Lesson 6 of 9
Esters: join two fragments and keep track of their origins
The name has an alcohol part followed by an acid part.
O-Level 6092 (2026) / SEC G3 K324 (2027)
Pure onlyI can explain ester formation.Syllabus
Syllabus K324, 11.3(i). Explain ester formation
I can deduce ester names/formulae and precursors.Syllabus
Syllabus K324, 11.3(j). Deduce ester names/formulae and precursors
Esters: join two fragments and keep track of their origins
The name has an alcohol part followed by an acid part.
Warm a carboxylic acid with an alcohol and a little concentrated sulfuric acid catalyst to form an ester and water. The reaction is reversible. Ethanoic acid and ethanol give ethyl ethanoate: CH3COOH + HOCH2CH3 reversibly form CH3COOCH2CH3 + H2O. The ester linkage is -C(=O)-O-.
| Starting family | C1 | C2 | C3 | C4 |
|---|---|---|---|---|
| Unbranched acid part | Methanoate | Ethanoate | Propanoate | Butanoate |
| Alcohol part, -OH at chain end | Methyl | Ethyl | Propyl | Butyl |
The acid contributes the part ending in -oate, including its C=O carbon. The alcohol contributes the group attached through the ester oxygen. For an alcohol with -OH on a middle carbon, retain that attachment position when drawing the product; a numbered group such as propan-2-yl may be used when naming it.
Worked exampleName and reverse an ester
Identify the acid and alcohol that form CH3CH2COOCH3.
Try it first. Then show the worked stepsHide the worked steps
- Split at the bond between the single-bonded O and the alcohol-derived carbon group.
- CH3CH2CO- is the three-carbon acid fragment: propanoic acid.
- -CH3 attached to the ester O comes from methanol.
Methyl propanoate, made from propanoic acid and methanol. Count the carbonyl carbon when counting the acid carbons.
Check your understanding
Which ester forms from butanoic acid and ethanol?
Show the answer
Ethyl butanoate, CH3CH2CH2COOCH2CH3
The alcohol gives the first word (ethyl) and the acid the second (butanoate). The formula is often written starting with the acid part.