hydrogen left in the water).

Structural formulae like CH3COOC2H5 are accepted where the question says “structural”; “displayed” means every bond drawn, the same rule as throughout organic chemistry.

Worked exam question

Propanoic acid reacts with methanol to form an ester. (a) Name the catalyst used. [1] (b) Name the ester and the other product. [2] (c) Draw the displayed formula of the ester. [2]

Model answer, mark by mark:

• (a) M1: concentrated sulfuric acid. (“Sulfuric acid” usually earns it; “an acid” does not.)
• (b) M2: methyl propanoate (methanol gives methyl, propanoic acid gives propanoate, in that order). M3: water.
• (c) M4: correct skeleton: CH3−CH2−C(=O)−O−CH3 with the C=O shown. M5: all hydrogens drawn with their bonds (eight of them). A missing hydrogen or a stray O−H forfeits this mark.

Note the symmetry trap: methyl propanoate and propyl methanoate are different esters from different reactants. The acid’s carbons carry the C=O.

The mistakes that cost marks

1. Name halves reversed. Ethanol + methanoic acid makes ethyl methanoate, not methyl ethanoate. Anchor it: alcohol → -yl → first.
2. Water forgotten. Esterification has two products. “Ester” alone in a products question is half marks at best.
3. Catalyst named as the reactant acid. The carboxylic acid is a reactant; the catalyst is concentrated sulfuric acid. Writing “ethanoic acid catalyses it” confuses the roles.
4. Displayed formulae with an extra OH. The acid’s O−H and one H from the alcohol leave as water. If your ester still shows an O−H bond, it is not an ester.

How examiners want it phrased

Student wording	Mark-scheme wording
”Mix the acid and alcohol with acid"	"Heat the carboxylic acid with the alcohol and a few drops of concentrated sulfuric acid as catalyst"
"It smells nice so it’s used in perfume"	"Esters have sweet smells and are used in perfumes and food flavourings"
"The two join up"	"The acid and alcohol join with the loss of a water molecule, forming the ester linkage −COO−"
"It can go backwards"	"Esterification is reversible; the equation is written with ⇌”

If you are an Extended candidate, esters pair naturally with carboxylic acids in revision: same reagents, one extra step. Unsure whether the Extended paper is the right call for you at all? That is the first question we answer in a free trial lesson, with your mock scripts in front of us.

Test yourself

All three are Supplement level, like this page. Answer first, then click to check.

Q1 (3 marks). Ethanol is heated with butanoic acid and a catalyst. Name the ester formed, the other product, and the catalyst.

Show answer

• ethyl butanoate (alcohol half first) [1] • water [1] • concentrated sulfuric acid [1]

Q2 (2 marks). Name the alcohol and the carboxylic acid needed to make propyl ethanoate.

Show answer

• propanol (gives the propyl half) [1] • ethanoic acid (gives the ethanoate half) [1]

Q3 (2 marks). Write the structural formula of methyl ethanoate, and explain why the esterification equation is written with the ⇌ symbol.

Show answer

• CH3COOCH3 (the CH3CO half from ethanoic acid, the OCH3 half from methanol) [1] • the reaction is reversible [1]