Esters: the –COO– group and esterification
An ester contains the ester functional group . Esters form by esterification: a carboxylic acid reacts with an alcohol, with concentrated sulfuric acid as the acid catalyst, in a reversible reaction that also makes water. The classic example is ethyl ethanoate, from ethanol + ethanoic acid:
Naming esters: alkyl + -anoate
Name an ester from the two parts that built it. The alcohol gives the first word (the alkyl group): methanol → methyl, ethanol → ethyl, propanol → propyl. The carboxylic acid gives the second word, changing -anoic acid to -anoate: ethanoic acid → ethanoate, propanoic acid → propanoate. So propanol + ethanoic acid → propyl ethanoate. Working backwards, methyl propanoate must come from methanol + propanoic acid.
Esters: volatile flavourings & perfumes
Esters are volatile — they evaporate easily, having low boiling points relative to the acids and alcohols they come from — and have distinctive, pleasant, often fruity smells. These two properties make them widely used as food flavourings and in perfumes: the volatility lets the molecules reach the nose readily, and the sweet smell makes them attractive — the same combination that gives many fruits their natural aroma.
Drawn from real examiner reports.
Drawing the acid, not the ester
Asked for the ester, candidates often draw the carboxylic acid (, ending in ) instead. The ester has no : the acidic hydrogen is replaced by the alkyl group from the alcohol. Check the linking oxygen bonds to a carbon on each side (), not a hydrogen.
June 2024 Paper 2C Q5(c): many candidates drew or wrote ethanoic acid (or a bare molecular formula) instead of the ester structure, so the mark could not be awarded.
Ester group: missing O, –OH end, 5 bonds
The group is a carbon with a C=O double bond AND a single bond to a second oxygen that links to another carbon (). Common errors: leaving out the linking oxygen, ending in (that is an acid, not an ester), or giving a carbon five bonds.
June 2024 Paper 2CR Q7(c)(i): candidates lost marks for an –OH on the end instead of the –COO– linkage, a missing linking oxygen, or carbons with more than four bonds.
Displayed formula: show every bond
In a displayed formula every single bond must be drawn — each C–H, each C–C, the C=O double bond and the C–O bonds. Candidates lose marks by omitting bonds, especially the O–H of the alcohol or acid. A condensed formula such as does not answer a displayed command.
June 2024 Paper 2C Q5(b)(iii): marks lost for not showing every bond in a displayed formula.
Miscounting atoms in the molecular formula
Condensing a displayed ester into a molecular formula, candidates miscount by a carbon or two hydrogens. Esterification removes only one from the combined alcohol + acid, so every other atom is kept: add all atoms of both, then subtract . Self-check: a simple ester fits .
June 2024 Paper 2C Q5(d)(ii): a common wrong answer was the molecular formula C5H12O2 instead of the correct C6H12O2.
Naming: alcohol first, acid second
The order of the two words trips candidates: the alcohol gives the first word (the alkyl group) and the carboxylic acid gives the second word (-anoate). Naming from the acid first is wrong — ethanol + propanoic acid gives ethyl propanoate, not "propyl ethanoate". Fix the alcohol as the alkyl and the acid as the -anoate every time.
(common Edexcel chemistry error)
Forgetting -anoic acid → -anoate
Candidates keep the acid ending unchanged. The acid's -anoic acid ending must become -anoate in the ester name: ethanoic acid → ethanoate, propanoic acid → propanoate, methanoic acid → methanoate. Leaving it as "ethanoic", or writing "ethanoicate", loses the naming mark. Only the alcohol keeps a simple alkyl form (methyl, ethyl, propyl).
(common Edexcel chemistry error)
Name-or-build routine for esters
Fix a routine: identify the two parents first. The alcohol gives the alkyl (first) part — methyl/ethyl/propyl; the acid gives the -anoate (second) part. Reverse the rule to read the parents off an ester name. Settle both halves before drawing so they are never swapped.
Writing the esterification equation
If an equation is asked for, show both products and conditions: ester plus water, over a concentrated sulfuric acid catalyst, with a reversible arrow (). The water, catalyst and arrow are the usual lost marks.
Structures and molecular formulae checks
For a structure, draw the link with the C=O double bond and C–O–C single bonds, and check each atom's bonds (C: 4, O: 2, H: 1). For a molecular formula, add both reactants' atoms and subtract one .
What functional group do all esters contain?
Ethyl ethanoate has the molecular formula . Calculate its relative formula mass (). (: C = 12, H = 1, O = 16)
