AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
The structures of: carboxylic acids esters.
Full specification wording
Carboxylic acids and esters (A-level only): The structures of: carboxylic acids esters.
A carboxylic acid carries the -COOH group: one carbon holding both a C=O and an O-H, so the two oxygen atoms sit on the same carbon and are never bonded to each other.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 1 of 8
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A carboxylic acid carries the -COOH group: one carbon holding both a C=O and an O-H, so the two oxygen atoms sit on the same carbon and are never bonded to each other. Names end in -oic acid, and the acid is weak but still strong enough to liberate carbon dioxide from a carbonate, which is the standard test-tube check for it. An ester replaces that acidic hydrogen with an alkyl group, giving R-COO-R′: the C=O stays put and the second alkyl group hangs off the single-bonded oxygen. The name runs the opposite way round from the drawing, so ethyl ethanoate takes ethyl from the alcohol and ethanoate from the acid. With no O-H left, ester molecules cannot hydrogen bond to one another, so an ester boils lower than the acid of similar size and smells sweet rather than sharp.
What earns the marks
- A carboxylic acid drawn with the C=O and the O-H on the same carbon atom.
- An ester drawn with the alkyl group on the single-bonded oxygen, not on the carbonyl oxygen.
- Naming an ester as alkyl alkanoate, the alkyl taken from the alcohol.
- Effervescence with a carbonate identifying the carboxylic acid rather than the ester.
Common mistakes and how to put them right
Drawing -COOH as C-O-O-H, which is a peroxide and not an acid.
Attaching the ester's second alkyl group to the carbonyl oxygen instead of the single-bonded oxygen.
Naming CH₃COOCH₂CH₃ as ethanoate ethyl, or as ethanoic ethanoate.
Answering with a molecular formula such as C₃H₆O₂, which AQA rejects as ambiguous between an acid and an ester.
Exam tips
- Draw the bond from the chain to the carbon of -COOH, never to one of its oxygens; an ambiguous bond loses the structure mark.
- If asked to distinguish an acid from an ester, give the reagent (sodium carbonate solution) and the observation (fizzing), not just the word 'test'.
- In a displayed formula show every bond, including both O-H and C=O; bond angles do not have to be right.
Try the lesson
The start of the Carboxylic acids and esters (A-level only): The structures of: carboxylic acids esters. lesson, made from checked teaching notes for this specification point.
Have a go first
Which functional group is present in every carboxylic acid?
Listen
A carboxylic acid carries the -COOH group: one carbon holding both a C=O and an O-H, so the two oxygen atoms sit on the same carbon and are never bonded to each other. Names end in -oic acid, and the acid is weak but still strong enough to liberate carbon dioxide from a carbonate, which is the standard test-tube check for it. An ester replaces that acidic hydrogen with an alkyl group, giving R-COO-R′: the C=O stays put and the second alkyl group hangs off the single-bonded oxygen. The name runs the opposite way round from the drawing, so ethyl ethanoate takes ethyl from the alcohol and ethanoate from the acid. With no O-H left, ester molecules cannot hydrogen bond to one another, so an ester boils lower than the acid of similar size and smells sweet rather than sharp. A strong answer earns marks for these points. a carboxylic acid drawn with the C=O and the O-H on the same carbon atom. an ester drawn with the alkyl group on the single-bonded oxygen, not on the carbonyl oxygen. naming an ester as alkyl alkanoate, the alkyl taken from the alcohol. effervescence with a carbonate identifying the carboxylic acid rather than the ester. Exam tip: Draw the bond from the chain to the carbon of -COOH, never to one of its oxygens; an ambiguous bond loses the structure mark.
Key words
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carboxyl group
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ester linkage
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alkyl alkanoate naming
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See every definition
- carboxyl group
- A group in which one carbon holds both a C=O and an O-H, so the two oxygen atoms sit on the same carbon and are never bonded to each other.
- ester linkage
- A structure in which the acidic hydrogen is replaced with an alkyl group, giving R-COO-R′, with the C=O staying put and the second alkyl group hanging off the single-bonded oxygen.
- alkyl alkanoate naming
- The name runs the opposite way round from the drawing, so the alkyl is taken from the alcohol and the alkanoate from the acid.
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